3-V Accelerometer Featuring TLV2772
Contents
1. END CONV TRO A Interrupthandler for the external INT3 IRQ which is the end of conversation signal of the ADC The Routine will enable the Chip Signal again and send the next sample instruction contained in ADWORD to the ADC END CONV TRO MAIN IRQ ROUT TN LDP ADWORD LACL ADWORD XF Enable Chip Select move ADWORD into DXR ETE Return to Wait loop KKK KKK KKK KKK KKK KKK KKK KKK KK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KK ECEIVE IRQ Interrupthandler for serial receive of the SPI interface The Routine will disable the Chip Signal and store the received sample into the by ADMEM specified memdory location It also checks how many samples were made2. 10 100 1k 10k 100k f Frequency Hz Figure 4 4 Bode Plot H1 s Vp Vi 4 1 4 2 Component Values for H2 s Vo Vp To minimize phase shift in the feedback loop caused by the input capacitance of the TLV2772 it is best to minimize the value of feedback resistor R4 Also to reduce the required capacitance in the feedback loop a large value resistor is required for R4 A compromise value of 100 kQ is used for R4 To set the upper cutoff frequency the required capacitor value for C2 is C2 1 27 x upper cutoff frequency Hz x R4 Q C2 1 6 28 x 500 Hz x 100 kQ 3 18 nF A more common 2 2 nF capacitor is used for C2 This changes the upper cutoff frequency to 724 Hz Gain dB Phase deg 10 100 1k f Frequency Hz Figure 4 5 Bode Plot of H2 s Vo Vp Using the amplifier gain calculated above Figure 4 5 shows the bode plot approximation to the transfer function H2 s Vo Vp for the x and y channels The z channel is the same except the gain is slightly higher in the pass band 4 1 4 3 Component Values for H3 s Vadc Vo Resistor R5 and capacitor C3 cause the signal response to roll off further To set the fr
3. Figure 4 9 SPICE Simulation Schematic The SPICE simulations show agreement with the hand ACH 04 08 05 does not specify the role off calculations and the bode plot approximations characteristics of the shock sensor s frequency The analyses above do not consider the frequency response but in general it is expected that there will be response of the shock sensor which falls off below more attenuation in the signal below 0 5 Hz and above 0 5 Hz and above 5 kHz The data sheet for the T UE E a TE PIE EE ELE t x FURUade 51408 IR CI AUENULLATGUU zn L Enis di ndr a PRCE Tein JARDIN Figure 4 10 SPICE Simulation Results 3 V Accelerometer Featuring TLV2772 5 Circuit Realization The shock sensor and signal conditioning circuits are through board to board connectors Standoffs and built on area 100 of two Universal Operational Amplifier screws secure the two boards together Figure 5 1 EVM boards SLOP 120 1 One EVM board holds a shows the schematic diagram with reference TLV2772 an ACH 04 05 08 a TLV431 and required designators for using area 100 on two Universal ancillary devices The other board has only a TLV2772 Operational Amplifier EVM boards and required ancillary devices The signal conditioning circuit is constructed by installing required components and wiring The two boards share signals and sources Tables 5 1 and 5 2 summarize reference designator part description
4. SIXe 7 UAL iagram OL ere H zsa o XaNou Sex 2S Sj 1no viva 0 ISIS 9 X cS a S X LS A TL QN58 SIXe Iviva ObPSIAIL OAL N so 19 O I LYVLSO W1OANI 434 9OA 2 System Description z zzsia Accelerometer System D oepelul O I L z tesa OW 02 AVS dany anzzo even 2 1 SXe g S0 80 70 HOV CN N N 2 um d EH L Ke GL LL f GU GL E O E lt se jn Figure AL CC 9u 19J H VY L uoMpuo9 euis Buiuonipuo jeubis pue josuag 1ejnduio WAAXSO0ZESWL INN3OQVtvS LATIL JeuosJeqd JosseooJg ejeq JelleAUOD ejeq Figure 2 1 shows the accelerometer system diagram 3 V Accelerometer Featuring TLV2772 2 1 Sensor An AMP ACH04 08 05 shock sensor converts mechanical acceleration into electrical signals The shock sensor contains three piezoelectric sensing elements oriented to simultaneously measure acceleration in three orthogonal linear axes The sensor responds from 0 5 Hz to above 5 kHz An internal JFET buffers the output Typical output voltage for x and y axis is 1 80 mV g Typical output voltage for the z axis is 1 35 mV g Refer to AMP s web site at http www amp com sensors for in depth in
5. 3096 more gain is required in channel z The results of the analysis show the dominant noise source is the sensor The noise specification for the sensor is given in mg VHz and converted to equivalent input noise voltage Therefore the noise voltage as calculated in this analysis will be higher in channel z but not significantly so It is typical in amplifier circuits to identity equivalent input noise voltage per root Hertz for system components This is then multiplied by the equivalent noise bandwidth and amplifier noise gain The result is the total system noise It is convenient to work in RMS values to compute the total system noise then to convert to peak to peak pk pk and compare to 1 LSB to see the effect on the ADC s output The following formula is used Total System Noise in RMS Volts k gt ENB en An m 1 Where ENB is the equivalent noise bandwidth in hertz the subscript m indicated an individual equivalent input noise source en in RMS volts per root Hertz An is the noise gain of the amplifier and k is the total number of noise sources referred to the input Normally noise in RMS voltage is multiplied by numbers ranging from 5 to 7 to convert to peak to peak pk pk voltage This accounts for the large crest factor seen in noise signals The pk pk value of noise voltage is of concern when driving the input to an ADC converter A multiplication factor of 6 is used in this analysis to convert RMS in
6. amplification is the lowest at the upper cutoff frequency At 1 kHz the open loop amplification is about 75 dB or 5600 Using this value 1 ab 0 000016 and the deviation of the amplifier gain from ideal is 0 00296 The error caused by the op amp gain setting resistors is the tolerance of the resistors In this case 196 resistors add an error of 1 6 1 3 ADC Gain Errors Errors caused by sampling of the signal by the ADC also need to be analyzed Figure 6 1 shows a simplified schematic to use for analysis 3 V Accelerometer Featuring TLV2772 Set initial conditions where Vadc Vo and Vsample 0 To simplify the analysis notice that the time constant R5C3 0 22 ms is much longer than the time constant RadcCadc 55 ns because C3 is 4000 times larger than Cadc In this system the TLV1544 s I O CLK is being driven at 5 MHz The sampling period 6 I O CLK periods 1 2 us Because C3 Cadc when the switch closes during sampling C3 acts like a voltage reservoir for charging Cadc via Radc During the sampling period very little charge is transferred from Vo The sampling period 1 2 us 29 x 55 ns or 29 RadcCadc time constants Therefore at the end of the sampling the period the voltage across Cadc and C3 is essentially equal the sample voltage is set by the ratio of C3 to Cadc LV1544 Sampling Circuit 0 Rdac VSample V Figure 6 1 Sampling Input Model Vsample Vaal C3 0 999
7. 04 05 08 shock sensor states that the internal JFET used to drive the output is similar to the industry standard 2N4117 To model the sensor a signal source is used to drive the gate of a 2N4117 JFET and proper bias is applied Modeling the signal conditioning circuit is straight forward except that most available SPICE versions do not have a library model for the TLV2772 This is easily remedied Place a similar part on the schematic and modify its model with the model editor to match that of the TLV2772 Figure 4 8 below shows the TLV2772 sub circuit model as printed in the TLV2772 data sheet 3 V Accelerometer Featuring TLV2772 SUBCKT TLV2772 X 123 45 C1 11 12 2 3094E 12 C2 6 7 8 0000E 12 CSS 10 99 2 1042E 12 DC 53 DE 54 5 91 POLY 2 3 0 4 0 0 5 5 POLY 5 VB VC VE VLP VLN 0 19 391E6 1E3 1E3 19E6 19E6 12 150 80E 6 99 7 5576E 9 DC 116 40E 6 VLIM IK 10 JX1 10 JX2 100 0E3 6 6315E3 6 6315E3 17 140 17 140 4 5455E3 1 7182E6 DC 0 DC 1 peo DC 0 DC 47 92 DC 47 MODEL DX D IS 800 0E 18 MODEL DY D IS 800 0E 18 Rs 1m Cjo 10p MODEL JX1 PJF IS 2 250E 12 BETA 195 36E 6 VTO 1 MODEL JX2 PJF IS 1 750E 12 BETA 195 36E 6 VTO 1 ENDS wo IOo E KD R O ON Figure 4 8 TLV2772 Sub Circuit Model Figure 4 9 shows the schematic used for SPICE simulation Figures 4 10 show the simulation results 3 V Accelerometer Featuring TLV2772 ACH 04 05 08 Model
8. 25 maximum Signals from the signal conditioning circuitry were routed to the TLV1544 ADC via coaxial cables Sine wave acceleration was used and data was collected at various acceleration levels and frequencies on all 3 axis Data for each axis was plotted over frequency and acceleration using the mean sensitivity The data was analyzed and the errors calculated Tables 8 1 through 8 6 and Figures 8 1 through 8 6 show the result Figure 8 7 shows the output vs input for each axis when averaged over frequency A maximum cross axis sensitivity of 17 696 from y axis to x axis was observed Resonance was observed in the z axis at 2 kHz Figure 8 1 X Axis Output vs Input 3 V Accelerometer Featuring TLV2772 Table 8 2 X Axis Error Table 8 3 Y Axis Output Vs Input Using Y Axis gf 10Hz 20Hz 50Hz 100Hz 500Hz Mean Sensitivity 1 35 mV g 1 20 00 20 00 20 00 5 00 17 50 10Hz 20Hz 50Hz 100Hz 500Hz 10 15 00 10 00 5 60 13 00 20 9 30 4 70 12 75 10 50 50 7 60 2 30 14 80 1965 19 18 20 10 Ls 15 X axis Error 96 Y axis Output Figure 8 2 X Axis Error Figure 8 3 Y Axis Output vs Input 3 V Accelerometer Featuring TLV2772 Table 8 4 Y Axis Error gf 10 Hz 20 Hz 50 Hz 100Hz 500 Hz 2 50 2 50 10 50 2 50 12 50 5 00
9. LOOP Ck Ck ck ck ck ck ck ck KKK KKK ck Ck Sk KK ck ck ck ck ck ck ck ck Sk ck Ck ck KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KOK 3 V Accelerometer Featuring TLV2772 start the final sampling of the accelerator TRIG SPLK MAIN CHANNELO ACT CHANNEL set the next INTERRUPT BRANCH ADDRESS to the MAIN sampling LDP FINITO LACC FINITO SUB 00001h BCND M1 LT sampling is done wait until user interrupt Dk ck ck ck ck ck 0k ck ck ck ck 0k ck ck Ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ockock ck ok ck Sk Sk ko Sk kv Mk Sk kv ko M2 no operation B M2
10. MAIN_CHANNEL2 ACT_CHANNEL L2 AD_DP channel_2 ADWORD data_loc_pointl ADMEM 1 data loc pointl 00001h ADCOUNT MAIN_CHANNEL3 ACT_CHANNEL L3 AD DP channel_3 ADWORD data_loc_point2 ADMEM 1 data_loc_point2 00001h ADCOUNT MEMCOUNT 1 MEMCOUNT INITISIMO EQ F MAIN_CHANNELO ACT CHANNEL 00001h FINITO CHANNEL_END ACT_CHANN TLV1544 POWER DOWN AFTER TRANSITIONS CHANNEL_END LDP SPI SPI AD_DP power_down ADWORD 0 ADCOUNT point to AD Data Page SELECT CHANNEL load actual memory pointer increment ACC save new memory pointer NUMBER OF DATA set new jump level point to AD Data Page SELECT CHANNEL load actual memory pointer increment ACC save new memory pointer NUMBER OF DATA set new jump level point to AD Data Page SELECT CHANNEL load actual memory pointer increment ACC save new memory pointer NUMBER OF DATA increment ACC ADCOUNT ADCOUNT 1 if ADCOUNT not 0 jump to K set new jump level set finish sign set new jump level point to AD Data Page SELECT CHANNEL 0 NUMBER OF DATA 0 3 V Accelerometer Featuring TLV2772 SPLK CHANNEL_END ACT_CHANNEL set new jump level RET KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KK KKK KKK KKK KK KKK KK
11. Model X Axis Acceleration Graphed in Excel X Axis Output Vs Input X Axis Error Y Axis Output vs Input Y Axis Error Z Axis Output vs Input Z Axis Error Average Output vs Input Over Frequency for Each Axis Tables Board 1 Universal Operational Amplifier EVM Area 100 Board 2 Universal Operational Amplifier EVM Area 100 Board 1 Universal Operational Amplifier EVM Area 100 ACH 04 08 05 Connections Interface Between Signal Conditiioning Circuit and TLV1544 EVM X Axis Output Vs Input Using X Axis Mean Sensitivity 1 16 mV g X Axis Error Y Axis Output Vs Input Using Y Axis Mean Sensitivity 1 35 mV g Y Axis Error Z Axis Output Vs Input Using Z Axis Mean Sensitivity 1 01 mV g Z Axis Error 3 V Accelerometer Featuring TLV2772 1 Introduction Accelerometers are used in aerospace defense Figure 1 shows a block diagram of a typical analog automotive household appliances instrumentation data collection system This application presents audio transport material handling etc This application information about the sensor signal conditioning ADC report develops a data collection system for processor display and memory acceleration in 3 axis Power Supply Display Distribution Signal Signal Conditioning Conditioning Figure 1 1 Typical Analog Data Collection System AOL Arv eBuey Alddns LZSdL duo L T aday GGAq SISIL Y
12. THE CUSTOMER S RISK In order to minimize risks associated with the customer s applications adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards Tl assumes no liability for applications assistance or customer product design TI does not warrant or represent that any license either express or implied is granted under any patent right copyright mask work right or other intellectual property right of TI covering or relating to any combination machine or process in which such semiconductor products or services might be or are used Tl s publication of information regarding any third party s products or services does not constitute TTS approval warranty or endorsement thereof Copyright 1998 Texas Instruments Incorporated 3 V Accelerometer Featuring TLV2772 Application Report Jim Karki Advanced Analog Products Advanced Analog Applications Group Abstract This paper describes a complete solution for digital measurement of acceleration An AMP accelerometer sensor is used for the conversion between mechanical acceleration and electrical analog This electrical signal is then conditioned using Texas Instruments TLV2772 op amp on the Universal Op Amp EVM digitized using the TLV1544 ADC EVM and processed with the TMS320C5X EVM This provides the user with a quick and easy way to evaluate a complete 3 axis accelerometer solution Contents Introduction Syst
13. is insignificant compared to the input impedance of the op amp Therefore Vref will appear at the positive input to the Figure 4 2 DC Circuit Model op amp Assuming the ADC input does not impose a significant load on the circuit the voltage divider formed between the ADC input and R5 can be disregarded 3 V Accelerometer Featuring TLV2772 The dc model shown in Figure 4 2 is based on the assumptions made above The gain of the amplifier with reference to the negative input Vn is Vdc _ _R4 vref P Vref Ha 2 Vdc Vref B3 The gain of the amplifier with reference to the positive input Vp is Vde _ HA R4 Vret A or Vdc Vref S Superimposing the positive and negative dc gains of the amplifier results in Vdc Vref The output of the amplifier is referenced to Vref The ac response is superimposed upon this dc level 4 1 2 AC Analysis For ac analysis break the circuit into 3 parts and determine the transfer functions e Hi s Vp Vi e H2 s Vo Vp e H3 s Vadc Vo Combine the results to obtain the overall transfer function e H s Vadac Vi Where Vi is the input signal from the shock sensor and Vadc is the output signal to the analog to digital converter Figure 4 3 shows the ac model with the dc Sources shorted Output to ADC Figure 4 3 AC Circuit Model 4 1 2 1 H1 s Vp Vi Capacitor C1 and resistor R2 form a passive high pass filter from the sensor input
14. of the actual channel and if all done it will call the procedure CHANNELS to load the next channel specifications RECEIVE IRO save AR7 before handling the IRQ routine ldp isr save sar ar7 isr save Save AR7 in isr save MAIN IRQ ROUTINE Data collection from ADC CLRC SXM Clear sign bit LDP 0 3 V Accelerometer Featuring TLV2772 load content of DRR in ACCh shift 10 AR7 ADMEM save sample in the memory AR7 ADMEM ADMEM AR7 ADCOUNT AR7 ADCOUNT ADCOUNT 1 increment ACC ADCOUNT ADCOUNT ADCOUNT 1 KEEP_ON N if ADCOUNT not 0 jump to KEEP_ON ACT CHANNI load addr to set new sampling channel jump into the subroutine XF Disable Chip Select restore AR7 before jump back into the user software ldp isr save lar ar7 isr save load old ARO RET Return from Interrupt copy channels asm end KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KK TITLE TLV1544C ADC Interfac
15. the x axis output channel 2 samples the y axis output and channel 3 samples the z axis output Make the following modifications to the TLV1544 EVM REV Channel 0 Modifications Remove U4 R26 R27 and R28 Jumper U4 2 to U4 3 Place a 1 kQ resistor for R28 Place a 22 uF capacitor between AO and ground Channel 1 Modifications Remove U5 R29 and C19 Jumper U5 3 to U5 6 Place a 1 kQ resistor for R29 Place a 0 22 uF capacitor for C19 Channel 2 Modifications e Place a 1 kQ resistor for JP3 e Place a 0 22 uF capacitor across D6 Channel 3 Modifications Place a 1 kQ resistor for JP4 e Place a 0 22 uF capacitor across D8 Switch SW1 to operate the TLV1544 EVM at 3 V Set the TLV1544 reference voltage input to 2 4 V by installing jumpers JP1 1 2 and JP2 1 2 Use the 3 V power supply on the TLV1544 EVM to supply power to the shock sensor and the signal conditioning circuit Table 5 4 shows the interface connections between the signal conditioning circuit and the TLV1544 EVM REV Table 5 4 Interface Between Signal Conditiioning Circuit and TLV1544 EVM SIGNAL CONDITIONING CIRCUIT TLV1544 EVM Board 1 V1 AVpp Board 1 GND1 GND Board 1 VREF1 signal reference JP19 U2 2 Board 1 A1OUT X axis output Board 1 B1OUT Y axis output Board 2 A1OUT Z axis output Analog Input 1 Analog Input 2 Analog Input 3 Figure 5 5 is a schematic drawing for the interface between the signal conditioning
16. 1544 is a low voltage 2 7 V to 5 5 V dc single supply 10 bit analog to digital converter ADC with serial control 4 analog inputs conversion time 10 us and programmable 1 uA power down mode Refer to Texas Instruments web site at http www ti com to download a TLV1544 data sheet literature SLAS139B TLV1544 EVM User s Manual literature SLAUO14 and related information 2 4 Processor Memory and Display The TMS320C5x EVM controls and collects data samples from the TLV1544 Refer to Texas Instruments web site at http www ti com to download the TMS320C5x EVM Technical Reference literature SPRU087 and related information The TMS320C5x EVM installs into a PC platform The PC provides programming and control of the TMS320C5x EVM and provides resources for file storage or other processing of the collected data 3 System Specification Requirements The following system specification requirements were derived to guide the design 3 1 G Force Measurement Requirements Range 50g Noise 0 05g pk pk equivalent input noise measured in g Resolution 0 1g Frequency Response 1 Hz to 500 Hz min 3 dB BW 3 2 Power Requirements Input Voltage 3 V 10 Noise 30 mV pk pk 20 MHz BW Input Current 5 mA max power requirements for sensor and signal conditioning circuitry 3 V Accelerometer Featuring TLV2772 4 Sensor and Signal Conditioning Design Circuit design is a 3 step process Input powe
17. 2 integer A n f char AA byte tin file of byte tout text offset byte nb byte number longint number low longint number high longint begin repeat clrscr writeln writeln TEXAS INSTRUMENTS C SOFTWAR writeln 1998 writeln write hard drive for input binary hexadecimal file 16bit na readkey writeln na write file name readin ffile ffile nat ffile assign tin ffile i reset tin Sit i IoResult if i lt gt 0 then begin repeat writeln file not found new Input Name Y N n upcase readkey until n2 Y or n N end if i20 then n w until n N or n w if n N then begin 3 V Accelerometer Featuring TLV2772 inpfile ffile repeat writeln write hard drive for decimal output file na readkey writeln na write file name readin ffile ffile nat ffile assign tout ffile i reset tout Sit i IoResult if i 0 then begin repeat writeln file already found should I overwrite it Y N n upcase readkey until n Y or n N end else n Y until n 2 Y rewrite tout writeln writeln tout binary Hex dec convertion made by TI writeln tout Input file inpfile read tin nb reset tin for i 1 to 70 do read tin nb repeat read tin nb number low nb read tin nb number high 2nb number number_high 256 number_low
18. 2 00 3 00 1 50 1 10 20 1 75 4 13 0 50 50 0 70 6 00 5 10 20 X axis Error 96 Figure 8 4 Y Axis Error Table 8 5 Z Axis Output Vs Input Using Z Axis Mean Sensitivity 1 01 mV g 10 Hz 20 Hz 50Hz 100Hz 500 Hz ies s 20 40 ol ass 59 10 Z axis Output Figure 8 5 Z Axis Output vs Input g 3 V Accelerometer Featuring TLV2772 Table 8 6 Z Axis Error f 10 Hz 20 Hz 50 Hz 100 Hz 500 Hz 1 1 11 00 3 00 3 00 3 00 13 00 0 18 00 16 50 18 00 17 00 20 16 00 17 25 17 00 50 17 4096 16 90 18 20 Z axis Error 96 Figure 8 6 Z Axis Error Average Output 10 15 20 25 30 35 40 45 50 Input Figure 8 7 Average Output vs Input Over Frequency for Each Axis 9 References ACH04 08 05 Data Sheet htto www amp com sensors TLV2772 Data Sheet literature SLOS209 Universal Operational Amplifier EVM User s Manual literature SLVU006 TLV1544 Data Sheet literature SLAS139B TLV1544 EVM User s Manual literature SLAU014 Interfacing the TLV1544 Analog to Digital Converter to the TMS320C50 DSP Applications Report literature SLAA025 7 TMS320C5X Evaluation Module Technical 8 9 Reference literatur
19. 75 Vade Cadc C3 The error attributed is 0 025 The other sources of error in the ADC can be read from the data sheet As stated earlier the system is essentially an ac system and dc errors are not included in the error analysis Therefore only the maximum linearity error from the data sheet which is 1 LSB is included With 10 bits of resolution or 1024 codes the error 0 0976 6 1 4 Total Gain Error From this point on in the system the signal is digital and it is assumed there are no errors which cannot be corrected for Calculating the total error as the square root of the sum of the error squared results in V259 2 0 002 2 1 2 0 025 2 0 0976 2 25 02 The uncertainty associated with the shock sensor output dominates the error analysis Calibration is required to insure accurate measurement of acceleration 6 2 System Noise Analysis The first step to analyzing system noise is to qualify and quantify noise sources within the system Next the components must be added to see the overall effect on the system In this system the noise sources are the shock sensor the resistors and the amplifier Since all three channels are essentially the same the noise analysis is performed on one channel Reference designators from Figure 4 1 are used with the component values as calculated above for channel z Because the z axis of the shock sensor has about 3096 less sensitivety than either x axis or y axis
20. D TEXAS INSTRUMENTS 3 V Accelerometer Featuring TLV2772 Application Report 1998 Advanced Analog Products SLVA040 IMPORTANT NOTICE Texas Instruments and its subsidiaries Tl reserve the right to make changes to their products or to discontinue any product or service without notice and advise customers to obtain the latest version of relevant information to verify before placing orders that information being relied on is current and complete All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement including those pertaining to warranty patent infringement and limitation of liability Tl warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with Tl s standard warranty Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty Specific testing of all parameters of each device is not necessarily performed except those mandated by government requirements CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF DEATH PERSONAL INJURY OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE CRITICAL APPLICATIONS TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED AUTHORIZED OR WARRANTED TO BE SUITABLE FOR USE IN LIFE SUPPORT DEVICES OR SYSTEMS OR OTHER CRITICAL APPLICATIONS INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE FULLY AT
21. EL1 ACT CHANN MAIN_CHANNELO LDP SPLK LACL SACL ADD SACL SPLK SPLK RET AD_DP channel_0 ADWORD data_loc_point3 ADMEM 1 data_loc_point3 00001h ADCOUNT MAIN_CHANNEL1 ACT_CHANNEL NUMBER OF DATA 0 set new jump level point to AD Data Page The first Word to send is fast conv to wake up the ADC from power down received byt DATA LOCATION THRES because first is meaningless NUMBER OF DATA 0 set new jump level point to AD Data Page SELECT CHANNEL 0 DATA LOCATION 0 NUMBER OF DATA 0 set new jump level point to AD Data Page SELECT CHANNEL 0 DATA LOCATION 0 NUMBER OF DATA 0 set new jump level point to AD Data Page SELECT CHANNEL 0 DATA LOCATION 0 NUMBER OF DATA 0 set new jump level point to AD Data Page SELECT CHANNEL load actual memory pointer increment ACC save new memory pointer NUMBER OF DATA set new jump level MAIN_CHANN LDP SPLK LACL SACL ADD SACL SPLK SPLK RET MAIN_CHANN LDP SPLK LACL SACL ADD SACL SPLK SPLK RET MAIN_CHANN LDP SPLK LACL SACL ADD SACL SPLK LACL SUB SACL BCND SPLK RET FINITISIMO SPLK SPLK RET 3 V Accelerometer Featuring TLV2772 AD_DP channel_1 ADWORD data_loc_point0 ADMEM 1 data_loc point U 00001h ADCOUNT
22. K KK KKK TITLE TLV1544C ADC Interface routine FILE values ASM FUNCTION N A PROTOTYPI N A CALLS N A PRECONDITION N A POSTCONDITION N A DESCRIPTION contains the control data for the TLV 1544 the starting address to save the samples to the DSP memory and the number of samples according each channel AUTHOR AAP Application Group Dallas CREATED 1998 C BY TEXAS INSTRUMENTS INCORPORATED TMS320C5x User s Guide TI 1997 Data Aquisation Circuits TI 1998 ck ck ck KKK KKK KKK KKK KKK KKK KKK KKK KEK KKK KK KKK KKK KKK KKK KKK KKK KKK KKK KKK KK KKK KK KKK KKK ck ck ck ock ck ock ck kk ko ko KKK KKK KK channel U Se 0000h Select TLV1544 channel channel_1 Se 2000h Select LV1544 channel 0 1 channel_2 Se 4000h Select TLV1544 channel 2 3 channel_3 Se 6000h Select TLV1544 channel power_down Se 8000h Software Power Down fast_conv Se 9000h Fast Conversion Rate slow_conv Se 0A00h Slow Conversion Rate test 200 Se OBOOh Vreg Vreg 2 test 000 Se OcOOh Vreg tets 3FF Se OD000h Vregt num data 0 Se 1FFh Number of Data from channel 0 num data 1 Se 200h Number of Data from channel 1 num data 2 Se 200h Number of Data from channel 2 num_data_3 Se 200h Number of Data from channel 3 tresh Se 1000h address to waste the first input sample after initialization data_loc_0 se 01000h Start data location for channel data_loc_l s
23. N BODY FOR A PROGRAM UNRELATED TO AD CONVERTING PROCESSOR SLEEP AT NONACTIVITY TIMES INSTEAD OF THE IDLE INSTRUCTION USER CAN RUN THEIR OWN PROGRAM KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK ck ck ck KKK KKK KKK KKK KK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KK USERINTERFACE SPLK 00000h FINITO sampling not finished yet testing the input samples against trigger values to start i the final sampling of the accelerator ck ck ck ck ck 0k ck ck ck ck ck ck ck Ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck koc ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ckock ck ck ck ck ko ck ko ck Mk Sk kv Pk Sk ko ko TRIP LOOP LDP KTST CHI LACC TST_CH1 SUB TRIP_HIGH BCND TRIG GEQ LACC TST_CH1 SUB TRIP_LOW BCND TRIG LEQ LDP TST_CH2 LACC ST CH2 SUB STRIP HIGH BCND TRIG GEQ LACC TST_CH2 SUB TRIP_LOW BCND TRIG LEQ LDP TST_CH3 LACC TST_CH3 SUB TRIP_HIGH BCND TRIG GEQ LACC TST_CH3 SUB TRIP_LOW BCND TRIG LEQ B RIP
24. OUTI FSX generated by DSP OPL 080COh SPC activate transmitter and receiver INITIALIZE USER INFORMATIONS FOR DATA AQUISITION CALL CHANNELS load informations about ADC channel and memory location for data saving Enable Interrup LDP 0 OPL 00014h IMR Unmask RINT and INT3 CLRC INTM enable global interrups FIRST SEND OPERATION MODE TO TLV1544 LDP ADWORD LACL ADWORD CLRC XF Enable Chip Select SAMM DXR move ADWORD into DXR DXR fast conversation mode initialize MEMCOUNT and data_loc_pointx KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KOK 3 V Accelerometer Featuring TLV2772 SPLK data_loc_0 data_loc SPLK data_loc_1 data_loc SPLK data_loc_2 data_loc SPLK data_loc_3 data_loc SPLK TRIG_RI ST_CH SPLK TRIG_RI ST_CH SPLK TRIG_RI ST_CH SPLK numb_data_all MEMCOUNT KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KKK KKK KOK MAI
25. R105 Y Axis A102 R107 AAA Z Axis Out 10 ko A10UT A103 R119 ANN V1 8 kd X Axis 1M 4 1 2Dual Op Amp A104 R109 MV 02 Z Axis Not Used B10UT 1 2 Dual Op Amp Figure 5 1 Signal Conditioning Schematic using Two Universal Operational Amplifier EVM Boards 3 V Accelerometer Featuring TLV2772 5 1 Test of Signal Conditioning Circuit After constructing the signal conditioning circuit and before connecting the shock sensor tests were run to verify the signal conditioning circuit performed as expected An AP Instruments network analyzer was used to measure the gain and phase response Figure 5 2 shows the results are very much the same as the SPICE simulation predicted Mag dB 18 00 11 00 400 3 00 10 00 17 00 24 00 31 00 38 00 Phase deg 340 00 300 00 260 00 220 00 180 00 140 00 100 00 60 00 20 00 1k 10k Figure 5 2 Network Analyzer Display 5 2 Test of Shock Sensor and Signal Conditioning Circuit After connecting the shock sensor the measurement of acceleration was verified A spring loaded test fixture was constructed to induce simple harmonic motion into the sensor Figure 5 3 shows a diagram of the test fixture By measuring the deflection from center and the frequency of the oscillation the acceleration is calculate
26. circuit and the TLV1544 3 V Accelerometer Featuring TLV2772 5 4 Interfacing the TLV1544 EVM to the further details refer to Interfacing the TLV1544 TMS320C5C EVM Analog to Digital Converter to the TMS320C50 DSP Applications Report literature SLAA025 Figure 5 6 shows the connections required to interface the TLV1544 EVM to the TMS320C5X EVM For S 3 r Signal TLV1544EVM Conditioning iko TLV1544 VREF1 TP19 AA A0 1 kO X Axis Output 1 Analog Input VWv Al 1 kO Y Axis Output E 41 Analog Input 2 VV A2 1kQ Z Axis Output Analog Input 3 VV A3 AT 0 22 uF 0 22 uF Figure 5 5 Schematic Signal Conditioning to TLV1544 ADC TLV1544 EVM TMS320C5X EVM TLV1544ADC TMS320C50 DSP EOC INT3 VO CLK CLKR CLKX DATA IN DX DATA OUT DR cs XF FS Figure 5 6 Interface Between TLV1544 EVM and TMS320C5X EVM 5 5 The TMS320C5X EVM Source Debugger User s Guide literature SPRU055B The TMS320C5X EVM plugs into an ISA slot in a PC IOEUSSPIMORDISNOIE Refer to TMS320C5X Evaluation Module Technical Reference literature SPRU087 for detailed information on its use The TMS320C5X C Source Debugger is also required Refer to TMSS20C5X C 3 V Accelerometer Featuring TLV2772 6 Error and Noise Analysis Error analysis can predict the errors to be expected under normal operating conditions Two analyses are
27. d as a 2 x cos ot 0 Where a is acceleration o is frequency of oscillation in radians x is the maximum deflection from center is the phase Figure 5 3 Spring Test Fixture While monitoring the output of all 3 axes with an oscilloscope the system was set into motion along the x axis Figure 5 4 shows the result displayed on an oscilloscope Channel 1 is the x axis channel 2 is the y axis and channel 3 is the z axis About 0 25 mechanical deflection was measured visually An oscilloscope was used to measure the frequency of oscillation and the peak voltage amplitude Using the above formula to find the peak acceleration expected from the mechanical movement ignoring the sign a peak 27 x 16 2 0 25in s 2524in s 6 58g Where g 32 ft s To check the output of the signal conditioning circuit use the formula Peak voltage output Sensor output Gain a peak 9 Me 128 mV T80 mn 6 46 g Using the typical x axis output 1 80mV g there is good agreement between the two calculations 3 V Accelerometer Featuring TLV2772 JA 128mv 1 4mv E E Do qd o od Sa Toons The io0mV amp Mso oms Chi 7 92mMV 5 jun 1998 s 3 100mV Vs 08 29 25 Figure 5 4 Output Displayed on Oscilloscope Note that output is seen on the y axis and z axis The ACH 04 08 05 data sheet specifies a transverse sensitivity of 1596 Fifteen percent of the x axis pea
28. decimal numbers into decimal ASCII format Source code listings are included in Appendix A With the data converted and loaded into Excel it can be converted into equivalent acceleration in g using the formula p 42 Sample x ADC Ref Volts 210 Sig Ref vols 1 x ee output x sa Figure 7 1 shows the x axis acceleration graphed in Excel from data collected when testing the signal conditioning and shock sensor circuits It can be seen that the results match the results as presented previously X AXIS ACCELERATION 9335 10053 10771 11489 1220 SAMPLE NUMBER Figure 7 1 X Axis Acceleration Graphed in Excel 3 V Accelerometer Featuring TLV2772 8 Calibration Data Analysis 8 1 Calibration Table 8 1 X Axis Output Vs Input Using X Axis The acceleration sensor and the signal conditioning Mean Sensitivity 1 16 mV g circuitry were housed in a Bud CU 234 box which was g f 10Hz 20Hz 50Hz 100Hz 500 Hz machined to mount on a fixture plate for calibrated 1 20 testing The calibrated acceleration source used was an Unholtz Dickie T 1000 vibration tester The 10 11 50 11 00 10 56 8 70 specifications for this machine are 21 86 17 45 1 inch maximum displacement 50 53 80 51 15 42 60 60 70 in s maximum velocity 75g maximum sine acceleration 10 operational tolerance 5 Hz to 5 kHz frequency range 1 4 Hz frequency operational tolerance Cross axis output
29. e SPRU087 TMS320C5X C Source Debugger User s Guide literature SPRU055B TMS320C5X EVM Technical Reference literature SPRU087 3 V Accelerometer Featuring TLV2772 Appendix A Source Code Listings TITLE ADC Interface routine FILE ACCELER1 ASM FUNCTION MAIN PROTOTYPI void MAIN CALLS Wait Channels PRECONDITION N A POSTCONDITION N A DESCRIPTION AUTHOR AAP Application Group Dallas CREATED 1998 C BY TEXAS INSTRUMENTS INCORPORATED TMS320C5x User s Guide TI 1997 Data Aquisation Circuits TI 1998 ck ck ck ck ck ck 0k ck ck 0k ck 0k ck ck Ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck ck o ckck ck ck ck kk ck ko Sk kk kv Pk Sk ko ko mmregs Sect vectors copy vectors asm m data values asm J MAIN ACT CHANNI ADWORD ADCOUNT ADMEM isr save FINITO ta loc pointO0 ta loc pointl ta loc point2 ta loc point3 EMCOUNT TST_CH1 TST_CH2 TSI CH3 AD DP usec
30. e 04000h Start data location for channel data loc 2 Se 07000h Start data location for channel data_loc_3 Se OA000h Start data location for channel numb_data_all Se 03000h how many samples per channel to TRIP_HIGH Set 023bh high trigger value 1 34 V U RIP LOW Set 01e7h low trigger value 1 14 V RIG REEF Se 0211n mean value of no triggering 1 24 V TITLE TLV1544C ADC Interface routine 3 V Accelerometer Featuring TLV2772 FILE vectors ASM FUNCTION N A PROTOTYPE N A CALLS N A PRECONDITION N A POSTCONDITION N A DESCRIPTION defines the interrupt vector table if RINT occurs vector points to RECEIVE_IRQ subroutine if INT3 occurs vector points to END_CONV_IRQ subrout AAP Application Group Dallas CREATED 1998 C BY TEXAS INSTRUMENTS INCORPORATI REFERENCE TMS320C5x User s Guide TI 1997 KKKKK KKK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KK KKK KKK KKK KKK KKK KK KKK KKK KKK KK KKK KKK KK KKK KKK KK KKK 0x00 RESET 0x02 external us
31. e routine FILE Channels ASM FUNCTION CHANNELS PROTOTYPI void CHANN CALLS N A PRECONDITION N A POSTCONDITION N A DESCRIPTION AUTHOR AAP Application Group Dallas CREATED 1998 C BY TEXAS INSTRUMENTS INCORPORATED TMS320C5x User s Guide TI 1997 Data Aquisation Circuits TI 1998 text ELS UP AD_DP point to AD Data Page fast_conv ADWORD The first Word to send is fast_conv to wake up the ADC from power down tresh ADMI DATA LOCATION THRES because first received byte is meaningless SPI SPI SPI SPLK RET 3 V Accelerometer Featuring TLV2772 1 ADCOUNT one_more ACT_CHANN AD_DP channel_1 ADWORD tresh ADM 4 ADCOUNT CHANNEL1 ACT_CHANNI AD_DP channel_1 ADWORD TST_CH3 ADMEM 00001h ADCOUNT CHANNEL2 ACT_CHANN AD_DP channel_2 ADWORD TST_CH1 ADMEM 00001h ADCOUNT CHANNEL3 ACT_CHANNI AD_DP channel_3 ADWORD TST_CH2 ADMEM 00001h ADCOUNT CHANN
32. em Description 2 1 Sensor 2 2 Signal Conditioning 2 4 Processor Memory and Display System Specification Requirements 3 1 G Force Measurement Requirements 3 2 Power Requirements Sensor and Signal Conditioning Design 4 1 Hand Analysis 4 2 Spice Simulation Circuit Realization 5 1 Test of Signal Conditioning Circuit 5 2 Test of Shock Sensor and Signal Conditioning Circuit 5 3 TLV1544 EVM 5 4 Interfacing the TLV1544 EVM to the TMS320C5C EVM 5 5 The TMS320C5X EVM Error and Noise Analysis 6 1 System Gain Error Analysis 6 2 System Noise Analysis System Test and Evaluation 8 Calibration Data Analysis 8 1 Calibration 9 References Appendix A Source Code Listings X TEXAS INSTRUMENTS 3 V Accelerometer Featuring TLV2772 Figures 4 7 4 8 4 9 Typical Analog Data Collection System Accelerometer System Diagram 1 Axis Accelerometer Sensor and Signal Conditioning Circuit DC Circuit Model AC Circuit Model Bode Plot H1 s Vp Vi Bode Plot of H2 s Vo Vp Bode Plot of H3 s Vadc Vo Bode Plot of H s Vadc Vi TLV2772 Sub Circuit Model SPICE Simulation Schematic 4 10 SPICE Simulation Results 5 1 5 2 5 3 5 4 5 5 5 6 Signal Conditioning Schematic using Two Universal Operational Amplifier EVM Boards Network Analyzer Display Spring Test Fixture Output Displayed on Oscilloscope Schematic Signal Conditioning to TLV1544 ADC Interface Between TLV1544 EVM and TMS320C5X EVM Sampling Input
33. equency for this roll off to begin at the upper cutoff frequency select 1 21 C3 F x R4 Q upper cutoff frequency Hz With R5 1 kQ and C3 0 22 uF the roll off frequency 1 6 28 x 0 22 uF x 1 KQ 724 Hz Figure 4 6 shows the bode plot approximation to the transfer function H3 s Vadc Vo 3 V Accelerometer Featuring TLV2772 Gain dB Phase deg 10 100 1k 10k f Frequency Hz Figure 4 6 Bode Plot of H3 s Vadc Vo 4 1 4 4 Transfer Function H s Vadc Vi Superimposing the previous bode plot approximations results in the bode plot approximation for the overall transfer function to be expected from the signal conditioning circuit as shown in Figure 4 7 Phase deg Gain dB 10 100 1k 10k 100k f Frequency Hz Figure 4 7 Bode Plot of H s Vadc Vi 4 2 Spice Simulation Spice simulation verifies the results of hand analysis and provides a more accurate result than what is practical by hand The proper models must be used to perform SPICE simulation of the shock sensor and signal conditioning circuit The data sheet for the ACH
34. er interrupt 1 0x04 external user interrupt 2 0x06 external user interrupt 3 0x08 internal timer interrupt E IRQ 0x0A Serial Port receive interrupt 0x0C Serial Port transmit interrupt 0x0E TDM receive interrupt 0x10 TDM transmit interrupt INT4 0x12 external user interrupt 4 14 16 0x14 0x21 reserved area TRAP 0x22 trap instruction vector b NMI 0x24 non maskable interrupt KK IR RR Kk Ck AA AAAI ECKE KC ECKE KC IAA ECKE KC IA A AIA A A Kk Ck Kk kk kk A ke ke ke ke ke ke ke koe ke ke oe x f TITLE Hexadecimal to ASCII Decimal Converter FILE BH C502 PAS J FUNCTION N A Wf PROTOTYPI N A d N A PRECONDITION N A POSTCONDITION N A 7 DESCRIPTION strip header from COFF file and convert k hexadecimal numbers into ASCII decimal RUP AAP Application Group Dallas m CREATED 1998 C BY TEXAS INSTRUMENTS INCORPORATED REFER S K NOTE Written in PASCAL KK IKK AK Kk kk Kk kk KC I KAKAK K Ck Ck k CK Ck k KCkCk KCK Ck CK Ck k KCk Kk k CK Ck k K Ck k K Ck k k k ck k k ck kck ck kck ck OK ck kk ke kk f uses crt dos printer 3 V Accelerometer Featuring TLV2772 type positionstyp record AAAA word END positionstyp satzl file of positionstyp inpfile ffile string 20 na c chnar s2 string
35. esistance in ohms Hz if frequency in Hertz R2 contributes 128 nV VHz R3 and R4 appear in parallel at the negative input for noise calculations Together they contribute 10 7 nV VHz Resistor R1 adds 40 nV VHz Resistor R5 adds thermal noise voltage V 4KTR Hz to the system The value of the resistor is small and the noise is not amplified by the amplifier Therefore the effect of R5 on the overall system noise is negligible Adding the foregoing is straightforward for all components except the amplifier s input noise Referring to the equivalent input noise voltage vs frequency graph in the data sheet for the TLV2772 Figure 39 it can be seen that the equivalent input noise voltage is characterized by 1 f noise over the bandwidth of the system s operation The input noise is a combination of the 1 f noise and the white noise of the op amp en K2 f ey where en is the equivalent input noise voltage per root Hertz K is a proportionality constant and f is frequency in Hertz and ey is the white noise specifiction for the op amp K can be calculated from the values on the equivalent input noise vs frequency graph to be about 450 nV By integrating the noise density over the frequency range of operation the input noise can be computed by fH 80a ENB a K In f Where ega is the equivalent input noise voltage over the bandwidth in RMS volts K is a proportionality constant equal to 450 nV RMS fj is the lower cutoff f
36. formation about this sensor piezo materials in general and other related products 2 2 Signal Conditioning Circuitry using the Texas Instruments TLV2772 operational amplifier provides amplification and frequency shaping of the shock sensor output Due to its high slew rate and bandwidth rail to rail output swing high input impedance high output drive and excellent dc precision the TLV2772 is ideal for this application The device provides 10 5 V us slew rate and 5 1 MHz gain bandwidth product while consuming only 1 mA of supply current per amplifier The rail to rail output swing and high output drive make this device ideal for driving the analog input to the TLV1544 analog to digital converter The amplifier typically has 360 uV input offset voltage 17 nV vHz input noise voltage and 2 pA input bias current Refer to Texas Instruments web site at http www ti com and search on TLV2772 to download a TLV2772 data sheet literature SLOS209 The Universal Operational Amplifier EVM is used to construct the ACH04 08 05 shock sensor and TLV2772 operational amplifier circuitry The Universal Operational Amplifier EVM facilitates construction of surface mount operational amplifier circuits for engineering evaluation Refer to Texas Instruments web site at http www ti com to download the Universal Operational Amplifier EVM User s Manual literature s SLVUOOG 2 3 ADC The TLV1544 EVM provides analog to digital conversion The TLV
37. g Sensor Sensitivity Acceleration 3 V Accelerometer Featuring TLV2772 To avoid saturating the op amp base the gain calculations on the maximum negative swing of 1 23 V and the maximum sensor output of 2 25 mV g for the x and y axis and 1 70 mV g for the z axis Therefore Gain x y 1 23 V 2 25 mV g x 50g 10 9 and Gain z 1 23 V 1 70 mV g x 50g 14 5 Choosing R3 10 kQ and R4 100 kQ gives a gain of 11 in the x and y channels Choosing R3 7 5 kQ and R4 100 kQ gives a gain of 14 3 in the z channel 4 1 4 Bandwidth Calculations To calculate the component values for the frequency shaping characteristics of the signal conditioning circuit use 1 Hz and 500 Hz as the minimum required 3 dB bandwidth from the specifications requirements 4 1 4 1 Component Values for H1 s Vp Vi To minimize the value of the input capacitor required to set the lower cutoff frequency a large value resistor is required for R2 A 1 MQ resistor is used here To set the lower cutoff frequency the value for capacitor C1 must be C1 1 27 x lower cutoff frequency Hz x R2 Q C1 1 6 28 x 1 Hz x 1 MQ 0 159 uF A more common 0 22 uF capacitor is used for C1 This moves the lower cutoff frequency to 0 724 Hz Figure 4 4 shows the bode plot approximation to the input transfer function H1 s Vp Vi Gain dB b e Phase deg
38. is Gain errors are errors that contribute to uncertainty in the system output given a known input The system is essentially an ac system in which dc errors can be compensated Therefore dc errors caused by reference voltage errors offset voltages and the like are not included in this error analysis 6 1 1 Sensor Gain Error The shock sensor s output varies and this contributes to uncertainty in the system output given a known input The data sheet for the ACH 04 08 05 states the output sensitivity CHARACTERISTIC X axis sensitivity 1 35 Y axis sensitivity 1 3 Z axis sensitivity 1 This is a 25 tolerance in the output voltage for the same acceleration input Thus the error assigned to this component is 25 6 1 2 Signal Conditioning Gain Error Errors in the signal conditioning circuit also contribute to uncertainty in the system output given a known input The analysis is limited to the pass band of the amplifier circuit In its pass band the gain of the amplifier is given by _ fi 1 A 5 ab Where a open loop amplification of the operational amplifier _ R4 b m B The term 1 ab is seen as an error term in this equation because it is desired to set the gain of the circuit with the 1 b term ratio of R3 to R4 To minimize this error the open loop amplification of the operational amplifier should be as large as possible so that 1 ab is very close to 0 In this application the TLV2772 s open loop
39. k signal output is 19 2 mV The peak y axis signal is 40 mV and the peak z axis signal is 26 mV Transverse sensitivity does not account for the observation While the primary motion was in the x axis motion was noticed in the other 2 axes as well 5 3 TLV1544 EVM For this application the fast conversion mode of the TLV1544 ADC is used and three of the four analog inputs are converted one after the other This results in a sampling frequency of 28 ksps By Nyquist s sampling theorem the frequency content of the input analog signal must be insignificant for frequencies 1 2 the sampling frequency and above In this system the signal is attenuated by over 40 dB at the Nyquist frequency 5 3 1 Interfacing the Signal Conditioning Circuit to the TLV1544 EVM The TLV1544 EVM provides a power supply analog signal conditioning circuitry ADC TLV1544 and circuitry to support the interface to a DSP or micro controller Refer to the TLV1544 EVM User s Guide literature SLAU014 for detailed descriptions of the board and its functions To interface the shock sensor and signal conditioning circuit to the TLV1544 EVM minor modifications are required on the TLV1544 EVM To ease circuit realization place the output filter for each axis signal R5 and C3 in Figure 4 1 on the TLV1544 EVM The TLV1544 has 4 analog inputs In this application channel 0 samples the reference voltage from the signal conditioning circuit channel 1 samples
40. on board 1 Table 5 3 summarizes connections between board 1 and board 2 The the connections required board to board connections are GND1 V1 and VREF Also pin 12 of the ACH 04 08 05 must be connected to A104 on board 2 This is accomplished by using board to board connectors in the breadboard area and connecting the appropriate nodes thereto MELT ESI Figure 5 1 is the schematic of the 3 axis realization of PIN NAME BOARD 1 CONNECTIONS the 1 axis circuit shown in Figure 4 1 except for output D S1 Y Vds filter components R5 and C3 To ease component D S2 Y placement the output filters for each channel are GND1 placed on the TLV1544 EVM For standalone testing of the signal conditioning circuit without the TLV1544 EVM temporarily place the filter components on board D S1 X Vis 1 and board 2 Remove them for final integration RGND X VREF1 GND1 GND1 0 RGND Z VREF1 To board Table 5 3 Board 1 Universal Operational Amplifier EVM Area 100 ACH 04 08 05 Connections Oo O ale DO D S1 Z D S2 Z To board 2 CTG GND1 RGND Y VREF1 11 12 13 4 1 3 V Accelerometer Featuring TLV2772 C105 2 2 nF V1 GND1 X Axis Out A10UT V1 Power Supply Bypass V1 C105 2 2 nF lL Y Axis Out B10UT R111 1 2 Dual Op Amp C105 2 2 nF Voltage Reference ACH 04 08 05
41. presented gain error and noise Gain errors contribute to uncertainty in the system output given a known input Noise is a random electrical event that occurs in all conductors Noise imposes a statistical uncertainty on the system s electrical signals The system can be calibrated to compensate for the effect of gain errors The uncertainty associated with noise on the other hand prohibits any type of compensation Two of the most widely used types of error analysis are worst case analysis and sum of squares analysis The major task in performing an error analysis is to identify and quantify the error sources Worst case analysis assumes that all errors compound one on top of the other Therefore the maximum error in all circuit elements is added to arrive at the total system error Sum of squares analysis assumes that some errors will tend to cancel each other By summing the square of all the errors and taking the square root the result takes this cancellation effect into account A sum of squares analysis gives a realistic idea of how well the typical system will perform This is the type of analysis that will be presented A sum of squares analysis uses the following formula Total System Error Where the subscript n indicates an individual error component and k is the total number of error components System gain error analysis is presented first followed by system noise analysis 6 1 System Gain Error Analys
42. r is 3 V and ground The TLV431 precision 1 Hand analysis voltage regulator when configured as shown produces 2 SPICE simulation a nominal 1 23 V reference voltage This voltage l l provides the signal reference for the signal conditioning 3 Circuit breadboard and lab testing circuitry and the bias voltage for the internal JFETs in The signal from the sensor must be amplified and the shock sensor frequency shaped to provide a signal that the ADC can TI The transfer function of the signal conditioning circuit is properly convert into a digital number derived by several means the easiest of which may be The schematic in Figure 4 1 shows the topology used by using super position Perform a dc analysis perform in this application for 1 axis of the sensor and signal an ac analysis and superimpose the results conditioning circuit VDD Output TLV431 Input From Sensor to ADC 4 Signal m C3 VREF Conditioning Voltage Referance Shock Sensor Figure 4 1 1 Axis Accelerometer Sensor and Signal Conditioning Circuit 4 1 Hand Analysis In hand analysis simplifying assumptions make solutions easier to derive If the circuit does not function as anticipated these assumption must be reevaluated 4 1 1 DC Analysis qe To perform a dc analysis assume all inductors are igna z 3 Conditioning short circuits and all capacitors are open circuits Assume that the resistance of R2
43. requency 0 724 Hz and fy is the upper cutoff frequency 724 Hz The result is eg4 1 32 uV RMS 6 2 3 Total Noise Analysis Adding the system noise sources results in Total System Noise in RMS Volts 14 3 x 803 0 27E6 128E 9 2 10 7E 9 40E 9 1 32E 6 0 124 mV RMS To convert to pk pk voltage multiply by 6 Then Total System Noise pk pk 0 743 mV pk pk The TLV1544 ADC has 10 bits of resolution With a voltage reference 2 4 V 0 743 mV pk pk of noise is equivalent to about 1 3 LSB 3 V Accelerometer Featuring TLV2772 7 System Test and Evaluation With the shock sensor and signal conditioning circuit interfaced to the TLV1544 EVM and the TMS320C5X EVM proceed with testing and evaluation of the system performance The program listings as published in the Interfacing the TLV1544 Analog to Digital Converter to the TMS320C50 DSP Applications Report literature SLAA025 can be used as a starting point Appendix A contains program listings based on these except they modify program flow so that the TLV1544 performs consecutive conversions of three of its four channels When motion in any axis is detected the program stores 3000h samples in memory from each channel Once the data is stored it can be saved as a COFF file using the store data utility provided with the TMS320C5X C Source Debugger To read the data into an Excel spread sheet a utility program is required to convert the hexa
44. s to construct the shock sensor and signal conditioning circuits Table 5 1 Board 1 Universal Operational Amplifier EVM Area 100 REFERENCE REFERENCE DESIGNATOR DESCRIPTION DESIGNATOR DESCRIPTION R106 Not used C106 Not used R108 1 MQ 1 SMT C108 Not used R110 Use 0 22 uF 10 X7R SMT Capacitor C110 Not used R112 Not used C112 Not used R114 2 2 KQ 5 SMT U102 TLV431ACDBV5 R116 Not used VREF1 Signal conditioning reference R118 Not used R119 0 Q or Jumper A101 Not used B103 Jump to VREF1 A103 Jump to VREF1 B101 Not used A104 X Axis input from ACH 04 05 08 pin 6 Y Axis output Table 5 2 Board 2 Universal Operational Amplifier EVM Area 100 REFERENCE REFERENCE DESIGNATOR DESCRIPTION DESIGNATOR DESCRIPTION R101 Not used C101 Not used R105 100 KO 1 SMT C105 2200 pF 5 NPO SMT 3 V Accelerometer Featuring TLV2772 Table 5 2 Board 2 Universal Operational Amplifier EVM Area 100 REFERENCE REFERENCE DESIGNATOR DESCRIPTION DESIGNATOR DESCRIPTION R107 75ka 1 SMT cm nouse R109 Use 0 22 uF 10 X7R SMT Capacitor C109 0 Q or Jumper R111 Not used C111 Not used R113 0 Q or Jumper U101 TLV2772CD R115 Not used Vis 3 V from Board 1 AUT zou B nousa Mor Notused B0 lo Aw umpowugi ao Nowed Z Axis input from ACH 04 05 08 pin 12 B1OUT Not used The ACH 04 08 05 shock sensor mounts in the Board to board connectors provide required breadboard area
45. t variabl ACT CHANNI usect variabl jump address to init new channel ADWORD usect variabl Send bytes to the ADC ADCOUNT usect variabl counter for one channel ADMEM usect variabl points to act memory save location 3 V Accelerometer Featuring TLV2772 rm isr save usect variabl 1 memory location to save AR7 during interrupts FINITO usect variabl 1 shows that the sampling is completed EMCOUNT usect variabl counter for samples per channel CHI usect variabl CH2 usect variabl CH3 usect variabl data loc pointO0 usect variabl data loc pointl usect variabl data loc point2 usect variabl data loc point3 usect variabl Sect text MAIN ck CK ck ck ck ck ck ck ck Ck ck Ck ck Ck Sk KK KKK ck ck ck KKK KKK KKK KK KKK KKK ck ck ck KK KKK KKK KKK KKK KKK ck ck KKK KKK KKK KK KKK KKK KKK KKK KK INITIALIZATION BODY KKKKKKKKKKKKKKKKKKK SI Ik e Ik e SI Ik Ik Sk Ik Ik kk kk ke kk kk Sk kk kk kk kk kk kk kk Sk Sk kk Sk kk Sk Sk kk ko ko ko kock ok DSP INITIALIZATION SETC INTM DISABLE GLOBAL INTERRUPTS LDP 0 OPL 0038h Configure PMST allocate IRQ APL 0000h clear PDWSR zero wait states APL OOFOh clear CWSR zero wait states SERIAL PORT INITIALIZATION SPI INI APL K07F38h SPC clear Res DLB FO XRST RRST and FR OPL 00038h SPC Set Burst Mode CLKX 1 4CLK
46. to pk pk The equivalent noise bandwidth ENB accounts for the noise transmitted above the upper cutoff frequency as a result of the finite roll off or tail of the frequency response curve For a 2nd order system ENB is 1 11 times the normal bandwidth For this system ENB 1 11 724 0 7 802 9 Hz It is convenient and acceptable to use ENB 803 Hz 3 V Accelerometer Featuring TLV2772 The amplifier noise gain is the noninverting gain of the amplifier which is equal to 14 3 6 2 1 Sensor Noise Analysis From the ACH 04 08 05 data sheet the shock sensor has a specification for equivalent noise at 100 Hz equal to 0 2 mg VHz Using the typical z axis value for sensitivity 1 35 mV g the equivalent noise in volts VHz is 0 27 uV VHz The data sheet gives no further details as to the spectral shape of the noise from the shock sensor For this analysis assume the noise voltage from the ACH 04 08 05 is flat over the bandwidth of the system 6 2 2 Signal Conditioning Noise Analysis For Vdd 2 7 V the TLV2772 data sheet specifies equivalent input noise voltage of 147 nV vHz at 10 Hz and 21 nV VHz at 1 kHz Because the TLV2772 is a CMOS input op amp the input noise current is insignificant with the circuit values being used The resistors in the signal conditioning amplifier s input circuit contribute thermal noise voltage _ 4kTR Hz Where k is Boltzmann s Constant 1 38 x 10 23J K T is temperature in Kelvin R is r
47. to the high impedance positive input to the TLV2772 where y m Vp V m1 1 sCi Vp R2 HS R23 1 sCi Or 4 1 2 2 H2 s Vo Vp The amplifier gain is found by solving for H2 s Vo Vp The solution is a non inverting amplifier with Vo Z H2 s Vo 1 amp Where R4 a 1 sC2R4 substituting _ Vo AR4 1 ree 1 iin F iem 4 1 2 3 H3 s Vadc Vo Assuming that the input impedance to the TLV1544 ADC is very high in comparison to the impedance of C3 and R5 C3 and R5 form a passive low pass filter where Vadc Vo or EE ME 1 sC3H5 H3 s Vadc _ 1 Vo 1 sC3R5 4 1 24 H s Vadc Vi Superimposing the results from above gives the overall transfer function Vadc _ H2 Hor PEU R31 sC2R4J 11 sc3F5 To find the complete response add the ac and dc components so that Vadc Vi H s Vref 4 1 3 Gain Calculation Since the TLV2772 is capable of rail to rail output with a 3 V supply Vout min 0 V and Vout max 3 V With no signal from the sensor Vout nom reference voltage 1 23 V Therefore the maximum negative swing from nominal is 0 V 1 23 V 1 23 V and the maximum positive swing is 3 V 1 23 V 1 77 V Model the shock sensor as a low impedance voltage source with output of 2 25 mV g max in the x and y axis and 1 70 mV g max in the z axis and calculate the required amplification of the signal conditioning circuit as follows Gain Output Swin
48. writeln number writeln tout number until eof tin close tin close tout readln end end A 97 0 48
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