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Struggle with computing bridge output voltage

Question asked by Tomitooo on May 7, 2018
Latest reply on May 8, 2018 by Emman.A

Hello everyone, i have put together one-channel data acquisition unit for my school project. Its task is to sense output voltage of Wheatstones bridge with strain gages and to generate AC excitation for it. Reading data from ADC and generating sine wave via DAC is controlled by dsPIC uC and communication with PC is made via uart-usb interface.

Everything works fine, i am reading big data comming from unit, as raw int16 numbers and i am plotting them into live plot GUI app written in python


I have struggle with creating mathematical formula which would connect bridge voltage with ADC output code


Input section of DAQ unit is circuit from Application note number 683 (attachment). There is instrumentation amplifier with gain set to 991. Gain of demodulator AD630 should be 2 with this configuration and output of AD630 should be rectified and averaged after cascade of low pass filters. Output of voltage buffer of this analog section is then devided with two resistors (8k, 2k -> 1/5).


There is ADS8867 ADC with THS4521 fully differential amplifier front end with 5V reference. Rf and Rg resistors set gain of THS4521 and in this application its set to 2.5. 


Some numbers:

AD8221 gain: 991 (Rg = 49.9 R)

AD630 gain: 2 

AD630 output with LP cascade : averaged rectified sine wave: 0.637*U

Voltage divider (8k, 2k): 0.2

THS4521 gain: 2.5

ADC: 16bit

adc_reference: 5V


My current approach for getting voltage at the ADC input (if i can compute value of voltage at the ADC input, i can then in the inverse manner compute Ubridge with digital code)


Ubridge * AD8221gain * AD630gain * MeanValue * Volt.divider * THS4521gain = Uadc_input

Ubridge * 991 * 2 * 0.637 * 0.2 * 2.5 = Uadc_input 


Is my approach correct please? I am not sure about that AD630 part of the circuit (gained by 2 and after signal goes through low pass filters, there is mean value of the input signal). Right now i am testing my setup with full strain wheatstone bridge (beam made to measure bending forces with range of 1kg) which has sensitivity 1.5 mV / V, and i am generating excitation 2.5V with 400 Hz frequency. 


Values of microvolts which i get from my readings are not that far from theoretical values, but I am not completely sure if that approach is correct.


Could someone share their procedure of this kind please or give me any hints?


Thank you very much