I need to implement a four electrode measuring circuit to interface with the Ad5933.
Ive used the suggested analog front end in the attached link, which is very popular throughout the references:
Im able to calibrate a 1k resistor but whenever I change the load after calibration it shows very little difference, for instance an avg of 1040 ohm instead of 1000 ohm when I plug a 2k resistor.
Sometimes it shows even a lower avg value.
For the opamps I ised TL072 and for the inamp I used INA118P. both are have suitable specs for the job.
I used 1M resistor for R current and R protect and used Rin of 2k and Rfb of 1k.
Ive fed the inamp and opamps with v+ of 10v and v- of negative 10v.
Ive attached all grounds in the system.
I short circuited all the ad5933 vdd pins and have them 5v vdd from an arduino, and did the same for the ground pins and attached to the common ground.
The ad5934 is controlled via the arduino by attaching a4 and a5 pins to scl and sda pins.
Finally ive attached a 1k resistor as the load between the four electrodes. Each pole of the resistor gets two parallel electrodes.
Ive checked my wiring a hundred times and Ive tried to add Rg to the inamp.
Ive tried to play with rfb and r in.
Ive chexked with a scope in every point to see that the signal is what its supposed to be. Im sure its good until the inamps input, where im not quite sure because I get very low voltage different across the input pins (20mv).
I would cery much appreciate guidance