Hi Sofy,
I see that you are using only 40 mVp of ADuCM350 DAC output in non-attenuation mode and have not optimized the values of RCAL and RTIA resistors.You need to increase the excitation voltage as close as possible to 600 mVp if you are not using attenuation.
To achieve higher accuracy and repeatability of measured sensor impedance using ADuCM350, the RCAL, RTIA , CTIA and the DAC output voltage during the measurement needs to be optimized.
The accurate results can be produced when the ADuCM350 SNR is higher. To achieve higher SNR, the DAC output needs to be around 600 mVp and the ADC input should be around 750 mVp.
600 mVp – Maximum allowable swing from DAC output.
750 mVp – Maximum allowable swing at ADC input.
Please refer to the application note “AN-1271-Optimizing the ADuCM350 for Impedance Conversion” for detailed explanation of this calculations.
Please refer to the calculator 2_Wire_RCAL_RTIA_RG_RLIMIT_Calculator.xlsx which can help you in calculating the RCAL,RTIA values based on your excitation frequency and the impedance under measurement.RTIA and RCAL values with DAC excitation voltage 600 mVp,the ADC input sees a swing of 750 mVp.
Please feel free to get back to us if you have any questions or clarifications
Thanks
Balaji