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AD5933 Calibration - Bioimpedance 4 electrodes with Analog Front-End

Question asked by ValterF on Aug 26, 2017
Latest reply on Aug 30, 2017 by ValterF

Hi, I'm developing an circuit with AD5933, I known I need to calibrate it to get a gain factor.
I am doing a bioimpedance system, with 4 electrodes, and a frequency sweep from 50KHz to 100KHz.

These are the characteristics of my sistem:

VDD in AD5933 = 5V.
Range = Nº 1
My signal = 3Vpp
Typical Output Impedance, ZOUT = >100 Ohms
Maximum Ratio = x45
PGA gain = 1
Internal oscillator: MCLK = 16.776 MHz
fSTART = 50K Hz
∆f = 1K Hz
Increments = 50
RFB = 1K Ohm


My Questions are:


1) How do I position my Known Impedance? This Way? (Image 1), with R1 and R2. Since I have 2 current outputs and 2 inputs of the Instrumentation Amplifier.

2) How do I choose the correct value for my Known Impedance (R1 and R2)? Ideally, it's just a resistor, right? The formula of the AN-1252 does not fit for me because I do not have Zmin.

The "Current-to-voltage amplifier gain" of the AD5933 datasheet is RFB resistor? Does it need to be equal to my Known Impedance Resistor? As in AD5933 datasheet? If this is true the value of my known impedance must be 1K Ohm?



3) How do I set the gain of my Instrumentation Amplifier (INA118) for Calibration? Since I am using only 2 resistors (R1 and R2), as in the Image 1.


4) I will only use 2 frequencies in my bioimpedance calculations, 50KHz and 100KHz. Do I need to calculate 2 different gain factors for each frequency? Or should I make an average as in AD5933 datasheet?



Thank you!!

Best Regards!