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?*

Image1

* 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!

Hi,

could be possible to describe how did you implement the current sources, please?

answering your questions,

1- Place the resistor between Iout, and measure the voltage across the terminals.

2- Yes, you should apply the equation in AN-1252. If you refer no Zmin as could be 0ohms, this is not true. The body always has some impedance but more important, the electrodes used has an impedance as well.

3- Set the gain to make sure that at worst case, Zmax xIout x amplifier GAIN = 5Vpkpk

4- Yes, I'd recommend to do so.

Regards,

Miguel