I need a reference circuit for 4_wire the impedance measurement using AD5934, range 50 to 250 ohm.

You may use this post as a starting point:

4 wire Impedance measurement

In the link, it is mentioned that setting Rcal = 20 can set the impedance range from 1 to 60. Can you please explain this, I did not get this. How can Rcal resistor set the range. 

https://ez.analog.com/dds/f/q-a/28201/ad5933-4-electrode-configuration-for-human-body-thoracic-impedance-measurement-0-60-ohm

Thank you,

Do I need some filtering on the two input pins?

I have another question,

Do I need any sort of filtering in the receiver end, before any amplification. 

Not sure if it is needed, does your application require galvanic decoupling of the impedance to be measured form the electronics?

It really depends on a particular use case - filtering is always tricky as it is not really known what to filter for unless the circuit is tested in real-life application and particular sources of interference are identified. The circuit itself in this 4-wire arrangement is reasonably resistant to environmental interference thanks to common-mode rejection. The high-frequency interference is likely to be sufficiently suppressed by relatively slow INAMP (250 KHz) and the AD5933 built-in 100 KHz low-pass filter.
What is the excitation frequency range you are planning to use? For impedance under test with capacitive response there might be a case for using an additional low-pass filter on the excitation side to filter out high-frequency spikes between the DAC steps in the synthesized voltage sine wave.

I got the desire signal. This is the signal I got on RFB resistor end. I will adjust amplifier gain to get the desire signal amplitude for AD5934's ADC.

Looks good, congrats!
Peak-to-peak voltage on this pin with PGA gain of 1x should  not exceed Vdd - 0.2V, and when PGA gain is set at 5x, (Vdd - 0.2V) / 5. This is likely to occur at the higher limit of your impedance under test. The little steps along the sine wave trace are the DAC steps from the frequency synthesizer I mentioned earlier.
The oscilloscope indicates signal frequency at  ~ 2.68 KHz, what is the clock frequency coming to the AD5934? Signal frequency might be potentially too low for the "DFT" to give accurate readings.

Snorlax said:

Looks good, congrats!

Thanks, your help was the main factor.

I will keep peak-to-peak voltage under the rated value.

Frequency is making troubles. Should I post my questions related to frequency and impedance measurement here or should I open a new issue?

I would just keep discussing it in this thread - forum moderators usually move questions to different groups/threads as they see fit, so the questions endup in the right place anyways.  

Ok, sounds great.

I follow the CN0349 design in which 1Mhz crystal is used. But now I noticed that, I can get around 7800Hz frequency (maximum) using 1Mhz crystal. Though, I can get higher frequency using sweep() function, but that one is also limited, I think max is 15KHz. For this part I already order 16Mhz crystal, so I will change the crystal. 

As you said.

"The oscilloscope indicates signal frequency at  ~ 2.68 KHz, what is the clock frequency coming to the AD5934? Signal frequency might be potentially too low for the "DFT" to give accurate readings".

I did calibrate my system with 100 Ohm resistor, with 100 Ohm resistor Vin was almost 1.09 V peak-to-peak. But when I measure 120 Ohm resistor I get impedance value 91. I check Vin for 120 Ohm, which was almost 1.2 V peak-to-peak.

Below is the print values for measuring 120 Ohm resistor.

Red 120 Ohm mean the actual value. 

My question is,

Is this because of low frequency or there can be some other issue?

Before getting into the weeds with the frequency, when you say Vin voltage, do you mean RFB voltage? From the screenshot we know the raw Re and Im  values, but what were  the raw Re and Im values when you connected the calibration resistor of 100 Ohms? Also, is the software that calculates the impedance something you wrote?  

Before getting into the weeds with the frequency, when you say Vin voltage, do you mean RFB voltage? From the screenshot we know the raw Re and Im  values, but what were  the raw Re and Im values when you connected the calibration resistor of 100 Ohms? Also, is the software that calculates the impedance something you wrote?  

Yes, I am checking the signal on RFB resistor. 

For 100 Ω the Re is 3113 and Im is 4171.

I wrote the software for this part. 

Abid said:

For 100 Ω the Re is 3113 and Im is 4171.

Output log shown in pic below.

The suspicion is that your code thinks that the output is inversely proportional to the impedance under test - which is correct when the AD5934 does not have the 4-wire analog circuits between Vout and Vin. With this 4-wire circuit the output is directly proportional to the impedance under test.

In abstract sense the circuit is converting the impedance into integers in the Re and im registers that constitute complex output Re + j*Im. The conversion factor  between physical Ohms and counts in those registers can be viewed as complex gain G. This complex gain can be calibrated with a known resistor: resistor impedance has only real component and zero imaginary component, so Zcal = 100 + j*0 Ohm. The correspondent output is Outcal = Re + j*Im = 3113 + j*4171 = G * (100 + j*0) Ohm, so G = (3113 + j*4171 ) / ((100 + j*0) Ohm). The "unknown" impedance Zx produces output,  which is proportional to Zx: Outx = 3400 + j*4574 = G * Zx, so Zx = Outx / G = (3400 + j*4574)*(100 + j*0) Ohm / (3113 + j*4171) ≈ 109.5 + j*0.21 Ohm. As expected from a resistor, the imaginary component is nearly zero. The real component is 109.5 Ohms, which is a bit far from the expected 120 Ohm, but it could still be within manufacturing tolerance of both the resistors used for calibration and as the "unknown."

Snorlax said:

With this 4-wire circuit the output is directly proportional to the impedance under test.

Yes, I missed that point. Now I am getting 109 Ω when measuring 120 Ω. 

Snorlax said:

The real component is 109.5 Ohms, which is a bit far from the expected 120 Ohm, but it could still be within manufacturing tolerance of both the resistors used for calibration and as the "unknown."

I ordered more resistors (different values) I will calibrate and measure a bunch of different resistors. 

Thanks once again.

This is the result.

Yes, it would be great if you could find some 0.1% or better yet 0,05% resistors. Only avoid wire wound type - those may have too much inductance for AC measurements.

I did test and calibrate using different resistor values. I noted that the measurement accuracy is better near the calibrated resistance value. I think this is not a problem, this I can improve using some tricks.

The problem now I face, is, when I use 4 wires and short 2 wires together (T1 with R1 and T2 with R2) see the picture.

In this case I can get reading. See the pictures as a reference.

4 wires, 2 wires short together.

This is the waveform in the saline water.

But, WHEN I use for wires or electrodes separated from each other, then the behavior changes completely. I can not get any proper reading. please. check the pictures below.

This is 4 wires, first (white) and last (brown) are T1 and T2. Second (Blue) and  third (black) are R1 and R2 (R stands for receiver)

This is the acquire waveform,

Saline water is used in both cases, with conductivity of 7.37 mS/cm.

Can you help with this.

Thanks

Abid said:

wires or electrodes separated from each other

I am not sure it is possible to separate the excitation wire Tn from the corresponding sensing wire Rn when in saline - it is going to break the DC working point of your circuit. Roughly speaking, the wires in saline are not galvanically connected to each other due to polarization layer forming at the interface. Very crudely, you can think of it as a capacitor between the wire surface and bulk saline, so the equivalent circuit approximating corresponding behavior would be something like this:

 So, the sensing side of your circuit does not have any DC coupling to the rest of the circuit and your INAMP inputs are hanging at whatever voltage these capacitors are charged to by the input pin currents and leakages. On the excitation side this polarization "capacitance" would break the DC feedback loop for U2, that is why some kind of R11 is necessary.

I will add a capacitor on each Rn wires. This might block DC values.