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

For this part, I got the answer. 

"The accuracy achieved is very much dependent on how large the unknown impedance range is relative to the calibration resistor, RCAL. Therefore, in this example, the unknown impedance of 10.3 Ω measured 10.13 Ω, an approximate 2% error. Choosing an RCAL closer to the unknown impedance achieves a more accurate measurement; that is, the smaller the unknown impedance range is centered on RCAL is the more accurate the measurement. Consequently, for large unknown impedance ranges, it is possible to switch in various RCAL resistors to break up the unknown impedance range using external switches. The RON error of the switch is removed by calibration during the RCAL gain factor calculation. Using a switch to select various RFB values can optimize the dynamic range of the signal seen by the ADC.

Consequently, for large unknown impedance ranges, it is possible to switch in various RCAL resistors to break up the unknown impedance range using external switches. The RON error of the switch is removed by calibration during the RCAL gain factor calculation. Using a switch to select various RFB values can optimize the dynamic range of the signal seen by the ADC. In addition, note that to achieve a wider range of measurements a 200 mV p-p range was used. If the unknown Z is a small range, a larger output voltage range can be used to optimize the ADC dynamic range."

Reference link below, page 3.

https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0217.pdf

Abid said:

How can Rcal resistor set the range. 

Rcal does not set the range, it is used to measure the system gain, e.d. to find the dependence between the impedance under test and the output values produced in response. If you are planning to work with the circuit from the link you mentioned the factors setting the range are:
1. The excitation voltage programmed into the AD5933
2. Value of the current-setting resistor  Rlimit
3. Gain of the instrumentation amplifier INAMP
4. PGA gain programmed into the AD5933

Abid said:

How can Rcal resistor set the range. 

Rcal does not set the range, it is used to measure the system gain, e.d. to find the dependence between the impedance under test and the output values produced in response. If you are planning to work with the circuit from the link you mentioned the factors setting the range are:
1. The excitation voltage programmed into the AD5933
2. Value of the current-setting resistor  Rlimit
3. Gain of the instrumentation amplifier INAMP
4. PGA gain programmed into the AD5933