How to correctly calculate the Rfb value in AD5933

Question

I was reading the datasheet of AD5933, and I did not understand a few things about the calculation for the Rfb value and other things. 
 This is the Rfb equation: 
 
 How do I find the value of Zmin? 
 I intend to power the AD5933 with 5V, use the 1st Output Range (3Vpp in this case) and PGA gain = 1. With a frequency sweep of 1K Hz up to 100K Hz: Internal oscillator: MCLK = 16.776 MHz fSTART = 1000 Hz ∆f = 1000 Hz Increments = 99 
 In addition, I will use a high pass filter with a cutoff frequency of 100Hz, a circuit with a current source controlled by voltage (1mA), and an instrumentation amplifier INA118 (Gain = 2). This circuit will use 4 Electrodes. Similar to this: 
 
 In AN-1252 APPLICATION NOTE , page 3 of 12, it is said that for range 1 (Adding external op amp) Zout is equal to "> 100 Ohms". And on page 5, The maximum ratio, ZMAX / ZMIN, for my case is x45. Right? If it is correct. How to calculate Zmax and Zmin for "> 100"? 
 Furthermore. What is "RCal" for? If I do not use it in any equation. 
 In the example on page 7 , the Zmin value is 4.7K and the Zmax value is 47K. Why? I dont understand.

musach · Accepted Answer

Hi, 
 Yes, you are right.. I did a mistake... the filter suggested is for bipolar supply... 
 You will need to use this approach in single supply, 
 
 Best Regards, 
 Miguel)

musach · Answer

Hi, 
 The AD5933 can measure in a limited impedance range, defined by Rfb 
 Zmin is the minimum expected impedance, and zmax is calculated using Zmin... as far as i remember, once Zmin is selected, ie 500ohms, Zmax is around 45x500 = 22.5k 
 In your circuit, Rfb is not limiting anything... as the current is measured thru the inamp so, you must guarantee that the inamp output is between 0 and 3.3V. 
 If you are planning to measure bio-impedance, I built a simpler circuit time ago which provides pretty good accuracy, you can find teh reference to the circuit in this forum... Let me know if you do not understand something, please 
 BTW... in bio-impedance you must be careful with the cell-potential of the electrodes which will affect your performance.

Snorlax · Answer

Using the first equation one calculates the Rfb at given Vpk and Gain values (programmed into the chip). At Gain=1, the Rfb is typically 15-20% higher than Zmin. The second equation is not very useful as Zmax is either arbitrary or defined by one’s specific application. There is nothing wrong with infinitely high Zmax: measuring it should produce zeroes in both Re and Im output registers, but the equation does not yield any practical value for Rcal. The suggestion would be to use Rcal = Zmin to begin with and worry about the subtleties later. 
 Yes, you can certainly use Gain=5 with the AFE.

Snorlax · Answer

With the AFE it is a bit different story as VDCOFFSET = VDD/2, so solving the Rfb equation above for Zmin in this case yields: Zmin = Rfb * Gain * Vpk/(Vdd/2 -0.2). Depending on Gain and Vpk Zmin can be smaller than Rfb.

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How to correctly calculate the Rfb value in AD5933