How to correctly calculate the Rfb value in AD5933

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 MHzfSTART = 1000 Hz∆f = 1000 HzIncrements = 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"?

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.

So, the value of the Rfb resistor in my case does not matter? 1KOhm is good value?And what is the value of Zmin indicated to measure body composition by bioimpedance? I did not find this information anywhere.

Sorry for the various questions. I realized that you understand much about it.In my case, do you believe that a gain of 2 for inamp (INA118) is enough? I'm fed it with + -5V.

Regarding circuit power, before the current source, I am using a high pass filter to remove the DC component of the signal, I am sending an AC signal to the electrodes.Regardless of the type of current source used, I believe that a in amp with a single supply, can not amplify negative signals because the power is not symmetrical. Right?

So are you saying that the Rfb formula in the application Note AN 1252, is for the design with the AFE?

Because I am tring to use the device without the AFE and the Rfb value does not seem to be linearly propotional to the Zmin. At the moment, with no AFE added, and Rfb as 10K ohms, my system saturates at measuring 5K ohms impedance. Reducing the Rfb doesnt not lower the minimum impedance I can measure. I cannot measure even 1k ohm!

the system is at:

VDD = 3.3 V

DC offset = 1.48 V

Vp-p = 1.98 V

PGA GAIN = 1

Rfb = 10k

and then with this the minimum Z I can get is 5K ohm. Any imüedance below this (like 2k ohm or 100 ohms) is not measured.I get values saturated at 5k ohm .

The formula just helps calculating the RFB value when Zmin is known. From the formula it follows that RFB is always lower than Zmin, so the chip is not supposed to work when RFB is 10k and Z is 5k and even less so when it is 1k. It works backwards as well: for given RFB you can always solve it for Zmin and see that Zmin is always larger than RFB. So for your 10K and other parameters Zmin is about 15k (provided that Zmin is just a resistor, for complex impedance things are more, well, complex).

I'm using my AD5933 with AFE which I found in AN-1252. On Page 7 in table 6 the RFB Value is 6.8 kOhms and it is bigger than ZMin in table 5 is 4.7 kOhms. it makes no sense for me. because ZMin should be bigger than Rfb...

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.

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.

It is difficult to discuss this matter without knowing what you application really is. On the surface, yes, it should work, but, depending on what you are shooting for, may or may not be accurate enough.

Digipots are not terribly reproducible device-to-device, not that linear and flat with frequency compared to discrete resistors, which, again, may or may not be an issue for your application. Parallel RC networks should be measurable, generally speaking. A potential issue you may or may not run into at the lower impedance values is relatively high current across Rfb, for example, at Vpk = 1.98 V and Zx = 5 Ohm this current is ~0.4 A, which your excitation voltage source must be able to deliver, Rfb must be able to pass across and the OPAMP with Rfb must be able to sink.