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# 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 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.

• 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.

• Hopefully with your main question being answered, a minor point is that sweeping below 10K Hz is not advisable: your data will be affected by the "DFT" artifacts.

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.

Thank you.

• Hi,

1- the Rfb should be same value that the resistance connected to Vin pin, otherwise internal TIA will amplify your input signal.

2-You can run all the circuit in single supply, +5V

3- The impedance depends on the distance between electrodes... typically harm to harm is around 1kohms + impedance of your electrodes.

4- For the current generator, I used this circuit,

The RC network are 4.7Mohms,1uF capacitors

The current injected is controlled using R7.

Regards,

Miguel

• Thank you again, Miguel.

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?

• 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

• Hi Kanika,

If you are not using an external AFE, the equation applies as well BUT in teh AN-1252 is explained that the ration between Zmax/Zmin must be lower.

What is your expected Zmin/Zmax range?

Regards,

Miguel

• Hi Miguel,

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 .

Perspective on this will be greatly appreciated!

Thanks

Kanika

• Thanks for the quick reply Miguel

So I am doing body impedance measurements and I expect those values to vary from 3.2 k to 3.6 k inclusing electrodes and the body. At the moment I am testing the chip only with resistor values.

But the results I am getting using range one and the parameters I defined above do not follow the equation in the AN1252. And I am ot measuring below 5k ohms.

• Also, any idea how to increase the minimum resolution of the system?like can we measure a difference 0.01 ohms for instance?

Thanks