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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.

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• 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

• 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

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