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AD5933 Gain factor in real Resistance and Reactance

Question asked by ValterF on Aug 5, 2017
Latest reply on Aug 7, 2017 by musach

I'm trying to develop a system for bioimpedance measurement with the IC AD5933.

The AD5933 is a high precision impedance converter system that combines a frequency generator with a 12-bit, 1 MSPS, Analog-to-Digital converter. The frequency generator allows an external complex impedance to be excited with a known frequency. The signal in response from the impedance is sampled by the on -board ADC and a discrete Fourier transform (DFT) is done by an on-board DSP engine.

The DFT algorithm returns both a real (R) and imaginary (I) data-word at each frequency point along the sweep. The impedance magnitude and phase are easily calculated using equations.

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To convert this number into impedance, it must be multiplied by a scaling factor called the gain factor. The gain factor is calculated during the calibration of the system with a known impedance connected between the VOUT and VIN pins.

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The measured impedance at the frequency point is given by:

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My problem is that I need to use only the real (Resistance) and imaginary (Reactance) information, rather than the Impedance Magnitude.

Because the equations of body composition require, for example:

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So, my question is: How do I apply the gain factor only in real (Resistance) and imaginary (Reactance) values?

Another thing, this doubt is very simple: 
Is it possible to program the AD5933 to work with a fixed frequency of 50K?

Tks!

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