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AD5933 - Question about Real & Imaginary data

Question asked by NamhoKim on Jun 11, 2018
Latest reply on Jun 22, 2018 by NamhoKim

Hello,

I am trying to design continuous bioimpedance measurement system using AD5933.

I designed frequency sweep function with single cycle, and now I am on validation.

 

I referred the formula in data sheet to decide the gain factor using 1 kOhm resistor. (Calibration using 1 kOhm)

I decided each gain factor for each frequency, and below is the result of measurement for 1 kOhm. (Measure 1 kOhm)

 

Frequency: 10.00 kHz; Impedance: 0.96 kOhm; Real: 12, Img: -5,
Frequency: 20.00 kHz; Impedance: 1.04 kOhm; Real: 6, Img: 4,
Frequency: 30.00 kHz; Impedance: 1.11 kOhm; Real: 10, Img: 3,
Frequency: 40.00 kHz; Impedance: 0.92 kOhm; Real: 18, Img: 6,
Frequency: 50.00 kHz; Impedance: 1.23 kOhm; Real: 15, Img: 11,
Frequency: 60.00 kHz; Impedance: 1.08 kOhm; Real: 18, Img: 18,
Frequency: 70.00 kHz; Impedance: 1.08 kOhm; Real: 16, Img: 27,
Frequency: 80.00 kHz; Impedance: 0.99 kOhm; Real: 16, Img: 37,
Frequency: 90.00 kHz; Impedance: 1.07 kOhm; Real: 14, Img: 38,
Frequency: 100.00 kHz; Impedance: 0.98 kOhm; Real: 7, Img: 49,

 

And I measured 1.8 kOhm, but the result was not good. (Measure 1.8 kOhm)

 

Frequency: 10.00 kHz; Impedance: 0.30 kOhm; Real: 31, Img: -28,
Frequency: 20.00 kHz; Impedance: 0.10 kOhm; Real: 70, Img: 29,
Frequency: 30.00 kHz; Impedance: 0.10 kOhm; Real: 78, Img: 82,
Frequency: 40.00 kHz; Impedance: 0.12 kOhm; Real: 43, Img: 141,
Frequency: 50.00 kHz; Impedance: 0.13 kOhm; Real: -4, Img: 175,
Frequency: 60.00 kHz; Impedance: 0.15 kOhm; Real: -59, Img: 178,
Frequency: 70.00 kHz; Impedance: 0.17 kOhm; Real: -115, Img: 162,
Frequency: 80.00 kHz; Impedance: 0.21 kOhm; Real: -147, Img: 123,
Frequency: 90.00 kHz; Impedance: 0.22 kOhm; Real: -179, Img: 88,
Frequency: 100.00 kHz; Impedance: 0.24 kOhm; Real: -196, Img: 45,

 

 

So I want to ask two questions:

 

1. The Magnitude (sqrt(pow(Re,2)+pow(Img,2))) of 1.8 kOhm is higher than that of 1 kOhm so the impedance of 1.8 kOhm became lower than that of 1 kOhm (because Impedance is in inverse proportion to Magnitude by given formula). Then why is the Magnitude of 1.8 kOhm showed higher value than that of 1 kOhm? 

 

2. I used only one resistor (1 kOhm) for calibration, but there was imaginary results that means the exist of the capacitance reactance. Should I assume that imaginary results as off-set?

 

 

Thanks,

Namho Kim

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