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

With a single-voltage power supply the DC is not where it needs to be, no use for the scope at this point. Assuming that the resistors in the divider biasing the OPAMP in the right bottom corner are of equal resistance you may consider using the output of that OPAMP as the reference for the other two OPAMPS: just disconnect the grounded end of the resistor at the first OPAMP and the "+" input of the second OPAMP and connect both to the output of the OPAMP at the bottom generating Vref.