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AD5933 impedance variation as frequency changes

Question asked by daniel.ishara on Aug 23, 2016
Latest reply on Aug 24, 2016 by daniel.ishara

Hi,

 

I'm using AD5933 to study the impedance variation in a frequency range. And I'm controling it via Arduino Uno.

 

The problem is that the chip data is showing a variation in the impedance even when I'm using only a resistor.

For exemple, first, for the calibration, I measured the impedance of a 10k resistor in 2 frequencies, and the results were:

 

Freq = 100Hz 

real[0] = -17450
imag[0] = 8336
magnitude[0] = 19338

 

Freq = 10kHz
real[1] = -1144
imag[1] = 9680
magnitude[1] = 9747

 

The diference between them were too significant, and when I took a closer look I noticed that there was a leap in the value, meaning that the Impedance x Frequency function was not continuous. (It would keep it's magnitude value around 20000 for the first few frequencies and then jump to arround 10000, changing dramatically).

(OBS: those are non calibrated data, because they were the ones I intended to use to perform a 2 point calibration)

 

As I noticed that the leap occour around 1kHz I tried not to use frequencies less then 1kHz, and though it worked for the calibration (while using a 10k resistor), when I changed to a supposedly unknown impedace (a 10M(ohms) resistor) the resistance measured would start with a value of 850k and descend to 285k as the frequency increased (this time the variation was almost linear).

 

I will leave attached the table with the data related to this last measurement I've made.

 

All the measurments were made with a 10k feedback resistor.

PGA gain = x1

Vout = 2V p-p (but I was working with a power supply of 5V which should make Vout = 3 V p-p)

Clock speed = 16Mhz (internal clock)

And I'm connecting the AD5933 in the simplest way possible, not using any other components besides the arduino and the resistors.

 

To summarize, my problems are:

1) I don't know why the impedance values are changing, as I change the frequency since I'm only working with resistors.

 

2) Why when I'm taking measurements at a small frequency the value is so different? Could it be that I'm working out of the linear region? (If so how should I set the feedback resistor, as it depends on the value of the unknown impedance)

 

Thank you,

Daniel

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