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 = -17450
imag = 8336
magnitude = 19338
Freq = 10kHz
real = -1144
imag = 9680
magnitude = 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)