Please explain the process of unknown impedance measurement using AD5933 EBZ
You may start by referring to AD5933 evaluation board user guide: http://www.analog.com/static/imported-files/user_guides/UG-364.pdf
The "performing a frequency sweep section" gives detailed sequence on how to use the eval board if you want to sweep frequency across the unknown impedance.
But the frequency sweep is enabled only after mid-point gain is calculated.
Mid point gain is not automatically calculated when resistor value is not specified in EVAL-AD5933EBZ software.
We have to give known resistor values specified in Calibration impedance section of EVAL-AD5933EBZ board software.
Then how can it measure the unknown impedance if there is only chance to specify known resistor or capacitor values ?
In order to measure an unknown resistance, the AD5933EBZ must be calibrated first.
You can start off by using 200k for Rfb and 200k for Rcal (denoted by Z in eval board).
In our set-up, we used 150k for Rfb and 210k for Rcal.
Calculate Zmin and Zmax using the formulas found in page 5 of AN-1252: http://www.analog.com/static/imported-files/application_notes/AN-1252.pdf
Zmin = 121k ; Zmax = 509k
That means that the value of the unknown impedance to be measured should be within Zmin and Zmax.
Enter 210k on Resistor value R1 then press Program Device Registers
Next, press calculate gain factor then start sweep.
The calibration resistor value should have flat response across the defined start and stop frequency.
Next, remove Rcal and replace it with the unknown impedance (but withn Zmin and Zmax). In our case, we connected 330k. we should expect it on the software. Just press "Start Sweep" ONLY.
Thank you very much for the explanation.
I just found one formula for Zmin and Zmax. But what are Zmin and Zmax ?
The only formula given is Rcal = (Zmin+Zmax)* (1/3)
How did you calculate Zmin = 121k ; Zmax = 509k in your set up of 150k for Rfb and 210k for Rcal. ???
I am unable to calculate Zmin and Zmax for my set up with Rfb=200K and Rcal=200K.
Please explain that.
In page 5 of AN-12, you can see these formulas:
to compute Zmin and Zmax, we have known values for:
Vdd = 3.3
Vpk = 2Vpp/2
Vdcoffset = 1.48 (Range No. 1)
Gain = x1
Rfb = 150k
Rcal = 210k
Solving for Zmin in 1st formula gives 121k.
Solving for Zmax using 2nd formula gives 509k.
Hope that's clear.
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