I am stuck with my project. I have to measure the frequency characteristics of an unknown impedance valued sensor.
How to do that ?
Apologies for the delay. I have overlooked your follow-up question.
The board should always be calibrated for a change in any of the following parameters:
The board is calibrated again because there would be a change in the gain factor when any of the above mentioned parameters is changed.
The Rfb is used so that the ADC will not be saturated.
We tried to measure an unknown capacitance by using 150k for Rfb and 210k for Rcal. The same procedure for calibration that was done in measuring an unknown resistance was used.
Then the capacitor was placed on Z and the Start Sweep button was pressed. This is what we obtained:
You can download impedance data and compute the capacitance by using the formula:
Xc = 1/(2*pi*f*C)
From the impedance data, we chose a frequency of 30 kHz and an impedance of 342765.3. We obtained a capacitance value of 15.48pF.
Hope that helps.
Hi,I've just started using the EVAL-AD5933EBZ and really looking for a how-to guide.I'm not finding the official users guide (UG-364) comprehensive for beginner like me and have a few basic questions that is not explicitly answered (or at least I failed to find it)1) The "Calibration Impedance" in GUI (v1.5) corresponds to which physical element in the development/evaluation board ?
2) The Vin/Vout referred in the following corresponds to what in EVAL board ?as according to the schematic shown belowIt is appears to be "RFB" is Calibration Impedance (ref. GUI) as I failed to find Rcal in schematic extracted from UG-364 or any element on EVAL Board marked as Rcal/RCAL.If Rcal is Rfb, where to place the unknown impedance that I want to find out, in the element "Z" of the EVAL board, or between "T7 and T8" as it is across Vin and Vout, as stated in GUI manual.
The documentation is indeed confusing, it is stemming apparently from the differences between the AD5933 datasheet and the evaluation board documentation, where the bits and pieces of the text and schematics from the former are scattered across the latter. The evaluation board contains AFE, that effectively isolates the chip and takes over some of its functions, e.g. the Rfb is now connected as a feedback resistor to U1-B OPAMP, while the internal OPAMP in the chip is configured to have unity gain.
Sandeepan said:1) The "Calibration Impedance" in GUI (v1.5) corresponds to which physical element in the development/evaluation board ?
The reference designators in this window should be ignored, the calibration impedance should be connected in place of the rectangle designated as "Sensor" on the schematic diagram in your message, right whether the wires are crossing the rectangle's border. There should be no capacitors and "z" physically present when you perform calibration and measurements.
Sandeepan said:2) The Vin/Vout referred in the following corresponds to what in EVAL board ?
Vin is the wire running from the top of that "Sensor" rectangle to U1-B OPAMP and Vout is the wire running from the bottom of that "Sensor" rectangle to U1-A OPAMP.
Sandeepan said:It is appears to be "RFB" is Calibration Impedance (ref. GUI)
No, the Rfb is the feedback resistor to U1-B OPAMP. Rcal is not a part of the schematic, it is the resistor you need to connect in place of that "Sensor" rectangle. You can plug your Rcal and subsequently your unknown impedances into the Z receptacles, just make sure that the capacitors C1, C2 and C3 are not installed on the board.
Hope this will get you started, best of luck with your measurements!
Thanks Snorlax for filtering this muddy water.Would you please guide me to any equation regarding how to choose Rfb for unknown Z within a given range.Regards
The upper limit for Z range is always infinity: if you do not connect anything in place of Z you circuits continue to function in linear regime and you can read valid data from the AD5933 chip.The focus should always be on the lower limit of the Z range because if Z is too low, the U1-B OPAMP output voltage will hit the power supply rails, the sine wave signal will be distorted/clipped and the "DFT" performed by the AD5933 over distorted sine wave will be erroneous. Zmin and Rfb are related through the formula on p.5 of https://www.analog.com/media/en/technical-documentation/application-notes/AN-1252.pdf . Since you have the AFE on your board, Vdcoffset is equal to Vdd/2 regardless of excitation voltage setting, so the denominator is simply Vpk. Most accurate calibration can be achieved when Rcal = Zmin.