Hi Friends,

I am stuck with my project. I have to measure the frequency characteristics of an unknown impedance valued sensor.

How to do that ?

Parents
• Hi.

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:

• Current-to-voltage gain setting resistor
• Output excitation voltage
• PGA gain

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:

Magnitude:

Phase:

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.

-Sittie

• Hi.

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:

• Current-to-voltage gain setting resistor
• Output excitation voltage
• PGA gain

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:

Magnitude:

Phase:

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.

-Sittie

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