Impedance Analyzer Internal Compensation

[Himmel]

Hi,

I'm trying to measure the capacitance of a sensor, and am using the impedance analyzer tool to do so. If I do not connect anything I notice it sees a 22pF parallel capacitance.

Is this because the program does not automatically compensate for the internal 24pF capacitance?

EDIT: I am using the Analog Discovery 2 with Waveforms 3.8.2

[Piotr Rzeszut]

Have you performed calibration (open, short, ...)?

Additionally such small capacitance can be a parasitic capacitance of cables, AD2 connector etc.

[Himmel]

@Piotr Rzeszut

I have done the device manager calibration, I notice there are open and short compensation options in the impedance analyzer, is this what you mean?

[Piotr Rzeszut]

Yes, I meant these options.

[Himmel]

Is there instructions on how to do this, for instance does it matter which setup I am using (W1-C1-R or W1-C1-DUT)?

I assume for open I am to just remove the load, and for short, short where the load would be?

Appreciate your help.

[Piotr Rzeszut]

Yes. First you should measure your resistance with a precise multimeter and enter it in Compensation window.

Then open compensation is without load, and short is with shorted DUT terminals.

Please be aware, that compensation should be repeated when you change frequency range, frequency step, etc.

[attila]

Hi @Himmel

The open compensation is important specially for high reference resistors values and high frequency measurements, the parameters needed to measure small capacitive values.
To perform open compensation, leave the IA terminals open or wire ends unconnected, press "Perform Open Compensation" and check "Enable Open Compensation" be used.

The application stores separate open/short compensation values for each resistor value, but only for one frequency range, like it is shown in the compensation dialog. Changing the start/stop frequency or samples requires repeating the compensation.
The precise resistor values can be specified here and the open/short compensation can be started also from this dialog.

Here you can see a 39pF capacitor with and without open compensation:

[Himmel]

Cheers Attila, Piotr, This tool is great, I performed the compensation and I get a nice flat capacitance measurement for open response (albeit a very spiky phase response).

I do have a question: The compensation tool allows you to input your resistor value for accuracy, does it matter that we are not given this option in the main window?

[attila]

Hi @Himmel

I'm not sure if I understand you question exactly...

The Impedance Analyzer tool is intended to be used with the https://store.digilentinc.com/impedance-analyzer-for-analog-discovery/ module. That's why you have a specific list of reference resistor values.
The IA tool can also be used without such module, with custom setup/resistors. You can type in the exact value in the compensation dialog, like instead 100k you can use a 49.999k resistor.

In the IA tool you can select the currently used resistor.

 

[Himmel]

Cheers Attila, I see now, I hadn't actually entered the custom resistance value and only assumed. Many Thanks.