analog input DAQ impedance matching phase shift

[Jun_l]

Hi,

I am using a two analog input of a USB 6366 DAQ to measure my voltage and current signals across a DUT (Air capacitor with C = 100 pF). Sample rate = 2MHz, number of samples 1M. Frequencies 350Hz - 1950 Hz.

Because my voltage divider is currently out of order, I'm measuring my voltages with a 100:1 test probe (input impedance 100M // 5pF), with a 1MOhm resistor connected parallel to the DAQ channel input to lower the input impedance of the DAQ. I used the 1Mohm resistor cause the test probe worked well with the oszilloscope, which had an 1Mohm input impedance so i assumed it would be sufficient.

I seem to be able to get the correct RMS values, but I am getting a phase shift between the two signals that are > 90°. I'm expecting values around 89.89°. Does this mean that my impedance matching using the 1Mohm is incorrect? Are there any other ways to improve this?

Thanks

[Henrik_Volkers]

Have you compensated / adjusted the probe capacity to your input imdedance?

Have a look at the schematic of your probe, you will find n 'unknown' parts.

Now draw all parts (plus parasitic impedances ?) in a schematic with your DUT

Do some measurements with known DUTs , calculate the unknown impedances.

....

Guess why serious LCR meters use four channels and open/short calibration 😉