CMRR reducing on NI PXI 4070 or 4072

[j_dodek]

Hi,

I am completely disapointed about a measurement task with my 4072 or 4070

Well the measurement take place in extrem rough conditions (measuring a pulsed coil).

I try to measure a arithmetic DC Voltage of PWM Signal (25HZ 90% Dutycyle Umax = 3V) over a 1mH coil.

The creation of the signal is done by a controller with a pulse rate of 10 kHz.

Watching the signal  on a Textroniox Scope (Ri = 1MOhm) will show no noise. Measuring  the PWM

 with a cheap DC-Handmeter(Ri10 MOhm) parallel to scope or stand-alone will ALWAYS measure

the desired value.

But if you tries this with NI-4072 via the Softfront panel most time you will fail because your 4072 goes to overload.

it doesent matter if you have selected the highes DC range. If you put on the scope you will see the disaster:

a huge storm of 10 kHz noise. If disconnect your DMM you are in eye of hurrican a see the blue sky.

Well adding the scope will produce another in Rin so only 50% of the measurements will fail with overload.

If you try this with NI-4070 there is no bigg strom, most of the measure bring up the desired value.

But for automated testing this is unuseable.

It seems the PXI system leads somewhere to ground an infectes my measurement.

Is there a way to configure my high-end PXI DMM in way that it acts like a cheap 50$ Handmultimeter?

Greetings

Juergen

[Mark_E]

Juergen,

It's unfortunate you've been experiencing some difficulty getting the measurements you want. We definitely want to get you going with your application. Would you please provide more details as to your setup connections? Specifically, a schematic of all the connections, including the PWM source and all ground connections, would be very useful for troubleshooting the source of the error. Are the devices all connected to the same power strip, is the probe grounded to the wall outlet, etc.

Thanks.

[j_dodek]

Hi Mark,

Thanks you for response.

First i did a mistake in my writings in the upper thread. I am not measuring the voltage over the coil! I am always measuring it over a shunt resisitor to get the resulting current.

At NI- Germany there should be all information available under SRC number 752742.

We have figured by ourself out that we can elimate the noise by adding a ring core double choke (51uH) 

http://www.epcos.com/inf/30/db/emc_00/02430246.pdf on the input pin of the dmm.

Greetings

Juergen