5620 - AC to DC couple?

[Shazlan]

Hi all,

I just acquired two PXI cards, PXI-5422 AWG and 5620 Scope. During some tests I did, I've connected my PXI-5422 AWG output directly to the input for PXI-5620 Scope. I used the fGen SFP to generate the signal and NI Scope SFP to see the signals I got. The results can be seen attached. When I generated a Sine Wave, the resultant signals measured by the scope is as expected - a sine wave. The only problem is the peak value of the measured signal is far less than the amplitude value of the generated signal (1V). That's one issue. The second issue is when I generated other than Sine Wave (Square wave, for instance), the resultant graph looked funny. The captured signal in frequency domain showed that there's a lot of harmonics. Just to confirm on the integrity of the generated signals, I then connected the AWG to a Tektronix Scope and resultant graph was as expected.

I was told that the both issues is due to the coupling. PXI-5620 only accepts AC coupling but I need DC coupling to actually see the same signal as generated - or so I was told. Does this means that the signals I was getting (as the attached document shows) are correct as how it should be? If so, how can I convert the funny looking graph to look like the input signals?

I've read the tutorials from ni.com on AC/DC coupling but still couldn't understand when to use AC and DC coupling. I hope the solution to this is NOT getting another PXI card with DC coupling capabilities. Please advice.

Thanks.

Shazlan 

[Henrik_Volkers]

Some keywords to level: Output impedance, input impedance, kirchhoffs law. Short: 1Vss with 50 Ohm output impedance measured with 50 Ohm input impedance will give a 0.5Vss reading.

The AC measurement is done by a highpass filter. the schematic should be in the manual/spec. And a signal with DC and/or f <= F_highpass will be affected.

Sorry. I avoid opening docx    ( a screenshot saved as png is prefered) 

More details about what you want to measure (and how)  would be helpfull.

If you don't want to give these details in an open forum, you could contact your local NI engineer.

[John_P1 · Accepted solution]

Hi Shazlan,

Measuring a 1 kHz square wave on an AC coupled device is not going to work out so well since you will lose your lower frequency components.  From your screenshots, it also looks like the sine wave you are generating is 1 kHz so it will be severely attenuated by the AC coupling.  The below graph from the 5620 Specifications illustrates what is meant by AC coupling in the frequency domain:

With the above specifications in mind, one can understand why you are seeing the results that you are.  If you were to generate higher frequency signals more in line with the specs of the 5620 then you should receive better results.  Unfortunately it sounds like you might have the wrong digitizer for what you want to do but it is hard to say for sure without more information about your application.  I hope this helps,

-John