DC offset at beginning

[nirvtek]

Hi

I'm capturing input from a signal generator on a USB-4431 @48 kS (The signal has a lower frequency) and a loopback from the 4431's ouput (which is also generating a sinusoid).

For some reason on both channels the amplitude are higher during the first 650 samples before levelling off to a normal level.

I can't determine if this is an issue with my code or something more sinister.

[johnsold]

Are you using AC coupling? If so, you may be charging the input coupling capacitor.

Lynn

[nirvtek]

I also have an issue with another channel where the sinusoid from a signal generator has a higher amplitude for during the initial ~600 samples. The DC offset on the other channel is also the same.

The upper part of the sine wave is pictured below (note: the sinusoid isn't shifted, the entire upper and lower sections during the initial period has a higher amplitude:

[johnsold]

If the simusoid is not shifted, then the issue is not DC offset, it is a gain or attenuation change.

Do you have an oscilloscope or other way of measuring to see itf this is a real change in the signal or some kind of measurement artifact?

Can you show the entire cycle before and after the transistion at time 600? I would like to see exactly when the transistion occurs.  Best would be if you could capture the data on a graph or array indicator and save it as default. Then post the VI.

Is this reproducible?  Is the signal generator running continuously or being triggered? What is the manufacturer and model of the generator? Is there any other circuitry connected to the generator or USB-4431?  Is there anythng else happening during that 600 ms?

Lynn

[nirvtek]

Actually the weird amplification during the beginning also occured with a signal generated from the USB-4431's output.

Here's a zoomed out view, the DC offset on an empty output channel ais also visible in black:

This isn't from a VI, but from a Win32 application using the Nidaqmx C API.

[johnsold]

What happens if you run your program with the signal generator disconnected from the USB-4431? Short all the inputs or terminate them with 50 ohm terminators.

Does your generator have the same chassis ground as the USB-4431?

Lynn

[nirvtek]

Thanks for your advice. I'll check out the generator next week.

THe issue with the amplitude also occurred with just a loopback from the output to to an input. Adjusting buffer sizes eliminated the problem, depending on the frequency.

[johnsold]

The vertical scale is cut off in your images. What is the amplitude of the signals?

I would look very carefully at everything which is happening in your program and with the equipment for about 10-20 samples before and after that transisition.  Is some big DC relay or a heater turning on about that time? 

Lynn

[nirvtek]

The black line going through the middle from ~600 samples is level 0. It's also the channel which has a DC offset for the same number of samples.

[nirvtek]

Might be a bug in my capture code.