6255 sampling rate causes the dc offset

I see a dc offset in the measures of analog input I select different sampling frequencies.

I have USB-6255 (mass termination) multifunction data acquisition and I use measurement and Automation Explorer to put in place my entries.

My raw analog input is-0, 6250 volts dc, I have set up a task that uses 4 differential channels with no custom scale.

I have defined the scope of the input signal to +/-0 .8v for you sure I get good resolution.

Acquisition mode is continuous, samples of read is 1 k and I play with the order of 10kS/s rates 50kS/s.

While this task runs in the MAX, I can put my cursor in the rate field and use the top and down arrow keys to change the sampling frequency. As I do, I can see the light changes reported as much as 150 MV rate from one to the other.

It is a significant change when the total time of entry is lower to +/-1v.

The direction of movement is independent of the increase or decrease of the sampling frequency.

For example,.

23kS/s, the declared value is - 0.540v,

24kS/s, she moves to-0.620v.

25kS/s, she moves to-0.690v.

26kS/s, she moves back to-0665v.

27kS/s, she moves back to-0.625v.

and 28kS/s, she moves to - 0.535v.

At first, I thought that the sampling change made a change of the input impedance and change the load on my source, however, all the time, my dc signal source remained at the - 0.625v (as measured with a multimeter fluke at the connection point to data acquisition).

Why this is happening and what can I do about it? I want to give my users the ability to choose their desired sampling frequency.

My guess is that I need to add an amplifier to fixed gain with a gain of 5 to 10 to make the input signal to use the maximum of the analog input level (+ / 10v).

I use MAX version 5.0.0f1

Thanks for any help,

Tobin

Hi Tobin,

What do you use to generate the signal-. 625 volt? If you are using a switching power supply, you can experience aliasing where the power supply is turned on and stop.

In addition, are see you the same tensions at the same sampling rate? See you always - .540v to 23kS/s or vary over time?

Finally, you have a second 6255 you can try to replicate this on? It could be that the unit is defective.

N

Tags: NI Hardware

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