NI 9205 signals have sampling rate spikes on O-scope.

[Kanonfodder]

I've read some of the other NI 9205 questions and they had more of issues of noise.  I can see 5 KHz pulses on the DC signals being input reflecting my 5KHz sampling rate.  Is this a known issue, or is there a work around?  I'm using differential mode, but the ( - ) side are all tied to common ground in the system.

Thanks.

[Beutlich]

Hi Kanonfodder,

It sounds like the noise is due to the wiring setup you have implemented. Differential mode is going to expect two dedicated wires for each signal, not necessarily tied to ground. If you wire the second wire to ground, then the signal you will receive is going to be the difference between the signal and a floating node, which will understandably be noisy. The wiring you have currently sounds more similar to a Referential Single Ended (RSE), so you could also try changing the property in your task, although a differential configuration is less noisy than an RSE configuration. If you are interested in learning more about different wiring configurations with regards to noise, I recommend reviewing the Field Wiring and Noise Considerations for Analog Signals.

[John_P1]

The input channels on the 9205 are multiplexed into a single ADC.  In some cases this might cause unintended behavior (ghosting), but typically if enough settling time is allowed the multiplexing will be transparent to the user when measuring inputs with the 9205 alone (i.e. the multiplexer is switched, a certain amount of time is waited, then the sample is taken after the measurement has settled).

However, if measuing the same signal connected to the 9205 with an external scope, you will pick up small glitches whenever the multiplexer is switched.  The scope is likely sampling faster than the 9205 and it does not know to wait for the multiplexer to settle before taking a sample.

If you measure one channel at a time with the 9205 that should eliminate the glitches you are seeing on your oscilloscope.  If you want to measure multiple channels simultaneously without using a multiplexer you will need to use a module that has simultaneous sampling such as the 9215.

Best Regards,