analog output noise PCI 6722

[JW88]

I am using a PCI 6722 for analog output. The signals is sent to a high voltage amplifier to drive 6 piezoelectric devices. I am seeing noise on the waveform output, which should be displayed in the plot I am attaching. The plot shows the waveform I am attempting to create on 6 of the 8 AO channels.  The waveforms of the six channels are identical, except that they are delayed in time so that the step-up voltage occurs for each channel at a different time. As shown in the plot, sometimes the waveform is accompanied by a 100mV noise signal, with 250 microsecond periodicity. The plot is of the AO CH2, which fires after CH0 and CH1. The noise starts at the exact time the CH0 waveform starts, so there might be a cross talk issue.  It is curious because I run this waveform repeatedly, however this additional noise signal is only present about half the time, as can be seen in the plot where 3 waveforms are shown, with only the middle one displaying the noise signal.

any thoughts?

[dousley]

Hi JW,

Nothing obvious appears to me at the moment, but I have some thoughts.

1) It is an important observation that the noise in your graph begins at the same time that Ch0 has a rising edge type event. Here are some questions related to that:

a) You are using only six channels right? At what rate are  you updating?

b) If you generate on fewer channels do you still have the same phenomenon? Is the period of the noise the same? What about if you only generate on a single channel.

c) Do you see the noise simultaneously appear on all channels? Or does it show up on each channel at different times?

2) Could you give a little description of the grounding you have downstream from the AO.

a) What does the output noise look like when you disconnect from the high voltage amplifier?

Anyway, those are a few thoughts.

-Dousley 

[JW88]

Hi Dousley, thanks for the response. to address your questions

1a. yes, I am using only six of the available 8 AO ports. I create the waveform using a LabView code that utilizes the "DAQmx Timing (sample clock) vi" and I set the rate for this VI usually between 5kHz and 50kHz.  I made the observation that this noise signal is dependent on this rate that I set. Is there possibly some clock pulse signal that is getting into my signal?  If so, is there a better VI to use?

b. I applied to 1 and 2 channels and the signal does appear to be there

c. It appears that the noise is correlated among channels. Referring to the image in my initial message, there are some iterations of the waveform where no noise signal is present. It appears that when observing multiple channels, the noise signal is either present or absent on both channels during the same iteration.

2. The testing I have done recently was with the HV amplifier disconnected. 

[dousley]

--> "I made the observation that this noise signal is dependent on this rate that I set. Is there possibly some clock pulse signal that is getting into my signal?" 

That's a good observation. It certainly sounds like the sample clock is getting into the output signal somehow. When you run at 5kHz, what is the period of the noise pulse? Is it also 5kHz? or is it slower/faster?

It's very curious that it only appears occasionally. I wonder if it is systematic... and periodic. Does it occur in regular intervals, like every 10 cycles or something? Does it occur in regular time intervals, like every 3ms?

As to your question of a different VI, well, I'm not sure maybe someone else has some good suggestions about that. However, as a basic sanity check, I would try to go into the Measurement and Automation eXplorer (MAX), go to the 6722 device in the device list, and open up a test panel. From there I would try to generate a few sine waves on one of the channels to see if you still see the behavior. If you still see the noise behavior there, then I think you can confirm that software is not the cause of the behavior.

--> "b. I applied to 1 and 2 channels and the signal does appear to be there"

So when you reduce the number of active channels all the way down to one, so there is only one active channel and all the other channels are idle,... even then you still see the noise appear? Does the period of the noise pulse change depending on the number of channels? How about the duty cycle? The reason I ask about this is because it may be traceable to a mux that switches the DAC between channels. If you increase the number of channels the mux switches then it would need to switch faster. So anyways, if there is noise related to a mux, I would expect it to change in some way when you change the number of active channels.

Dousley 

Conditioned Measurements Hardware