Why is the A/D channel of my UMI-7774 noisy?

I am using a UMI-7774 mated to a PXI-7350 motion controller in a PXI-1002 chassis. I am testing A/D Read to the UMI with a short jumper between AI signal and AI ground pins on the Analog Input port. The result of A/D Read of this short-circuit is a DC bias offset of approximately 50 and spikes of 50-200 (see attached). I would like to know:
1) What is the unit of the A/D Read? For a channel I specified as +5V/-5V, I get readings of +30000/-30000 for inputs of +4V/-4V.
2) What is the source of DC bias and noise in the channel, and what are some techniques for compensation? Remember, I have jumped out this channel just to look at the noise in the UMI/controller. The only controls I see on this channel are en
able/disable and a choice of fixed ranges.
I want to use the A/D Read function to read a conditioned LVDT signal, then use the value to command a servomotor, but now the signal is too noisy. I would like to avoid extensive averaging, as this is a real-time application (and the spikes have too great an effect even on the average). The LVDT signal after conditioning (3rd-party hardware) appears clean in comparison.
Thanks.
Laine
Attachments:
A2D_Loopback.JPG ‏69 KB

Hello!
What is the unit of the A/D Read? For a channel I specified as +5V/-5V, I get readings of +30000/-30000 for inputs of +4V/-4V.
The A/D outputs the 'bit level of the A/D converter,' or what we would call raw data. The A/D is a 16 bit converter, so it has a range of 32767 to -32768. Depending on what voltage range you have selected, these values correspond to different voltages. For example => +5V corresponds to 32767, and -5V corresponds to -32768.
What is the source of DC bias and noise in the channel, and what are some techniques for compensation? Remember, I have jumped out this channel just to look at the noise in the UMI/controller. The only controls I see on this channel are enable/disable and a choice of fixed ranges.
The mot
ion controller A/Ds are not specifically designed for Data Acquisition, so they can have some behavior like this. The source of the noise is likely just system noise, etc., as the ground plane is shared among other signals. The best that you will be able to do is to do some DC offset calibration in software, and to over-sample and average your readings to reduce the noise.
I hope this helps!
Regards,
Colin C.
Applications Engineering
Colin Christofferson
Community Web Marketing
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