Multifunction DAQ

cancel
Showing results for 
Search instead for 
Did you mean: 

DAQ 9209 Analog input non-zero offset

Solved!
Go to solution

Greetings everyone! 

I have a spot of trouble that I hope you can all help me out with. I am using a NI 9209 analog voltage input running 16 channels differential mode. All data cables are from the same twisted pair macrocable. The COM channel is connected the foil shield in the macro cable, which is again grounded on the physical measurement end of the cable. I run in high resolution mode using DAQmx to set up the virtual channel in Labview, with DAQmx sampling clock at continuous sample mode and a 1 Hz per channel frequency (conversion time per channel is 52 ms in high resolution mode = 832 ms total for 16 channels. But 1 Hz just looks better to my eyes...), and the input impedance is stated to be over 1 Gohm. 

 

Here's the issue then. I noticed some discrepancy with my data where channels that should be "identical" were not, and noticed that for most of the differential channels, the signal is as close to true 0 as you can reasonably expect when short circuited. (This is around a few uV from 0 over a long averaging period, comparable to the max resolution of the NI 9209). However, for a few of the channels, when averaging over thousands of measurements, they sit at around -20 uV. This is the case even while its neighbouring channel is ~0, so it does not appear to be a common mode voltage issue. I get that the NI 9209 has a noise floor of around these 20 uV, but I very much expected the noise floor to be random around 0 and not random around an off-set. I have tried 'varying' the common mode source to be a direct ground connection as well, to no effect. 

 

My question now is:

  1. Can I expect this off-set to be constant with time, and therefore 'calibrate' my data aquisition in Labview to counteract the offset, thus getting my accurate measurement? 
  2. Is this something that should be expected - after all, I am very close to true 0 and it currently appears to be a stable offset - or is there something wrong with my DAQ device? 
  3. Can there be an issue with the grounding? I would expect the offset to appear on all channels if this were the case, but it only appears on certain channels. 

 

All help and suggestions you might be able to give me are much appreciated! 

0 Kudos
Message 1 of 5
(184 Views)
Solution
Accepted by topic author Thraundil

Hi Thraundil,

 

The NI 9209 specs a typical offset error of 0.003% of range (10.4V). The offsets you're seeing are within that spec.

 

To narrow down if it follows the DAQ channel or the cable, you can:

1. Short the input of the channel with offset to COM at the front of the module. What does it read? If the offset is there it is the NI 9209.

2. If you move the cable that was on that channel to another channel that had no offset, does the offset appear?

 

If the offset follows the channel of the 9209 you can offset null the channels.

Alex
Hardware Engineer
Message 2 of 5
(153 Views)

Hi Algarcia,

 

That makes sense! Just as I thought I had looked at everything in the specs - always another thing to look out for. 

1. When you say short the channel with offset to COM, do you mean the COM channel on the NI 9209 itself? I tried doing this - over a 100 kohm resistor as shown in the getting started guide for floating differential measurement (well, the guide says 1 Mohm but we only had the smaller one at hand). The offset persisted. 

2. Moving the cable, as you suggested, to a 2nd channel, and the offset is not there. So the offset seem to be channel specific. Channel 0 has the offset, channel 1 does not. 

 

Does that mean I can offset null the specific channels with offset and trust that the offset will be steady for the lifetime? Just want to make sure I am not missing anything before I lock in and deploy the system. Thank you for your reply! 

0 Kudos
Message 3 of 5
(138 Views)

When I mentioned shorting the channel, I meant connecting AI+ and AI- to COM directly on the 9209 for CH0. This would show the offset error inherent to the channel.

algarcia_0-1651773110582.png


@Thraundil wrote:

Does that mean I can offset null the specific channels with offset and trust that the offset will be steady for the lifetime? Just want to make sure I am not missing anything before I lock in and deploy the system. Thank you for your reply! 


No, that would not necessarily be constant over the lifetime. Offset errors drift with temperature changes and are also only valid during the calibration interval of the device. Offset errors are only a portion of the total measurement uncertainty.

 

Alex
Hardware Engineer
0 Kudos
Message 4 of 5
(117 Views)

Another point is thermoelectricity if you dig into µV 🙂 

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


0 Kudos
Message 5 of 5
(72 Views)