PCI-6024E AI 0 is measuring negative values when connected to a SCB-68A chassis ground lug.

[robojeff]

I am having a problem with am LabVIEW  test application where I am getting negative values measured from a
PCI-6024E and I decided to isolate everything by only connecting the PCI-6024E that is inside my PC
to a NI SCB-68A breakout box.

I then connected a jumper wire from the SCB-68A chassis ground lug to pin 68 (AI 0) as this is the line that
I am measuring with my application and hardware test fixture and all measured values are Negative which
is why my application is failing.

With the AI0 line being grounded, shouldn't the value being measured from the NI MAX test panel measure zero volts?

Here is what I am seeing.

Any suggestions on how to correct this issue are appreciated.

[AnalogKid2DigitalMan]

20V span with 12 bit resolution, 1 A/D count is 4.88mV. So you are within 1-2 counts of accuracy at presumably 0V.

This can be expected.

-AK2DM

[robojeff]

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@AnalogKid2DigitalMan wrote:

20V span with 12 bit resolution, 1 A/D count is 4.88mV. So you are within 1-2 counts of accuracy at presumably 0V.

This can be expected.

-AK2DM

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Thanks for the reply...

Is there a way to set this up so that the 1-2 counts of accuracy doesn't dip below zero?

[robojeff · Accepted solution]

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@robojeff wrote:

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@AnalogKid2DigitalMan wrote:

20V span with 12 bit resolution, 1 A/D count is 4.88mV. So you are within 1-2 counts of accuracy at presumably 0V.

This can be expected.

-AK2DM

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The answer is a 0.01 ufd capacitor between the AI0 signal and ground helps a lot... 

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