PCI 6221: AI- Signal is drifting away as a consequence of another AI-signal

[Steveo123]

Hello all,

Operating System: Windows 7
NI-Software: LabVIEW 2010 SP1
Driver: DAQmx 9.1.7
NI-Hardware: NI PCI 6221 (68 PINs)
external Hardware: 2 Sensors, 1 Initialsensor, 1 Accelerationsensor

General Description:
There is a NI PCI 6221-Card at a teststand. 2 Analogue Voltagesignals are acquired with this 2 sensors. It is 1 initialsensor
(to measure the speed of rotation) and a accelerationsensor (to measure the occurent acceleration). With every rotation the Ini-
tialsensor is sending a short 5 V impulse to the card. As long as there is no acceleration available, the acc-sensor is sending
0 V to the card, as expected.
The initialsensor is differential connected ( e.g. AI0 with AI8). The accelerationsensor is connected with ground (e.g. AI1 with AI GND).
The signals are experimental measured and displayed in MAX.

Occuring Problem:
As soon as the initialsensor is sending 5 V to the the acqisitioncard, the signal of the acc-sensor is drifting above with a maximum of 1 V.
Is the inisensor on 0 V again the signal of the acc-sensor is going down again to 0 V. This behaviour repeats always.

The following attempts were already tried:
- change both sensors to the differential method -> signal behaviour is the same
- ini-sensor to ground (RSE) and acc-sensor to differential -> signal behaviour is the same
- both sensors to ground (RSE) -> signal behaviour is the same

Of course, the corresponding measuring method was also adapted in MAX.
A check with the DAQ Diagnostic Utility deliverd no problems or errors with the AI.

It seems like the PINS on the card are influencing themselves, regardless of which method is selected.

There is picture of the signal drift in the attachment. The longer the ini-sensor is sending 5 V the higher the acc-signal will go
until it reached 1 V at maximum.

By what is that drift caused? The card seems not to be defect.

Many thanks.

Best Regards.

Stefan Geiger

[GovBob]

Possibly a ghosting issue?

Hope this helps

[Steveo123]

hi again,

thank you very much for the links.

we tried already the following steps:

1. decreasing the sampling rate to minimum, which is much under our using requirements (1000 Samples/s) -> the same effect

2. increasing the settling time to maximum -> the same effect

3. rearranged the signals - first the acc-sensor then the initalsensor ->the same effect

4. created several dummy-channels to increase time between ini-sensor and acc-sensor ->the same effect

the only step which we have not already tried is implementing a voltage follower or buffer circuit. is it recommended to implement it? if yes, how to connect it to the device?

is there any tutorial?

As a information, there is no difference between the ground of the device and the ground of the case(so, no ground loop possibility).

by the way we are you the following transducer for the acc-sensor:

http://www.pcb.com/spec_sheet.asp?model=482A21&item_id=12217

thanks in advance.

best regards

Stefan Geiger