ICP Excitation control issues

[Chris_NZ]

We are having problems with ICP control after some initial tests on two NI-4472s trying to drive accelerometers have resulted the following behaviour:

- ICP does not appear to work irrespective of input configuration - differential, reference single ended etc (when ICP is enabled we ensure AC Coupling, when ICP disabled we ensure DC Coupling)
- However, when say chn 1 is configured for ICP, and chn 2 has its negative wire grounded then ICP functions. So there seems to be some input configuration setting issue to do with grounding.

Any idea what might be wrong or things we might check/try?

Note the following applies to NI-4472s:
The board actually has 'Unbalanced Differential Inputs' which is basically differential mode with
a bias resistor to ground on the negative input built-in. This means floating signals are generally preferred - which includes accelerometers as mentioned - and are already ground referenced internally. The board will also accept externally grounded sources but only if their ground potential is not more than 2.5V different to the 4472 ground (regardless of AC or DC coupling).

Thanks,
Chris

[Shea C]

Chris,

What exacly happens when you say that ICP doesn't appear to work? Are you enabling excitation in the scan list after configuring the task? What functions are you using to program the board? You should be using AC coupling with excitation enabled. Take a lookat the following KB and the related link at the bottom of this document.
How Do I Know the Integrated Circuit Piezoelectric Excitation For My 4472 is On?">


Figure 2-3 on Page 2-7 of the NI 4472 User Manual shows how to connect floating and grounded signals to the NI 4472 input terminals. The negative lead of each input channel of the NI 4472 board is connected to the chassis ground of the boar
d through a 50 ohms resistor. The chassis ground of the board is connected to the ground of the computer itself. If you have a grounded signal that is not referenced to the same ground than the NI 4472 board, you could create a ground loop that would introduce errors to your readings. I hope this resource helps.

Shea C
Applications
NI

[Chris_NZ]

Hi Shea
I am aware of the ref you provided thanks - tapping a connected ICP transducer is also an easy way to tell.

We are using traditional NI-DAQ & LV6.1 and are using a DSA excitation control VI downloaded from NI which is placed after AI Config.

We only did the grounding of the second channel as something to try, the unusual thing was that ICP worked when this occurred.

Any further comments appreciated. Also looking for insights on the "input config (no change:0)" settings on AI Config. Having ICP enabled with AC coupling and no change or ref. single ended has worked empirically on another system although those transducers may have been grounded...

Thanks,
Chris

[Shea C]

Chris,

The "no change" setting on the AI config is used as a selection for which the previous channel setting were the same. Therefore not taking time to configure the board for a setting that it is already configured for. The first element in this array should always use values other than "no change". The reason I suggested the previous document was to ensure that ICP is being enabled at the board regardless of what the accelerometer is returning. If the accelerometers are indeed floating the differential setting should be selected. Make sure that you have configured a setting in each element of the "Coupling and Input Config" array. The behavior that you are seeing seems to indicate that the correct setting for each channel has not been configu
red. If the problem persists after checking these things, please contact NI support at http://www.ni.com/support/

Shea C