PI PXI 4472 shows a constant bias in readings with AC coupling

[Sid11041979]

Hi I'm using a NI PXI 4472 card for acquiring (8 channels) input from voltage accelerometers. When using AC coupling (either in MAX or in my DAQ software) the accelerometer readings show a constant dc negative voltage bias (~3mV). This is not a time constant thing with the high pass filter characteristics of the AC coupling i.e. the offset persists always and doesn't go away after a few seconds.

Any suggestions as to why this could be happening?? Do i need to recalibrate my ADC converters on the card or are there any easier solutions/quick fixes.

Thanks

[EBL]

The 4472 uses A/D converters that are not particularly stable at DC. More than likely the offset has drifted with time and temperature and you're seeing the result (the spec indicates a max offset of 3 mV, coincidentally). Try running a self-calibration on the board; that ought to take care of the problem.

Usually a small amount of DC is not a problem when acquiring data with AC coupling. The purpose of the AC coupling is either to allow DC offsets which would otherwise exceed the input range or to suppress low frequency noise. I use AC coupling myself, for example, when I'm looking at power supply noise. The vast majority of the time, I'm using some sort of frequency-domain processing, such as a power spectrum, and DC is ignored altogether.

Offsets might cause trouble when looking at time-domain artifacts, particularly if you're counting on a residual DC level for triggering. In general, I would not expect DC offset performance better than a few mV on a dynamic signal acquisition card such as the 4472.

Hope this helps,
Ed L.