Using a SCXI Scan my first few inputs values are off: ideas?

[DW]

I'm scanning 25 channels on a SCIX-1100. Setup for differential input, connected to my MIO board. When I run the scan, the first inputs should be 0, but I'm getting non-zero numbers.

They start high, and disappear in a few channels. If I change the sample period, the second and third values drop, but the first remains (but slightly reduced).

If I use the calibrate function, I can ground the PGIA for a delayed time, it helps but doesn't take the whole value away.

I found that if I scan the module twice (in the same scan operation), it seems to not have the problem.


Any ideas? Chassis problem? Software? Module?

[Bharatinder Sandhu]

Hi:

This might be occuring because your signals have high impedances. You can remedy that by using a op-amp in the voltage follower configuration before the signals go into the SCXI 1100. Please look at the following Knowledge Base for more information on using a voltage follower:

Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device

Regards,
Bharat Sandhu
Applications Engineer
National Instruments

[DW]

Yeah, I tried shorting the 1M resistors, but I still had the same problem.

If I include a second scan in the same scan list it seems to work. (scan the first few channels, then scan them all.)

Otherwise I'm stumped. I'd try the unity gain buffer but that would involve significant redesign.