Unstable PXI/SCXI system

[Chris_hsl]

I’m using a SCXI 1100 with a SCXI 1300 module to read the output of 6 instruments and a test signal. These are used in conjunction with a PXI-6040E module and is housed in a PXI-1011 chassis. The majority of the instruments are 4-20ma and a 120ohm resistor is used across each 1300 input. The test signal is 0.3v taken from a battery-operated supply. When used with my CVI application, all signals seem to periodically dip to low value (0.2V) and then back again, although they are constant when read with a DVM. It doesn’t happen to all signals at the same time but often 2 consecutive channels will drop together. There are 3 other SCXI 1100/1300 modules in the system, with 32 voltage inputs connected to each. These seem to functioning correc
tly. Any ideas? Thanks

[BeckyB]

Hello Chris_hsl �

It seems like you are having incorrect software readings for these channels.

You may have a settling problem caused by either filters or high gains. First, check to see if you have enabled filtering. If you use a 4 Hz or 10 kHz filter on the SCXI-1100, you must scan channels at a slower rate. Because the filter circuitry is located after the amplifier, you must give the amplifier time to settle before switching to the next channel.

If you are not using the filter, check the amplifier gains you are using on the SCXI-1100 module and the DAQ board. Using a very high gain on your SCXI-1100 module and on some DAQ boards also has the effect of increasing your settling time. Decreasing your scan rate (increasing the delay between channels) should sol
ve this problem.

If these suggestions do not solve your problem, please reply to this post and I will look for a different solution to your dilemma. Have a great day!

Becky B.
Applications Engineer
National Instruments