PCI-6259 SCB-68 Analog Input Dying

[wpolk]

I believe my problem comes from the SCB-68 Connector Block.

I am using AO0 to send out an ultrasonic signal, and AI0 to read in an ultrasonic signal from a different source.

My problem is that every so often, the input signal just disappears. Putting a probe to look at the input signal shows all 0's for the input signal. Hooking an oscilloscope shows the system is still receiving a signal. When the signal is running, the software probe shows the DC offset of the signal start off at 0 and steadily rise as long as the system is running. Once the system died, just touching my finger to the line would make it work agian.

Hooking an oscilloscope to the system seems to make this probelm disappear overall, both the locking up and the rising DC offset. So the first solution I tried was to put a 5pf cap to ground, but that did not stop the system from dying. Then I tried a lowpass filter, it introduced what I believe to be a 0.3 V oscillation when the system does not have a signal in (I.E. reading in nothing). I then tried a resistor and capacitor in parallel to ground, trying to add an impedence load, but that produced the same ringing.

What should I do? I need it to be more stable! But that 0.3 V oscillation is unacceptable for the scope of what we're doing!

[AnalogKid2DigitalMan]

Have you tried removing the signal wires of the offending signal out of the SCB-68 and re-installing? I had an issue once where the wire was touching the top contact which also contacts the screw but not making a good connection to the bottom contact that feeds the signal into the DAQ device. Pulling/reseating and making sure the screw was tight solved the problem. You stated you fixed it by touching the wire- that is why I am suggesting this scenario.

Probing with a meter on the screws would show that the signal was always present (but poor connection to the bottom contact was preventing the signal from reaching the DAQ card). It drove me screwy for a few minutes