Strong oscillations on SC-2311 with 5B40

[fraffan]

I have an SC-2311 with a 5B40-2 module. I have a shielded thermocouple cable connected directly to the +- screw terminal and the shield is connected to the analog ground on the board. When I take measurements on the open thermocouple, I get a flat signal as expected, but once the thermocouple cable moves around for repositioning, for example, I get a strange oscillatory behavior (sometimes a square wave, sometimes a sine wave) in my signal. This oscillation is definitely triggered by cable motion, but sometimes it becomes permanent so immobilizing the cable won't help at all. As you can expect, this osccilatory behavior is also present when the thermocouple cable is connected to an actual thermocouple. Does anybody know what may be causing this behavior on the signal? I think it may have something to do with the +- screw terminals themselves since the problem persists among 3 channels on the board and on 2 different 5B40 modules (and with 3 different thermocouple cables).

[Henrik_Volkers]

> I get a strange oscillatory behavior (sometimes a square wave, sometimes a sine wave) in my signal.

Could that sine be hum (50/60Hz) ?

And the square wave just a totaly clipped sine hum?

[ColdenR]

Fraffan,

How strong is the signal (and as Henrik asked, approximately what frequency is it)? You might try looking at the Field Wiring and Noise Considerations for Analog Signals guide. Specifically, the "Minimizing Noise Coupling in the Interconnects" and "Solving Noise Problems in Measurement Setups" sections might be useful.

Also, especially since you are only seeing the noise when the cable moves around, make sure that none of the connections are loose in either of the communicating boards.

[fraffan]

It's definitely 60 Hz. I will read about field wiring to see if that takes care of it. The amplitude of the oscillations is on the order of 3-4 V after going through the module (as displayed by a VI that takes the output of the module and shows it scope style), so my bet is that the original noise is on the order of 30-40 millivolts, then amplified by the 5B40. Thanks for your replies and I will continue to troubleshoot this and post new messages with my findings.

[johnsold]

Power line noise of 30-40 mV is not uncommon.  If after reading the field wiring guide and following its recommendations you still have noise, try to determine if the coupling to your thermocouples is inductive or capacitive.  Properly connected shielding should eliminate most capacitive coupling.  If the coupling is inductive, ground loops may be the problem.

Lynn