Hello all, I am having a problem with excessive noise on one of the signals in my test system. The signal source is a third party device (interface box to a sensor) which produces a 0 - 1.0V signal proportional to the signal output. The signal is connected to a distribution box and then to my NI DAQ device via an SCB68 terminal block. I am using differential inputs. As far as I know all of the shielding is correct. I know that it would be ideal to tie the chassis ground to the signal ground at the source, but it is not feasible. The chassis of the signal source is not grounded, so there shouldn't be a current loop in the shielding. For a typical signal, I am getting about 20 mv (pk-pk) of higher frequency (>100 Hz) noise and >200 mV of lower frequency noise ~ 20 Hz. I have not been able to isolate the noise source or to reduce the noise levels any further. The lower frequency noise consists mostly of spikes not sinusoidal noise. Attached is a diagram of the wiring. I have the shield tied to a 10 megaohm resistor that is connected at the AI GND terminal. I know this is difference than what is recommend in the documentation, but I have read other resources that recommend this connection. If I remove the resistor, the noise doubles. Thanks for your time.

1. If the signal is grounded soemwhere, then you no longer have a differential signal. Although any difference in potential between the ground in the signal distribution box and the SCB68 may show up as though it is part of the signal connected to the - input. Either do not ground it or do not use the differential input configuration. 2. See 1. 3. OK. Floating one end of the shield often helps. I am concerned that the ground connection in the signal distribution box may be part of the problem. What was the reason for making that connection? Why there rather than at one end or the other, or no ground at all? What is the signal source? What kind of sensor? How many channels are you measuring? Lynn

Multifunction DAQ

Noisy signal and shielding techniques

Solved!

Great!

There must have been a ground loop through the 12 V power supply. They can be tricky to find.

Lynn

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Multifunction DAQ

Noisy signal and shielding techniques

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