60 HZ Hum on Analog Input PXI-6229

[mdricken]

Ther is a 600mV common mode noise hum being introduced somewhere on the circuit below. It is causing the PXI-6229 to read inaacurate measurements. Is there a way to configure the device to reject common mode noise? Or any other suggestions?

[johnsold]

What is the power source for the LM324? How is it grounded? Any possibility of ground loops involving the PXI chassis, the computer, and anything else atttached?

Is the 1N4002 in an epoxy package or a glass package? I have seen glass packaged diodes act like photodiodes under fluorescent lights, although the signal would be 120 Hz rather than 60 Hz.

Any possibility of inductive pick up?

You should make a serious effort to detemine how the interference is getting in before trying to figure out how to get rid of it.

Lynn

[mdricken]

I think the 60Hz is being introduced inductively. The diode and the resistor are inside of a metal box. When 115VAC 60Hz is applied to the metal box (to power other items inside the box) the noise increases from 100mVpp to 600mVpp. 

The opamp is powered by a seperate bipolar (+15/-15) linear power supply running off 115VAC, so there is a possiblity of a ground loop I suppose.

However the PXI-6299 quotes a 100dB CMRR for DC to 60Hz signals on the analog input. So shouldn't the 60Hz be rejected? I can see the 60Hz sine wave on the MAX test panel, so it is not rejecting the common mode 60Hz.

[John_P1]

For CMRR to apply the noise would have to be common to both leads on the analog input.

Best Regards,

[AnalogKid2DigitalMan]

Also try using a differential input configuration instead of single ended to improve noise rejection.

Minimal length wiring runs along with shieled twisted pairs would be of benefit.

-AK2DM

[johnsold]

Is the metal box connected to both the power system (safety) ground and the signal ground? This could be the source of the ground loop. Are the computer, PXI chasis, and the metal box power all connected to the same branch circuit and ground?

Try drawing a diagram showing every ground connection in the system.  Do not just use a ground symbol but actaully make it a wiring diagram which shows where every wire, chassis, box, conduit, or anything else grounded is connected. This can sometimes be useful in discovering ground loops.  I found one which included a wire attached to the mounting bolt for a toroidal transformer. It resulted in several amperes flowing through the housing of a very sensitive magnetic instrument!

As has been mentioned CMRR only applies to differential inputs. The circuit you posted is single ended so I think it is unlikely that a differential connection would help.  

Lynn