How to eliminate noise while acquiring the data using the DMM

[drkjreddy]

Sir,

In my circuit I have used op amps (LF356) as Amplifier, subractor and buffer and LF398 as sample and hold amplifier. After processing the input signal the output is taken at the last stage of the LF356 amplifier. As the input signal is in millivolts it is amplified and process with a small circuitry. I have checked at each output of the IC using Oscilloscope and found no noise. I am using Keithley 2700 DMM to read the output of the circuit. The moment I connect the negative terminal/ground of the DMM, I found noise at every stage of the op amp ICs. If I remove the ground terminal of the dmm, the signal out put of each section seems to be OK.
Kindly let me know why it happening. Kindly suggest me the best solution to read the data using DMM.

With thanks and Regards
K.J.Reddy

 

[dick_freebird]

DMMs do kick back noise. I have to disconnect mine when
doing transistor noise probing.

If your sample rate is low I would say to use DPDT relays
to connect DMM in+/in- only after S/H has entered hold
mode.

You might approach it as a calibration problem if the DMM
noise contribution is fixed; use a quiet 'scope channel to
measure voltage without DMM and with DMM attached,
and compare to DMM result. A scale and/or offset cal
factor may be derivable. A "golden unit" can be essential
for long term setup repeatability.