When you must measure a differential signal, perhaps from a thermocouple or bridge, the two signal wires will also pick up some unwanted signals. In a lab environment, those signals may appear negligible. But in real-world measurement systems you often must measure a small signal in the presence of large unwanted signals added through magnetic-, capacitive- and electromagnetic-coupling effects, as well as ground-potential differences. These unwanted common-mode signals appear on both differential-signal wires (see figure, below).
A high-quality instrumentation amplifier (in-amp) will reject common-mode signals to the extent specified by its common-mode rejection (CMR) or common-mode rejection ratio (CMRR) (see note). The unit-less CMRR equals the ratio between the amplitude of the common-mode input voltage and the in-ampís output voltage for that input alone. And, CMR = 20*log(CMRR), expressed as decibels. So an Analog Devicesí AD8295 in-amp, for example, with a CMR of 90 dB has a CMRR of 31,620 to 1. Thus a 1-V common-mode input signal would add about 31 ĶV to the in-ampís output.
An in-ampís CMR decreases as the frequency of the common-mode signal increases. Although the data sheet for the AD8295 lists a 90 dB minimum CMR, the data sheet also includes a plot of CMR that shows it drops from 90 dB at 0.1 Hz to about 80 dB at 10 kHz, for an in-amp gain setting of one. So as the frequency of the common-mode signal increases, the in-amp cannot attenuate it as much. Increasing the gain of the in-amp also increases its CMR. This frequency versus CMR relationship applies to all in-amps.
You can eliminate some common-mode ďnoiseĒ through proper routing and shielding of sensor signals. Keep cables away from electrical fields and electromagnetic sources such as motors and transformers. I prefer shielded twisted-pair cable, with the shield grounded at the measuring equipment.
Because CMR decreases with frequency, radio-frequency interference (RFI) can get into an in-amp where its components rectify the RF signal and add a dc offset to the in-ampís output. (I once saw electronic equipment go haywire every weekday between 7:30 and 8 p.m. due to a distant RF source.) You can use a low-pass filter in front of an in-ampís inputs to remove RFI. But the filter must provide matched performance for each in-amp input so as to not disturb the amplification of the differential signal. A carefully matched common-mode RF choke could also help.
Note: People often carelessly interchange the abbreviations CMR and CMRR. Pay attention to the unit-less CMRR and the dB units for CMR.
1. "Reducing RFI Rectification Errors in In-Amp Circuits," AN-671. Analog Devices.
2. "Understanding Common-Mode Signals," AN-2045. Maxim Integrated Products.