Process control and data acquisition systems often make differential measurements so that they can isolate the desired differential signal from unwanted common-mode signals. For example, when measuring a strain gauge in a factory environment, the signal of interest is the differential voltage at the output of the strain gauge, not the 50/60Hz common-mode signal that is picked up by the wires connecting the sensor to the instrument.
Most differential signals are measured using an instrumentation amplifier or difference amplifier. This article will discuss how these circuits achieve high common-mode rejection, CMR. More importantly, however, it will discuss how to quickly calculate the level of performance one can expect from off the shelf components.
Figure 1. Electromagnetic interference and ground potentials can be rejected because they are common to both inputs. The differential signal from the sensor is amplified.
What is CMR and Why it is Important
Common mode voltages are the signals that are the same at both inputs. Often, these signals are unwanted because it is the differential signal that is of interest. Sensors that output differential signals include load cells, strain gauges and pressure sensors.
Measuring current across a shunt resistor also requires a differential measurement. Figure 1 shows a schematic of an instrument measuring a Wheatstone Bridge. The wires are far from the sensor, and they pick up electromagnetically induced, 50/60Hz signals from the power mains.
Since it is the sensor that is of interest and not the 50/60Hz signal, the data acquisition system will measure the difference across the sensor and reject the 50/60Hz signal that manifests itself at both inputs.
Figure 2 Difference Amplifier (left) and a three op-amp instrumentation amplifier (right).
Difference Amplifiers and Three Op Amp In-Amps share a similar topology
Either a difference amplifier or instrumentation amplifier can condition differential signals. Figure 2 illustrates the differences. Difference amplifiers are used when the input signal is larger than the supply voltage of the op amp. When higher impedance is needed, an instrumentation amplifier is selected, because it has buffered inputs which offer high input impedance, typically in the GigaOhm range. A classic three op amp instrumentation amplifier incorporates a difference amplifier. This is relevant, because the same equations are used to calculate CMR for both types of amplifiers.