The Common-Mode Rejection Ratio (CMRR) indicates the ability of a differential amplifier to suppress signals common to the two inputs. Desired signals should appear on only one input or with opposite polarities on both inputs. These desired signals are amplified and appear on the outputs. Unwanted signals (noise) appearing with the same polarity on both input lines are ideally cancelled by the differential amplifier as these amplifiers are used as a means of suppressing common-mode signals. Such noise signals can arise from the following sources: (1) radiated signals coupled equally to both lines, (2) offset from signal common created in the driver circuit, or (3) ground differential between the transmitting and receiving locations.
The measure of an amplifier’s ability to reject noise is the CMRR. The ideal differential amplifier provides a very high gain for desired signals (single-ended or differential) and zero gain for common-mode signals.
Real differential amplifiers used in practice exhibit a very small common-mode gain (<<1), while providing a high differential voltage gain (usually several thousands). The higher the differential gain compared to the common-mode gain, the better the performance of the differential amplifier in terms of rejecting common-mode signals. A good measure of the diff-amp’s performance in rejecting undesirable common-mode signals is the ratio of the differential voltage gain (Av(d)) to the common-mode gain (Acm). This ratio is the CMRR.
A very high value of CMRR means that the differential gain Av(d) is high and the common-mode gain Acm is low. Thus the higher the CMRR, the better. A well-designed differential amplifier typically has a high differential gain and low common mode gain, resulting in a high CMRR. The CMRR is often expressed in decibels (dB) as
A CMRR of 10,000 (80dB) means that if the amplitudes of the differential input signal and the common-mode noise are equal, the desired signal will appear on the output 10,000 times greater in amplitude than the noise. With very high CMRR, noise or interference will be essentially eliminated.
Measuring CMRR
Several methods are used to measure the common-mode rejection ratio. The one shown in figure 2 makes use of four precision resistors to configure the operational amplifier as a differential amplifier. A signal is applied to both inputs, and the output is measured. To have no change in the output, the amplifier would need to have an infinite CMRR.
The key disadvantage in this circuit is that the resistors must match within 1ppm to measure higher CMRRs (greater than 100dB). For example, a mismatch of 0.1% between resistor pairs will result in a CMRR of only 66 dB, regardless of the actual performance of the amplifier. Other methods of measuring the CMRR do not require accurately matched resistors but involve more complex circuits. In this circuit in Figure 3, switching the power supply voltages changes the common-mode voltage.
Supply and common-mode ranges can be accommodated by changing voltages as required. In the circuit above, the power supply values shown in the circuit are for a ±15V DUT op amp, with a common-mode voltage range of ±10V.
Why should it be high?
The CMRR is one of the most vital specifications of an operational amplifier. Nowadays, electronic designs are moving toward higher bandwidths in which design issues related to noise and electromagnetic interference (EMI) are far more critical. A differential mode of operation at the input side enables the op-amp to reject various frequency components constituting common-mode input (CMI) and, thus, suppress unwanted noise and EMI. That shows why a high CMRR is critical in empowering an op-amp to attenuate any CMI elements. Ideally, an op-amp should have an infinite CMRR. However, in practice, it is not achievable. This is why op-amps should be designed to have CMRR as high as possible. The higher the CMRR, the better the op-amp’s ability to reject unwanted noise and EMI.
