Column: Design problem solvers
Increasing the CMRR of differential amplifiers
By Thomas Brand,
Field Applications Engineer, Analog Devices
D
ifferential amplifier circuits (Figure 1) are required by many applications; for example, in systems requiring an extremely high level of measurement accuracy. To achieve this accuracy, it’s important to minimise any error sources, such as offset and gain errors, as
well as noise, tolerances and drift. For this purpose, high- precision operational amplifiers are used. Also important are the external components of the
amplifier circuit, especially the resistors, which should have matching ratios rather than be arbitrarily chosen. Ideally, the resistors in differential amplifier circuits
should be selected to have identical ratios (R2 /R1 = R4 /R3 ),
anything else may lead to undesired common-mode error. The ability of a differential amplifier to reject this common- mode error is given in terms of the common-mode rejection ratio (CMRR), which indicates how the output voltage changes with matching input voltages (common-mode voltages). In the best case, the output voltage should not change because it only depends on the difference between the two input voltages (max CMRR). However, this differs in practice.
CMRR CMRR is an important characteristic of differential amplifier circuits and is usually given in dB. For differential amplifier circuits such as the one shown in Figure 1, the CMRR is determined by the amplifier itself, as well as the externally- connected resistors. The resistor-dependent CMRR is denoted by “R” in the remainder of this article and is calculated using the following equation:
(1) For example, a desired gain of G = 1 and using resistors
with a tolerance of 1% matched to 2% in the amplifier circuit yields a CMRR of:
or in dB: (2) At 34dB, the CMRRR is relatively low. In this case, even if
the amplifier has a very good CMRR, high accuracy can’t be achieved since the chain is only as strong as its weakest link.
Precisely measured Accordingly, for precise measuring circuits, the selected resistors must also be extremely precise.
10 September/October 2020
www.electronicsworld.com
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