Instrumentation Amplifiers: CMRR, You Stole My Precision

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Instrumentation Amplifiers: CMRR, You Stole My Precision [Copy link]

Instrument amplifiers, or instrument amplifiers for short, are usually used for high-precision low-frequency signal detection, such as temperature, pressure, and other bridge differential measurements, current sampling, and amplification of weak differential signals such as bioelectricity. These signals have common characteristics: differential signals, small amplitudes, high source impedance, and large common-mode voltage changes. Amplifying these signals usually requires high DC accuracy, and offset voltage and offset current are usually the parameters we pay attention to. However, there is another very important parameter, CMRR, common-mode rejection ratio, which will also have an important impact on the accuracy of the instrument amplifier.

The common mode rejection ratio describes the change in output voltage caused by the change in the common mode voltage of the amplifier, and is usually described in dB. For example, a common mode rejection ratio of 80dB means that if the common mode voltage changes by 1V, the input offset voltage changes by 0.1mV. If it is amplified by 1000 times, the corresponding output offset voltage will change by 100mV.

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Where Vout is the output offset voltage; Vcm is the input common-mode voltage, that is, the difference between the common-mode voltage of the input terminal to ground and Vref; Gain is the gain of the instrument amplifier. From the formula, the change in output offset voltage caused by the common-mode voltage of the instrument amplifier is related to the gain. In fact, the CMRR index of the instrument amplifier is different at different gains. For example, TI's typical high-precision zero-offset instrument amplifier INA188 has a minimum CMRR of 84dB when the gain is 1, and a minimum CMRR of 118dB and a typical value of 130dB when the gain is 1000. So repeat the above example, use INA188 to amplify 1000 times, and the common-mode voltage changes by 1V. Under typical circumstances, the output offset voltage changes by only 0.3mV, which can meet the application requirements of most applications.

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The figure above shows a typical bridge measurement solution, which includes a pre-stage differential instrumentation amplifier, a level shift limit buffer driver ADC and other structures to provide high-precision small signal sampling. Typical instrumentation amplifiers can refer to INA188 , INA826 , and INA333 , which can cover all occasions from high voltage to low power consumption, and the minimum CMRR is greater than 100dB when the amplification factor is 10. OPA196 is TI's new generation of E-trim operational amplifiers, and the DC offset voltage is below 100uV, and OPA191 even reaches 5uV. Such high CMRR and low offset voltage can improve system accuracy while reducing system design difficulty and cost, and enhance product competitiveness.