Instrumentation amplifier, referred to as instrumentation, English name is Instrument Amplifier, usually used for high-precision low-frequency signal detection, such as temperature, pressure and other bridge differential measurement, current sampling, bioelectric and other weak differential signal amplification. The common characteristics of these signals are: differential signal, small amplitude, high source impedance, and large common-mode voltage variation. Amplifying these signals usually requires high DC accuracy. Offset voltage and offset current are usually the parameters we are concerned with. However, there is a very important parameter. CMRR and common mode rejection ratio also have an important impact on the accuracy of the instrumentation amplifier.
The common-mode rejection ratio, which describes the change in output voltage due to changes in the amplifier's common-mode voltage, is usually described using the dB value. For example, a common-mode rejection ratio of 80dB means that the common-mode voltage changes by 1V, and the input offset voltage changes by 0.1mV. If the amplification is 1000 times, the corresponding output offset voltage will change by 100mV.
Where Vout is the output offset voltage; Vcm is the input common-mode voltage; that is, the difference between the input-to-ground common-mode voltage and Vref; Gain is the gain of the instrumentation amplifier. From a formula perspective, the output offset voltage change caused by the common-mode voltage of the instrumentation amplifier is related to the gain. In fact, the CMRR specifications of the instrumentation amplifier are different at different gains. For example, TI's typical high-precision zero-offset instrumentation amplifier, the INA188, has a minimum CMRR of 84dB at a gain of 1, and a minimum of 118dB at a gain of 1000, typically 130dB. So repeat the above example, using INA188 to amplify 1000 times, 1V common mode voltage change, typically the output offset voltage change is only 0.3mV, which can meet the application requirements of most applications.
The figure above shows a typical bridge measurement scheme, including a pre-stage differential instrumentation amplifier, a level shifting, limiting buffer, and an ADC that provides high-precision small-signal sampling. Typical instrumentation amplifiers can refer to INA188, INA826, INA333, covering all applications from high voltage to low power consumption, and the minimum value of CMRR at a magnification of 10 is greater than 100dB. OPA196 is TI's new generation of E-trim operational amplifiers, the DC offset voltage is below 100uV, and the OPA191 even reaches 5uV. Such high CMRR and low offset voltage improve system accuracy while reducing system design difficulty and cost, and enhancing product competitiveness.
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