CMRR improvement - Instrumentational amplifier

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• Satyawati Patil   -Posted on 09 Oct 15
  The combination of operational amplifiers and the configured stable and accurate resistive feedback network,which set the precise value of desired gain by using a single external resistance, is known as 'Instrumentation Amplifier'.
  
  It is essential to improve CMRR of instrumentation amplifier so as to yield better signal to noise ratio. Improvement of CMRR in three op-amp instrumentation amplifier involves two techniques such as the introduction of guard, reference and sense terminals & the bootstrapped power supply.
  
  In the method which comprises guards,sense and reference terminals, the buffered common-mode signal is used as a guard voltage in order to minimize the effects of cable capacitance and leakage.
  
  Besides these, the sense and reference terminals allow the sensing of output voltage at the load so that the feedback operates in such a manner that the losses in wiring or external circuit are eliminated. The reference terminal also allows to offset an output signal by means of DC level and hence the impedance from reference terminal to ground should be kept small to avoid CMRR degradation.
  
  Bootstrapping method also results in the elimination of input common-mode signal due to absence of any swing (of common-mode signals) at their inputs with respect to the power supplies. This technique is quite beneficial for CMRR at DC level.
  

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