Common Mode Rejection Ratio (CMRR) and The Operational Amplifier

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Introduction

• The CMRR(Common Mode Rejection Ratio) is the
  most important specification and it indicates the
  how much of the common mode signals will present
  to measure. The value of the CMMR frequently
  depends on the signal frequency and the function
  should be specified. The function of the CMMR is
  specifically used to reduce the noise on the
  transmission lines.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
What is a CMRR

• The CMRR in an operational amplifier is a common
  mode rejection ratio.
• Generally, the op amp as two input terminals
  which are positive and negative terminals and the
  two inputs are applied at the same point.
• It will give the opposite polarity signals at
  the output.
• Hence the positive and the negative voltage of
  the terminals will cancel out and it will give
  the resultant output voltage. 
• The ideal op amp will have the infinite CMRR and
  with the finite differential gain and zero common
  mode gain.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
What is a CMRR

• The ideal op amp will have the infinite CMRR and
  with the finite differential gain and zero common
  mode gain.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Common-mode Rejection Ratio Formula

• The common mode rejection ratio is formed by the
  two inputs which will have the same sign of DC
  voltage. 
• If we assume one input voltage is 8v and the
  other 9v here
• The 8v is common and the input voltage should be
  calculated through the equation of V V- .
• Hence the result will be 1v but the common DC
  voltage between the two inputs has a non-zero
  gain.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Common-mode Rejection Ratio Formula

• The differential gain Ad magnifies the
  difference between the two input voltages.
• But the common mode gain Ac magnifies the
  common mode DC voltage between the two inputs. 
• The ratio of two gains is said to be as a
  common mode rejection ratio. 
• The value of the format is in dB. The formula
  of a common mode rejection ratio is calculated by
  the following equation.

• CMRR 20logAo/Ac dB

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Power Supply Rejection Ratio

• The power supply rejection ratio is defined as
  the changes in input offset voltage per unit
  changes in the DC supply voltage. 
• The power supply is also calculated in the
  format of dB.
• The mathematical equation of the power supply
  rejection ratio is given below.

• PSRR 20log?VDc/?Vio dB

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Common Mode Rejection Ration of Op Amp

• The common mode rejection ratio is
  a differential amplifier and the op amps are
  amplified in with the differential input. 
• Hence the CMMR ratio can be applied to the
  operational amplifier. 
• Using the condition of common mode rejection
  ratio, i.e. 
• When both the input of the amplifier has same
  voltage.
• Then the output of the amplifier should be zero
  or the amplifier should be rejecting the signal.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Common Mode Rejection Ration of Op Amp

• The following image shows the amplifier of MCP601
  of common mode rejection ratio.

Common Mode Rejection Ration of Op Amp
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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Offset Error of a CMRR of The Op-Amp

• The CMRR can build parallel out offset voltage in
  op amps configured in
• The non-inverting amplifier which is shown in the
  below figure. 

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Offset Error of a CMRR of The Op-Amp

• The non-inverting operating amplifier will have a
  small amount of CMRR error because both the
  inputs are connected to ground.
• There is no presence of CM dynamic voltage.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Offset Error of a CMRR of the Op-Amp

• Error (RTI) Vcm / CMRR Vin / CMRR
• Vout 1 R2/R1 Vin Vin/ CMRR
• Error (RTO) 1R2/R1 Vin/CMRR

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Measuring Common Mode Rejection Ratio

• There are different ways to measure the common
  mode rejection ratio.
• In the below figure we will discuss the four
  precision resistor to configure the op amp as a
  differential amplifier. 

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Measuring Common Mode Rejection Ratio

• A signal is applied to the both inputs.
• Changes in the output are measured and an
  amplifier with infinite CMRR also no changes in
  the output.
• The inherent difficulties of this circuit are
  that the ratio match of the resistors is
  important as the CMRR of the op amp. 

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Measuring Common Mode Rejection Ratio

• The 0.1 mismatch is between resistor pair and
  the result will be in CMR of 66 dB.
• Hence the most of the amplifiers will have a
  low frequency of CMR is between the 80dB to
  120Db.
• In this circuit, it is clear that there is only
  marginally useful for measuring the CMRR.

?Vout ?Vin / CMRR (1 R2/R1)
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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
CMRR without Using Precision Resistors

• The following circuit is more complicated by
  comparing with the above circuit and it can
  measure the CMRR by without using a precision
  resistor.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
CMRR without Using Precision Resistors

• By switching the power supply voltage the common
  mode rejection ratio is changed. 
• Practically, the circuit can be implemented
  easily  and by using the same circuit.
• We can apply different power supply voltages to
  measure the power supply rejection ratio.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
CMRR without Using Precision Resistors

• In this circuit, the power supply is from the
  -15 DUT op amp with the common mode voltage
  range of -10V. 
• From this circuit, the integrated amplifier A1
  should have high gain.
• Low Vos and low IB and the op amp is 097 devices.

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Common Mode Rejection Ratio (CMRR) and The
Operational Amplifier
Conclusion

• The common-mode rejection ratio (CMRR) of
  a differential amplifier (or other device)
  measures the ability of the device to reject
  common-mode signals, those that appear
  simultaneously and in-phase on both amplifier
  inputs. An ideal differential amplifier would
  have infinite CMRR this is not achievable in
  practice. A high CMRR is required when a
  differential signal must be amplified in the
  presence of a possibly large common-mode input. 

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