What is the Maximum Common Mode Voltage for an Instrumentation Measuring System?

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SUMMARY

The maximum common mode voltage (Acm) for an instrumentation measuring system utilizing a differential amplifier with a Common Mode Rejection Ratio (CMRR) of 120 dB and a differential gain of 105 is calculated to be 0.1 volts. The formula used is CMRR = 20 log10(Adiff/Acm), leading to the conclusion that Acm = 10^5/1000000. However, the calculation raises questions regarding the conversion of a gain ratio to voltage, as the original CMRR value does not validate the derived Acm when substituted back into the equation.

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  • Understanding of differential amplifiers and their specifications
  • Knowledge of Common Mode Rejection Ratio (CMRR) and its significance
  • Familiarity with logarithmic calculations in electronics
  • Basic principles of voltage and gain relationships in instrumentation systems
NEXT STEPS
  • Review the principles of Common Mode Rejection Ratio (CMRR) in detail
  • Study the relationship between voltage, gain, and noise in differential amplifiers
  • Learn about the implications of amplified noise voltage in measurement systems
  • Examine the tutorial provided by Analog Devices on differential amplifiers for deeper insights
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Electronics engineers, instrumentation specialists, and students studying differential amplifiers and measurement systems will benefit from this discussion.

Ianlamb
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Hi,
I have this question.

An instrumentation measuring system uses a differential amplifier having a CMRR of 120 dB and differential gain of 105. The maximum differential input signal is 60 μV. If the amplified noise voltage is specified to be not more than 1% of the maximum output voltage, calculate the maximum common mode voltage that can be present in the input to the amplifier.

Just need to know if this is correct or I am on the right track.

I have the formula

CMRR=20log10(Adiff/Acm)

Then

120=20log10(10^5/Acm)

Then

120/20=log10(10^5/Acm)

Then

6=log10(10^5/Acm)

Then taking anti logs

1000000=10^5/Acm

Transpose this into

Acm=10^5/1000000=0.1volt

Is this correct? as when I plug the 0.1 back into the original equation I do not get the original 120dB answer.

Thanks.
 
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