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

• Ianlamb
In summary, the conversation discusses the use of a differential amplifier with a CMRR of 120 dB and a differential gain of 105. The maximum differential input signal is 60 μV and the amplified noise voltage should not be more than 1% of the maximum output voltage. The maximum common mode voltage that can be present in the input to the amplifier is calculated using the formula CMRR=20log10(Adiff/Acm). After solving, the maximum common mode voltage is determined to be 0.1 volts. However, there seems to be a discrepancy when plugging this value back into the original equation. A tutorial is suggested to help clarify the issue.
Ianlamb
Member advised to use the homework template for posts in the homework sections of PF.
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

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.

## 1. What is a maximum common mode voltage?

A maximum common mode voltage is the highest voltage that can be applied to both inputs of a differential amplifier without causing distortion in the output signal.

## 2. Why is it important to consider maximum common mode voltages?

It is important to consider maximum common mode voltages in order to ensure the accuracy and reliability of the amplifier's output signal. Exceeding the maximum common mode voltage can lead to distortion and potentially damage the amplifier.

## 3. How is the maximum common mode voltage determined?

The maximum common mode voltage is typically specified by the manufacturer and can be found in the amplifier's datasheet. It is often determined based on the amplifier's input voltage range and the components used in its design.

## 4. Can the maximum common mode voltage be exceeded?

Exceeding the maximum common mode voltage can lead to distortion and potential damage to the amplifier. It is important to stay within the specified range to ensure the accuracy and reliability of the output signal.

## 5. How can the maximum common mode voltage be controlled?

The maximum common mode voltage can be controlled by using components such as resistors and capacitors to reduce the voltage at the amplifier's inputs. Additionally, selecting an amplifier with a wider input voltage range can also help to control the maximum common mode voltage.

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