# Questions tagged [cmrr]

Common-mode rejection ratio (CMRR) is the measure of rejection by the device of unwanted input signals common to both input leads, relative to the wanted difference signal.

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### Do differential ADC's really reject 100% of common mode noise?

Say you have differential signals that are perfectly phase and amplitude balanced, but obviously 180 degrees out of phase V+= Asin(2pif*t)+ Vcommon_mode_noise V-= -Asin(2pif*t)+ Vcommon_mode_noise The ...
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### Can someone help me understand why exactly high CMRR is so important, particularly in medical instrumentation like ECG?

So I have spent days reading about specifics of op-amps, instrumentation amps, and ECG circuitry and it just isn't clicking for me. As I understand it, the common mode signal gets amplified and added ...
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### Capacitor Array Tolerance of Caps in Same Package

Consider an MLCC capacitor array like CA064X471J5GACTU. Its tolerance is 5% but does this apply to capacitors within the same package? More specifically, I am wondering if the tolerance of caps within ...
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### Calculating and plotting CMRR

I'm trying to find and plot the CMRR of the following circuit: I'm getting zero for common mode gain, that gives an infinite CMRR for the circuit. When I followed normal feedback laws, I get (2/SRC)+...
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### How to prove $A_d / A_{cm} = V_{cm} / \Delta V$ in the CMRR equation

CMRR is defined by the ratio of differential gain to common-mode gain: $$CMRR = {A_d \over A_{cm}}$$ And CMRR is measured by the change in output voltage ($\Delta V$) when a common-mode input ...
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### How to quantify the effect of the common mode input for a given CMRR?

The definition of CMRR is 20×log10(Aol/Acm) where Aol is the open loop gain and Acm is the common mode gain. If we have a buffer with 20dB CMRR opamp and if we apply 1V differential input along with ...
270 views

### How much will the overall CMRR degrade due to impedance imbalance?

If the CMMR of a differential amplifier is given as 100dB at 50Hz in its datasheet, this CMRR value assumes the applied voltages wrt to common ground are equal. However in practice, if the line and ...
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### How can I model/understand this isolation module's CMR rejection ability?

Before asking the question here I have to show how I was demonstrating the CMR simply: The first circuit above represents a 100dB CMRR and the second circuit shows how the 1k Ohm source impedance ...
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### Variance in ECG lead resistance ; its effect ; CMRR

We are designing custom ECG electrode with flexible printed wires as to reduce wires clutter from different electrode. The manufacturer has made the patch but the resistance variation between all the ...
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### Is it possible to very roughly estimate the CMRR degredation of a differential amplifier due to source imbalance given its datasheet

In the datasheets of differential input amplifiers there is a figure of merit called CMRR. This is normally experimentally measured at a particular frequency like 60Hz and presented in the datasheet ...
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### CMRR of Comparators

I have been looking around for the Common mode rejection ratio value on the datasheets for comparators but there don't have one. Why is this? This ratio is crucial in terms of choosing a particular ...
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### Why does CMRR degrade in an instrumenation amplifier when the feedback resistor is increased?

Why does CMRR degrade in an instrumenation amplifier when the feedback resistor is increased, keeping all other resistances constant at a fixed value? What mathematical relationship links the feedback ...
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### Effect of bias resistor value on common-mode rejection in a differential or instrumentation amplifier

In the below schematics a source is coupled to a differential amplifier. Rb is a bias resistor for bias currents to flow: Since in instrumentation amplifiers it is a remedy to add resistors for bias ...
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### High-side current sensing with a rail-bound instrumentation amplifier?

I know that current shunt monitor ICs and high common-mode voltage differential amplifiers like the INA117 and AD629 exist but I'd like to know if it's possible to use a regular instrumentation ...
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### How can the CMRR of an opamp be negative?

I was reading a paper from 1977 regarding the analysis for the CMRR of a three opamp instrumentation amplifier, I got the paper through my Unviersity but I cant post the paper since its copyrighted, ...
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### Measuring Transformer CMRR

I am wondering how to properly measure the CMRR vs. frequency of a(n) (audio) transformer circuit. I have found a couple of methods but not sure I understand them entirely. The main idea I am going ...
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### PSRR and CMRR in a diffferential amplifier

Please, can anyone help me how to find the PSRR and CMRR in this simple differential amplifier circuit as shown below in figure 1? I have searched on the internet and I decided to follow this ...
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### Why is noise is considered to be a “Common Mode Signal”?

I learnt that the magnetic interference due to the Earth causes noise in the cable but I wonder why this is considered to be a 'Common Mode Signal' and why differential amplifiers are used to ...
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### Divide Sequencial Parameter Step in Spice

I've been designing a (very) simple MOSFET op-amp for school, and one of the things I need to do is determine the CMRR from my simulations. For the assignment, I'm just going to use the value at 1kHz, ...
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### Minimum CMRR design requirement

I've just started studying op amps recently and I am having trouble with the following problem from my textbook: Design the difference amplifier circuit to realize a differential gain of 1000, a ...
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### Scenario in which the opamp Input offset voltage is more than inverting input?

we are designing a circuit with inverting inputs in the order of millivolts. The aim is to build an amplifier so that the signal can be raised to a digital level (3.3V) Now, the question is this: ...
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### Common mode choke to boost cmrr of transformer input

I was wondering, can a common mode choke be used to enhance the CMRR of a transformer coupled amplifier Even further? What are the drawbacks?
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### Op Amp CMRR problem

I'm studying on my own for an exam. I bumped into this problem that really got me. My answer just doesn't match. Find the CMRR of the operational amplifier above, consider Ad = 1000000 (Answer CMRR= ...