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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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6 votes
4 answers
334 views

Common Mode Rejection tests

What's the best equipment for testing the common mode rejection of an op-amp? Like can I use an ordinary signal generator and just connect the one output to both inputs of the op-amp? Or is there a ...
2 votes
1 answer
369 views

Why does my common mode test still have tiny retained sine wave?

I used this equipment the BMA-200 with the AMP01 having a CMRR of 125dB. I connected the signal generator output to both the In+ and In-. Yet the waveform still have retained sine wave of very small ...
2 votes
1 answer
415 views

How can I calculate CM and DM gains for this model?

Regarding the second page of this document, in the first example it calculates the DC CMR due to line imbalance Rs and an explanation is given in the first paragraph below (ignore the second example): ...
2 votes
2 answers
401 views

ECG / EEG Amplifier design tricks

First of all, sorry, I missed the links / images where I have found the solutions mentioned below (I hope, if you are familiar with this topic, you know them): I have to design an ECG / EEG analog ...
0 votes
0 answers
55 views

Test to determine the CMRR in a measurement device with 4-20mA input

I would like to measure the CMRR on a measuring instrument for 4-20mA current loops. The goal is to measure the CMRR of the entire system, not just the amplifier. That is, the relationship between the ...
1 vote
2 answers
260 views

Can CMRR of a data acquisition input be measured this way?

I want to check a data acquisition input or overall CMRR of a signal chain roughly by a simple method. Normally for a daq input CMRR is defined as: CMRR = 20×log10(Vin/Vout) where Vin is the common ...
4 votes
2 answers
1k views

Why do some laboratories use a multimeter on top of an aluminum plate connected to ground?

I was reading a laboratory to study some backgrounds about common mode rejection (CMR), when I saw this image: Does the aluminum plate connected to ground under the multimeter increase the reading ...
5 votes
5 answers
484 views

Why does this op amp input stage have single ended output?

Looking at the internal diagram of LM324: I see that the output of the the input stage, which is a differential pair, is single ended rather than being a differential output. How does then this input ...
0 votes
1 answer
73 views

Overall CMRR of analog front-end

My analog front end input consists of a chain of (instrumentation amplifier + differential amplifier + and ADC). Is the CMRR of the overall chain only considered based on the first element that is ...
2 votes
2 answers
383 views

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 ...
1 vote
2 answers
303 views

Common Mode Noise Attenuation

The question is below: Your raw sensor signal is 6.5 mV, and you are using an instrumentation amplifier to process it. The amplifier has a CMRR of 80 dB and a differential mode gain of 40 dB. If the ...
2 votes
1 answer
518 views

Why is CMRR defined differently in open loop and closed loop configuration of opamp?

I have recently been studying "Linear Integrated Circuits" and I am currently studying about opamp circuits. The books I am referring are Microelectronic Circuits (Seventh Edition) by Sedra ...
0 votes
0 answers
47 views

Is this set up to measure CMRR correct?

Is this set up to measure CMRR correct? Why did he not consider \$V_{out}/2\$ in his derivation? Also is there any reason to use plus-minus sign ± for the gain? ...
0 votes
5 answers
284 views

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 ...
0 votes
1 answer
316 views

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 ...
0 votes
0 answers
164 views

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 ...
1 vote
1 answer
117 views

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)+...
0 votes
2 answers
489 views

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 ...
3 votes
1 answer
1k views

Calculate output voltage using the CMRR

Im trying to calculate the output of an opamp, the data is Closed loop gain = \$20dB\$ CMRR= \$90dB\$ Common signal applyed = \$-60dBV\$ so the idea is to use the next equation \$v_{o}=A(v_{id}+\frac{...
0 votes
1 answer
189 views

How do I find CMRR?

How do I find the common-mode rejection ratio, CMRR for this circuit? I know I have to find differential gain and common-mode gain as 'differential mode gain/commonmode gain' but with current mirrors ...
1 vote
0 answers
76 views

What are the IEC standards/certifications to look for in a DSO while making measurements pertaining analog parameters like CMRR, SNR, THD etc

I am in the process of measuring CMRR for our mixed signal system that deals with bio-signal acquisition. I am using DSOX2014A keysight entry-level oscilloscope to make measurement to derive CMRR, SNR ...
0 votes
2 answers
843 views

Measuring CMRR of op-amp circuit

I am trying to calculate the CMRR of following circuit. I am aware that similar circuit has been posted here before but I have read through them and they don't answer my question. Based on my ...
0 votes
1 answer
567 views

How can I improve the common mode rejection ratio for TI ADS1299 ADC?

I am using 2 TI ADS1299 (home page, datasheet) devices with Raspberry Pi4 and I want to improve the common mode rejection ratio. I don't understand the proposed techniques such as the isolation ...
0 votes
1 answer
106 views

How calculate the error offset or the noise threshold for MyoWare and FSR sensors?

I am working on EMG and FMG signals to extract time domine features and i need a threshold value for some features. I must calculate it from the characteristics of MyoWare and FSR sensors.For the ...
2 votes
0 answers
191 views

First principles modeling of op amp CMRR

Introduction I'm trying to analyze an op amp circuit from first principles (i.e. without using the two rules of ideal op amp analysis -- virtual short circuit and infinite input impedance). The ...
3 votes
1 answer
527 views

What actually is the differential gain of an operational amplifier and why does its value change when we consider the common-mode gain?

Okay, this may be a stupid question, but at the beginning of my Applied Electronics course they taught us that the differential gain \$A_d\$ of an operational amplifier is ideally infinite; however, ...
2 votes
4 answers
2k views

operational amplifier CMRR measurement

I am trying to measure CMRR of an op-amp and I found an application note from Analog Devices, that suggests the circuit below. I understand what CMRR means, but I can't understand why in the formula ...
4 votes
1 answer
2k views

Why is CMRR effect multiplied by gain in this non inverting op amp configuration?

In this document Vout of a non inverting op amp is given as follows: But why \$\frac{V_{in}}{CMRR} \$ is also multiplied by the gain? I would write it as: \$ V_{out} = \big[1 + \frac{R2}{R1} \big] \...
0 votes
1 answer
411 views

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 ...
0 votes
2 answers
866 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 ...
1 vote
1 answer
190 views

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 ...
0 votes
3 answers
312 views

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 ...
0 votes
2 answers
258 views

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 ...
0 votes
2 answers
823 views

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 ...
0 votes
1 answer
294 views

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 ...
1 vote
2 answers
871 views

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 ...
2 votes
1 answer
3k views

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, ...
1 vote
2 answers
1k views

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 ...
2 votes
1 answer
633 views

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 ...
6 votes
2 answers
3k views

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 ...
2 votes
2 answers
3k views

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, ...
0 votes
1 answer
986 views

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 ...
0 votes
1 answer
126 views

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: ...
1 vote
0 answers
175 views

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?
2 votes
2 answers
3k views

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= ...
4 votes
1 answer
2k views

Should I connect ground for differential measurement with oscilliscope?

Suppose I need to view voltage between two arbitrary points of a circuit with an oscilloscope. For that I can use two probes and A-B math, but should I connect the ground lead to the circuit? How to ...
0 votes
1 answer
746 views

How to quantify the error introduced by CMRR given in dB?

Here is the extract from the manual: I want to have an idea about how much voltage error occurs when applied 10V signal. And is this CMRR zero if differential ended configuration is used? How can I ...