Questions tagged [common-mode]

A common mode signal is a signal that is common (i.e. has the same magnitude and phase) to both inputs of an electrical device, as opposed to a differential mode signal. It is often desirable to reject common mode signals, in which case the common mode rejection ratio (CMRR) is an important metric.

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What is "common-mode" noise?

Can someone explain what "common-mode" noise is, and how it can be problematic? I understand "noise" on a signal in general. If I have a "noisy" +5V rail on a circuit board, I'm not going to be ...
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What exactly happens to the signals hitting a common mode choke?

I'm trying to better understand the principles behind the common mode choke. I made a few drawings to clarify.   Differential Mode Signals Differential currents (driven by differential voltages)...
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How does an op-amp know where ground is?

Although I've been working with op-amps for a while, the following question never occurred to me before today. Consider first the op amp on the left (A). The negative terminal is connected to ground, ...
MGA's user avatar
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How does the CMRR of difference amplifier ICs change over time?

When designing precision analog circuits, I often come across parts which seem to be more than accurate enough for my purposes, but where the datasheet does not specify how key parameters will change ...
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How OPA333 withstand 400V bus without burning in this current sensing application?

I went through following app note, where OPA333 is used in 400V bus current sensing application, also i have seen current sensor which have term called common mode voltage, which is specify the max ...
kobi89's user avatar
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2 answers
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EMI Filtering on LVDS lines

This question is slightly related to: What's radiating on my PCB? These are Beckhoff's EtherCAT industrial IO modules. Each module is connected to its neighbours by 100mbps LVDS. Each module contains ...
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What is the common mode in common mode input voltage range?

The common mode voltage at the input of a difference amplifier is defined as $$\dfrac{V_1+V_2}{2}$$ and common mode input voltage range is the range of voltages at the input for which the opamp works ...
Henk's user avatar
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8 votes
4 answers
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Common Mode Rejection Ratio of op-amp

CMRR of op-amp is the ratio of the differential mode gain and common mode gain. What is the difference between these two ? What is the importance of CMRR in the performace of op-amp? How does CMRR in ...
nee's user avatar
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I designed and built an op-amp using 4 NPN and 2 PNP BJTs. What are the pros and cons, and how can it be improved?

There have been some questions asked recently about op-amps, and I was curious to see how to make one with just NPN and PNP transistors. I did a simulation in LTspice and it seems to work pretty well, ...
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Are the currents that travel on the outside of coax really "common mode"?

Consider a ladder-line transmission line. Given any currents on the two conductors, we can describe those currents as the current on each conductor individually, or: currents that flow one way on one ...
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Differential amplifier with differential output and common-mode shift

Summary: I would like to build a differential amplifier with differential output, but shift the common mode to a different level from the original. My current knowledge takes me this far: Take the ...
ultimA's user avatar
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What causes common mode currents in a coaxial cable?

I have been messing around with a signal generator, an RF current probe and an open ended coax cable. From about 50 MHz onwards, I can measure a significant common mode current if I attach some wires (...
Cyrus Tirband's user avatar
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Calculating Rout of a bjt while using correct RL

I'm studying the notes of my professor of electronics, and I'm noticing an inconsistency of \$R_{L}\:\$ compared to what I've seen around in books and on the web. Here's what is considered \$R_{L}\:\$...
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Is star grounding sources necessary for this data acquisition system?

Several single ended output and PSU powered transducers are tens of meters apart from each other outputting 0 to 10V DC like voltages. Some are temperature transducers some are force transducers ect....
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LTspice common mode choke differential noise

Edit, changed circuit diagram to make explicit the signal source is the load I'm designing a medical device and I'm in process of determining the line filter I'm going to need. I'm not especially ...
Barleyman's user avatar
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2 answers
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What causes the common mode gain in MOS differential amplifier

I went through many of the past questions in this forum and also a lot of other sites but none answered my question, so I made up my mind to post it here. What exactly causes the common mode signal ...
Bhuvanesh Narayanan's user avatar
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1 answer
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How do I properly size components for an EMC filter for a variable frequency drive power supply?

One of the products my company designs is, essentially, common bus power supplies for variable frequency drives (VFDs). Thus far we have only sold into the United States. We are considering selling ...
Stephen Collings's user avatar
6 votes
1 answer
2k views

Trifilar Transformer Common-Mode Rejection Circuit (Signal Recovery, Hybrid Combiner, RF)

I'm trying to develop a circuit that allows me to effectively subtract two different sources of RF noise from a single signal at the same time. I am planning to use something called a "trifilar" ...
iwantmyphd's user avatar
6 votes
3 answers
2k views

Is there a practical method to verify whether the noise have CM component in this setup?

An SMPS power supply unit namely Vsupply output can be composed of its desired DC output Vdc, differential mode noise Vdm and common-mode noise Vcm. If we have such a power supply and an earth ...
cm64's user avatar
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OpAmp with common-mode voltages and zero feedback current

Circuit A is a classic non-inverting amplifier with a common-mode DC voltage 1 V to both its inverting and non-inverting input. Since 1 V has to appear at the inverting input, there is no voltage drop ...
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Avalanche Pulse Generator: Common-collector alike setup with no input and fairly high voltage on the Common

I am wondering what would be purpose of a setup like this? simulate this circuit – Schematic created using CircuitLab If you notice there is no input in this setup and the voltage is fairly ...
Mehrad's user avatar
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5 votes
5 answers
481 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 ...
Jonathan Wick's user avatar
5 votes
2 answers
14k views

How to eliminate noise from switched power supply

I have an isolated switching power supply (120v in, 12v out) that gives an output shown in Figure 1 when referenced to the output ground (I'm not TOO worried about that noise). The bigger issue I have ...
Tyler Fontaine's user avatar
4 votes
6 answers
2k views

Can an op-amp's Vin+ and Vin- be outside the supply voltage as long as their difference is inside it?

An op-amp amplifies the difference between signals on Vin+ and Vin-. If I wanted to sample the differential voltage of a pair of signals whose common mode is outside and potentially significantly ...
J Collins's user avatar
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4 votes
2 answers
964 views

Why the common-mode gain of the differential pair is almost zero?

If you think of a better title, please do edit or let me know to edit, but this is how I thought I could word the question. I am finding difficulty in understanding how a differential pair exactly ...
midnightBlue's user avatar
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What is the practical advantage of using mirrored signal outputs eventhough what matters is balancing the wires?

Some devices output differential outputs since both wires carry varying signals which are mirrored. Most devices on the other hand output one signal carrying and one GND wire. There is also ...
user1245's user avatar
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4 votes
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Testing input common-mode voltage range of an op-amp

I have been trying to develop a set-up that could test the input voltage range and output swing of an op-amp. To measure the input voltage range, the output should swing within the limitations ...
Fateme's user avatar
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3 answers
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How does placing cap between signal GND and earth GND solve common mode noise?

Thanks in advance for your time. I am reading several articles about differential mode noise and common mode noise. Among which I found the below 2 pretty instructive: https://www.murata.com/~/media/...
momoxinduo's user avatar
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1 answer
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common mode ground

My doubt pertains to a certain document/app note about common mode and differential grounds- Link1 and Link2. What I read about common mode noise is that it is a noise generated on all signal lines ...
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Is this explanation of a common-mode choke correct?

I am trying to understand how common-mode chokes work. I get the basic concept: the signal lines are joined by a transformer, such that each line will aid or impede current in the other. But I am hazy ...
Sod Almighty's user avatar
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4 votes
1 answer
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Common mode choke in circuit with multiple ground connections

I'm designing an automotive circuit where I'm using a reference power design from TI. Together with protection circuitry, this design includes a common mode choke. The same circuit has an antenna ...
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4 votes
3 answers
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The best way to get rid off common mode noise for remote measurement

I measure DC pseudo-differentially over 1-2 m wires. In order to avoid any voltage drop in wires the built in ADC input buffers are used, thus there almost no current flow. I expect the wires to ...
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4 votes
2 answers
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Twisted-pair wire's effect on Common mode noise

In another answer here on the SE, this statement was made: Usually, my goal is to have all EM noise appear to a good amplifier as common mode, and use twisted pairs to achieve this." (emphasis ...
Adam Head's user avatar
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4 votes
1 answer
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12V 1A DC input filter

I have to design a power input filter of consumer device. It is powered with desktop AC/DC 12V power supply. Max current consumption - 1A. Device has a full metal enclosure. I am considering putting ...
Jurkstas's user avatar
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1 answer
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Why is the core resistance modeled as 2*Rp and capacitance as Cp/2 in a common-mode choke?

I directed my attention to a paper regarding the modeling of common-mode chokes. Within this paper, the authors structured the circuit in such a manner that they designated the core resistance (...
Alaska's user avatar
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1 answer
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Confusion with using non-inverting input to fix common mode voltage

Regarding this paper, and the following diagram: We can see above that the non-inverting input of U2 is connected to Vref of the DAC. Signal input goes to the inverting input and Vref to the non-...
ty_1917's user avatar
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4 votes
1 answer
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Ground planes disposition in Ethernet connection

I am designing a board that will have sensible analog signals and a processor communicating with the external word via Ethernet and I want to avoid that common mode currents couple into my analog ...
Luis Possatti's user avatar
3 votes
4 answers
976 views

How is the input offset voltage a function of common mode voltage?

I don't know about the internals of a opamps, but is it possible to explain how a change in common mode voltage changes input offset voltage Voff? I'm asking because when the signal Vin is fed to the ...
floppy380's user avatar
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What is the significance of Common Mode Signals?

I don't understand the physical significance of Common Mode Voltage. It is arbitrarily defined as the average of two voltages and is expected to be rejected by a differential amplifier. Why? What is ...
JGalt's user avatar
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3 votes
2 answers
355 views

Common-mode choke with inputs shorted together

I came across this configuration in a schematic, and it makes no sense to me. I have to admit common-mode chokes are not my strong suit, but is this even going to act as an inductor? simulate this ...
Aaron's user avatar
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3 votes
3 answers
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Why do we need bulky filters at the input stage of SMPS designs?

I usually see costly and large filters at the input stage of switch mode power supplies. Isn't the AC signal already filtered by the large capacitor (the 100\$\mu\$F one in this example) after the ...
hkBattousai's user avatar
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Does "Common Mode Noise" only affect single-ended signals?

In the above illustration the common mode noise is hitting to both + and - wires. Here is my understanding of the "common mode noise": In a Single-ended system: the - wire is GND and + wire is ...
floppy380's user avatar
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Why common-mode voltage is avarage of two input voltages in a differential amplifier?

I'm having hard time to understand the motivation behind the common mode voltage in differential amplifiers. I came to learn that common-mode voltage is chosen (v1+v2)/2 where v1 and v2 are input ...
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What exactly does "common-mode" mean in the context of op-amps?

I read this article about common-mode and rail-to-rail op-amps. For the common-mode, does the op-amp need to be configured in feedback mode such as a non-inverting amplifier? Should it be configured ...
curlywei's user avatar
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Instrumentation amplifier noise amplified

I built a simple circuit with an instrumentation amplifier (PGA204AP) in order to clean a differential signal from noise. The signal is generated few meters away from the acquiring point and the cable ...
Luca Daidone's user avatar
3 votes
3 answers
204 views

Can the DM inductors of an EMI filter be wound on the same core?

I want to implement the following EMI filter containing two separate LC filters such as DM and CM filters. Obviously, the CM inductors must be wound on the same core. Can I also wind the DM inductors ...
WeTech's user avatar
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3 answers
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Understanding Differential Mode Voltage of a Floating Circuit?

I am currently trying to work on studying for the ECE NCEE FE exam, but I came across the problem and don't quite understand the answer. Essentially we have an ideal transformer with an arbitrary ...
ReThink's user avatar
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2 answers
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Common mode input voltage range and comparators on an MCU

The comparators on my MCU, a dsPIC33FJ128GP802, give the common-mode input range as AVdd-1.15V. AVdd is 3.3V, so does this mean the comparators can't compare voltages in the range of 2.15V to 3.3V?
Thomas O's user avatar
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How to have a wider spectrum of FFT at LTspice?

I've used the FFT analysis tool to see the spectrum of DM and CM EMI noise in the range of 150K to 30MHz. But the analysis stops at about 3MHz automatically and there is no option to set the max and ...
WeTech's user avatar
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3 votes
1 answer
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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, ...
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