Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

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.

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. If your question deals with CMRR, consider tagging your question with both and .

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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 getting a constant value of +5, it will be bouncing…
Adam Head
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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) create equal but opposite magnetic fields B in the…
K.Mulier
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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, and a small voltage \$V_d\$ is applied between…
MGA
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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 over time. Right now, I am looking at datasheets…
Medo42
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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 an ET1200 ASIC which handles all of the…
Rocketmagnet
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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 voltage that can be applied to sensor inputs…
kobi89
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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 conductor, matched by exactly equal and opposite…
Phil Frost
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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, and I think it would be useful to illustrate the…
PStechPaul
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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 affecting the offset voltage and output voltage ?
nee
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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 traditional 3-opamp instrumentation amplifier, like…
ultimA
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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 as it's supposed to. But what is common mode…
Henk
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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 (even short ones, about 2 cm in length) to the…
Cyrus Tirband
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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. Their outputs all go to a single DAQ board which…
floppy380
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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}\:\$ in those notes: As you see, it's taking…
Neyo
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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 concerned about incoming noise but rather the…
Barleyman
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