# 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 the standard common mode voltage of Media Dependent Interface MDI[0:3]± signals?

I am working on the selection RJ45 connector for MDI interface between PHY and the link-partner. I am planning to use this PHY for my SGMII interface application. From various online readings, I ...
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### What can this CAN bus like protocol be?

I am currently probing what seems to me the CAN bus twisted pair of my car. But the signals are not what I expect. One of the pair is like a typical CAN HI signal, however the idle state is a 0V and a ...
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### How to simulate this circuit for common mode voltage effect?

Regarding my previous question and this paper, I wanted to simulate the circuit below to see whether it is rejecting common mode voltage as claimed. The paper says that sharing DAC's Vref with non-...
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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-...
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### EMI analysis for low power low voltage AC/DC

I've designed an 80W 220V-IN 400V-OUT PFC Boost converter in order to analyze its conductive EMI and design the proper active EMI filter for it. but as this is my first implementation project, working ...
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### Common mode input voltage in a 4-20mA transmitter

I designed and built the following circuit which is an almost straight-from-the-book 4-20mA loop transmitter. The idea would be to take an accelerometer axis (more or less 0.1 to 3.2V) and send out ...
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### Paralleling common mode chokes

Regular chokes can certainly be used in parallel so that multiple smaller chokes with a lower current rating can be used. But does this also work for common mode chokes? I would assume that ...
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### Why feed a V_OCM outpin pin of a FDA?

The Analog Devices reference design CN0540 (see diagram below) shows an AD7768-1 ADC together with a fully-differential ADC driver ADA4945-1. I am wondering why the common-mode voltage pin V_COM of ...
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### Common mode choke - Why only impedance?

Why are most common mode chokes only given with a impedance curve, inductance at a specific frequency and current rating (this is clear😅). Wouldn't be S-parameter better? I know it's more work, but ...
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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 ...
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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 (...
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### What is common-mode voltage in a differential amplifier?

I am using an LM2904 for my application. One of the input voltages, the voltage at the negative terminal of the op-amp (Vin-), can vary from -400mV to 500 mV. However, the voltage at the positive ...
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### Can an op amp Vin+ and Vin- be outside the supply voltage so 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 ...
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### Fully Differential Amplifier With Common Mode Feedback Network

I need to design the following circuit in a way that can provide a gain of 1000 and also has a power dissipation of 1.8 mW which, according to my calculations, results in a branch current of 250 μA. I'...
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### Capacitive coupling for common mode rejection?

I'm not finding any information on this method, so I'm thinking that I must be missing something. I have a 5V single ended RRIO opamp that I need to measure BLDC motor current with. The motor is ...
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### Differential and Common Mode Signals

if ı have amplifier and my ınput ac voltage and v is 2v haw can ı Guess and sketch output of common mode signal.
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### 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 ...
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### How to set up an LTSPICE test bench for Common Mode Feedback Loop Stability

Suppose we are given a fully differential (low voltage single supply) op amp with common mode feedback. It is common knowledge to break the loop in order to measure open loop ac performance for loop ...
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### How does an Instrumentation Amplifier reduce DC offset?

While researching different current measurement methods, I came across a particularly interesting circuit which has a very high input impedance and apparently reduces the effects of the input offset ...
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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 ...
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### Common Mode Voltage in Differential Amplifier used in Voltmeter Circuit

After many days of investigation, design and simulations I finally design this circuit used as part of my arduino voltmeter. This circuit uses AD8605 (https://www.mouser.com/datasheet/2/609/...
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### How does differential mode noise and common mode noise occur

I was reading this article on common mode and differential mode noise. But it is not mentioned anywhere on how the differential mode noise and common mode noise occurs. From the diagram, I kind of ...
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### Touch screen power supply problem

I am using Raspberry PI 4 Model B (4GB) with an official capacitive 7" display. I am powering the device using an external AC-DC (220VAC - 12V) power supply. I converter 12V to 5V using a buck ...
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### How accurate is this LTspice model of common mode signals?

The 0.9MHz voltage source at the bottom is what I want to use to model common mode noise on the signal. Is this the correct way to think about this? I am going to use this to do some analysis on that ...
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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 in this datasheet the Vcm vs Vout curve(or the 'diamond curve') is specified for 'all gains'?

From what I read so far and from the equations used for deriving the diamond plot for instrumentation amplifier, the shape of the plot changes with gain. Following is an example: However, in the ...
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### Why do I keep blowing this current shunt amplifier?

I'm using an INA293B3-Q1 current shunt amplifier in the standard high-side configuration as shown below. The supply is a variable DC power supply (0-20V). Rsense = 1 mOhm. And, the load is a ...
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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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### Does a rail-to-rail input fully-differential amplifier need Common mode feedback at Output nodes?

I am designing an Op-amp with rail-to-rail inputs and the operation being fully differential in nature. I am trying to understand whether I will still need a CMFB (common-mode feedback) circuit to fix ...
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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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### 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 ...
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### Common mode impedance in operational amplifier

While looking at this simulation, as they say no current flows at the non inverting terminal of the op-amp, but looking at the differential stage of the operational amplifier shouldn't there be some ...
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### SVPWM: space vector generation star point shifting - effects?

I keep seeing that the space vector generation (as a part of a FOC) is generated around DC Link / 2. What effects would be expected if the (in this case isolated) star point were permanently shifted? ...
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### Common mode filtering for three DC lines

I want to filter common mode noise on three DC lines: +5V, GND, -5V at 1A. Why are there so few options for common mode chokes with three lines? What is a good topology for this situation when using ...
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### Common-mode filtering long LED driver cable

I am designing a board to power a WS2815 style LED strip. This strip has three inputs: 12V, GND, Di. This wire -> LED strip will be up to 60ft away from the control board. As such, I want to filter ...
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### Common-mode voltage on the body

In the attached figure, input impedances of the right hand, right leg and left hand are respectively $20k,50k$ and $10k$ Ohms. The input impedance of the opamp is also assumed to be $1M\, \Omega$...
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### What is the meaning of Change in Output Offset Voltage between complementary output states

I have MAX9278 deserializer IC which is connected to a 3.1inch TFT display 800 (W) * 480 (H). Deserializer provides 4 LVDS lanes and 1 clock to the display. While I checking the electrical voltage ...
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### 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 ...
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### How does common mode voltage surges couple in wires

From doing some research and with the assistance of a user on this platform, I would like to know if my understanding of this concept is correct. Please note this post is an add-on to this post. A ...
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### Finite CMRR even with ideal tail current source (body effect)

In Razavi's Design of Analog CMOS Integrated Circuits 2nd edition page 123, the diff-pair below is included with a comment about how even if the tail current source is infinite, if the transistor pair ...
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### Can I rely on a the simulation of an op-amp based differential amplifier without looking at my op-amps common mode signal

Can I rely on what a simulator gives me for the output of an op-amp based differential amplifier without verifying that the common mode input to my op-amp is between the voltages of my op amp's power ...
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### How is a CAN Transceiver's common mode voltage range extended?

The CAN Standard states a max. common mode voltage of -2 V to +7 V. There are transceivers however, that allow for e.g. +-25 V common mode voltage range. How is that acheived? I imagine (I do not ...
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### What is input overdrive?

I'm looking for an affordable op-amp to be used as a low-frequency (<100Hz) low-output-power (5V 1mA) rail-to-rail voltage follower that can run on -0/+5Vdc. So far my best bet looks like the ...
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### Input common mode voltage of AC coupled differential pair

I was reading OnSemi's AN8173 app note and couldn't understand why input common mode voltage of AC coupled line is equal to VTERM. My thought was each input of the receiver, D and \D, will swing ...
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### Common voltage of 2 op amps instrumentation amplifier

I am trying to build an amplifier for a strain gauge sensor using the 2 op amps instrumentation amplifier design provided in the Texas Instruments note: https://www.ti.com/lit/an/sboa247/sboa247.pdf?...
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### What "mode" is static electricity (lightning?) Is it common-mode, differential-mode? It seems half-way, or "mono"

The common-mode or differential-mode designations assume two conductors, one in-phase, the other anti-phase, but a static circuit is different because the "conductor" is concentrated in thin ...
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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 ...
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### Conducted EMI of buck converter is too low even without filter

I have simulated a 48 V to 12 V buck converter with LISN in LTspice. However, CM and DM EMI is way below CISPR limits. I want to simulate a real world converter. How can EMI be increased? [2]
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### How to implement EMI filter in an N-redundant supply?

I am designing a dual supply powered system, where certain loads, depending on some conditions will be powered by one or other power rail, in order to achieve some safety levels. The problem arises ...
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### Common emitter amplifier's $R_C$ breaks
What happens if the collector resistor breaks (its value goes to infinity) in a textbook example circuit? I assume that $I_C$ hits zero, $U_{CE}$ decreases until the transistor goes into ...