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Questions tagged [common-mode]

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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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1answer
37 views

How to quantify the cable impedance for common mode voltages?

Since a coaxial cable is not twisted, in the case of a common-mode interference the positive and negative lines will pick up different voltages. This unbalanced impedance nature of the cable can be ...
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1answer
37 views

Common Mode and differential Mode gain of this Cmos diff Amp inverter

This is one of questions our prof gave to students in recent years and im preparing myself for this exam. What is Common Mode and differential Mode gain of this Cmos diff Amp inverter? i understand ...
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1answer
48 views

Trouble with instrumentation and simulation of the common-mode response of a filter

I have been struggling to obtain the Bode plot of common-mode response of a filter. I first simulated it using LTspice, then I used a USB network analyzer. Both do not match each other and to the ...
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2answers
41 views

Interpreting some functions of an optoisolator from its datasheet

I cannot interpret couple of things from the datasheet of this optoisolator. 1-) Is there and hint in the datasheet regarding the maximum CM voltage interference it can handle at its inputs? 2-) The ...
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0answers
31 views

Trying to view CM and DM components of a signal on a two channel digital scope screen

I want learn how to view the CM and DM components of a signal on a usual two channel scope. The reason for this to be able to verify whether a noise superimposed on a signal is CM orDM. Regarding my ...
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2answers
39 views

A question about oscilloscope and common mode noise

To ask this question I needed to simulate two circuits as an example. Imagine we want to measure the voltage across a device output. Call the differential voltage across its terminals Vd. And imagine ...
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1answer
43 views

Opamp input from transformer

Is it possible to connect the circuit like this. The circuit is from the resolver input circuit, It takes the sine (or cosine) signal from the output of resolver and reduce to appropriate voltage for ...
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1answer
24 views

Can I use cumulative winding in common mode chokes?

I have been designing a common mode choke for common mode noise filtering at the input of my SMPS. I have chosen the capacitance value to be 6.6nF on both Line to Ground and Neutral to Ground. I am ...
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1answer
26 views

Common Mode Voltage Range with Negative Feedback

In section 3.1 of AN21 from Apex Microtechnology it mentions to make sure to accomodate the common mode voltage requirements. But does the common mode voltage range mean anything if one of the inputs ...
1
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1answer
53 views

Does impedance balancing in the far end make no sense?

Below shows a conversion of unbalanced line to a balanced one: My question is about: if the unbalanced line is very long which means the signal source and the isolation amplifier is apart from each ...
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2answers
70 views

How does common mode range include the negative rail?

How do the devices like AD8220 handle the common mode voltage equal to the negative rail or even lower voltage (Take a look at figure 30 of AD8220 datasheet) ? Does the operation of the amplifier ...
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0answers
19 views

using directional coupler to cancel common mode signal?

Reading this paper, and other ones by the same authors, I encountered the idea of rejecting the common mode signal (the large background) using a directional coupler. The second paper linked here ...
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3answers
76 views

Common-mode voltage outside the specified range

I'm reading the datasheet of TL331. Here a summary of the relevant specification for my question: ...
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1answer
55 views

Why does LVDS (or any differential signaling for that matter) use a common mode voltage of 1.2V instead of 0V?

Why does LVDS (or any differential signaling for that matter) use a common mode voltage of 1.2V instead of 0V? Is there an easy way to shift the common mode voltage to 0V?
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3answers
54 views

Isolate low power i2c ADC

I'm using the MCP3421 to read voltages from thermocouples on battery power. I would like to get this working over USB but I think the low common-mode range of the ADC could be problem so I am ...
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1answer
26 views

x2y caps and common mode chokes (using both)

I am designing a ultralow noise PCB board. Common mode noise has to be minimized, so I have a question I am planning to use a CMC on power input and to further decrease the common mode noise I also ...
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1answer
126 views

Why CMRR degrades in Instrumenation Amplifier?

Why CMRR degrades in Instrumentation Amplifier when R3 (only the feedback resistor) is increased keeping all other resistances constant at a fixed value? What mathematical relationship links R3 and ...
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1answer
26 views

Netfilter (230VAC) without protective earth (PE)

I need to filter my mains input, but I don't have any protective earth (PE). Will a mainsfilter (schaffner for example) still be usefull? I think so. I think the mains filter despite of not being ...
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1answer
203 views

maximum possible common mode voltage for npn

First I would like to thank everybody here for providing the help. This great resource allows me to understand things beyond what my textbook only offers. I have the question below: Here is the ...
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2answers
420 views

Why is the MCP601 better suited than the LM833 for single supply applications?

Still not answered, please need some help. If you guys don't know the answer don't be afraid to admit it, but I want to generate discussion around potential solutions/calculations and not semantics ...
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2answers
72 views

Noise on the whole board after USB connection

I designed a device on which there is a HUB, FT232 , two external USB port and some periphery. I wanted to check the noise on the power bus. And I saw only white noise. But after I connect the USB ...
0
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1answer
76 views

Common mode range and DC common mode interference

Here is my basic understanding of common mode range for diff input amplifiers: As far as I understand the common mode voltage is nothing but the offset at which the diff signal is travelling above a ...
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2answers
52 views

Understanding a common mode signal conditioning module

As far as I understand, normally an ideal differential amplifier(or an instrumentation amp) can reject common mode(CM) voltages up to around its rail voltages. Let's say the rails of the diff. amp ...
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1answer
52 views

Common mode problem with floating sensor

So I'm trying to measure the signal from a potentiometric sensor, that has a working and reference electrode. Electrically it can be summarized with the picture below. Note that my circuit is intended ...
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1answer
216 views

USB chokes for USB power

We are designing a ultra low noise USB 2.0 hub. I am thinking to add a common mode choke to + and - of the USB connector, in addition to the D+ and D- choke to remove the common mode noise. I am ...
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1answer
56 views

Which diagram is better for common mode noise rejection?

I want to us this sensor(107LN type) as in the following signal chain with differential-input DAQ board and a STP CAT6 cable as below: Above the GND of the power supply is local/near to the DAQ. ...
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1answer
40 views

Would any voltage be induced in this scario due magnetic coupling?

Imagine a source with a 100 Ohm source impedance is coupled to an amplifier/receiver which has 1Meg input impedance and the transmission is done by using two parallel wires signal and return |XA| and |...
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2answers
104 views

Avoid in-amp common mode and ADC input protection

I'm designing a temperature measuring circuit using a PT100 on a wheatstone bridge. In the temperature range I'm measuring (0-45C) I should get 0-25.5mV from the bridge and then amplify the signal to ...
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2answers
101 views

Why is common-mode noise on a power line bad?

I've always been intrigued by common-mode noise, as it seems to be such a pertinent issue when it comes to op-amps, chokes, etc. used as solutions to remove it. I understand from a digital signal ...
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1answer
36 views

Isolation of voltmeter via DC->AC->DC conversion

I bought a combination ammeter/voltmeter which has the restriction that the measurement/display circuitry must be powered from a different source than the measured voltage. What is the underlying ...
0
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1answer
61 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 ...
3
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3answers
286 views

Commonmode voltage and measurement with voltmeter and an instrumentation amplifier

I'm trying to compare a voltmeter and instrumentation amplifier which both measures voltage. Below in Figure 1, Figure 2 and Figure 3 I made these diagrams to ask my question. Basically a device(on ...
0
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1answer
86 views

How do non-flowing common mode or bias currents cause common mode range to exceed/float?

Below is a very simplistic view of differential ended inputs of a data-acquisition channel's inAmp which measures a floating battery voltage V. To make concepts and explanations simpler I use two ...
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0answers
30 views

How can I create common mode interference for this setup?

I have a measurement setup where a battery is coupled to a single-ended data-acquisition board though a coaxial BNC cable. This board is powered and monitored by a laptop PC through a USB cable. The ...
3
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1answer
418 views

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 ...
0
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2answers
280 views

DM and CM noise filter

I have this long cable which is carrying two differential voltage signals (around 300kHz maximum frequency) through a very "noisy" environment. I need to acquire this signal through a differential op ...
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2answers
39 views

Truncating negative results of Op amp by rail limiting

I have a signal that will swing from -5v to 5v. I am interested in the 0 to 5v part only. I will set a reference voltage and give it and the signal into a comparator. The comparator's rails are 0, and ...
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0answers
45 views

Question about noise effects of parasitic common-mode coupling in a single ended system

This question is related to a previous question. Now I'm trying to understand a bit deeper about the phenomenon I will present. (to see bigger view please left click) To make it clear enough I used ...
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2answers
150 views

Why is a common mode bias required for this differential amplifier? [closed]

TI makes the ADS1118 which is an ADC with a programmable gain amplifier (PGA). In one of their application examples (page 4), they indicate that small differential input voltages (1-2mV) must have a ...
0
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1answer
354 views

common mode choke placement in smps

I am designing a hi end SMPS and I have 1 CMC on the input (ac line) and a second on the DC output. My question is where to place this second CMC. Usually its best to place filters right at the ...
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2answers
207 views

common mode choke on DC line

usually common mode chokes are used on AC lines to take care of common mode noise. If I am using a CMC on a DC line ..would the inductance degrade ?
0
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1answer
61 views

How to explain this solution to the common-mode voltages?

One of my previous question revealed that a variable voltage test device's power supply which is an AC/DC adapter, and is SMPS type which causes common-mode voltages. A solution given by tying the ...
0
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2answers
96 views

Is this definitely common-mode interference and how can I model and filter it?

A test device which can output voltage between 0 to 10V by a knob is powered by a DC adapter which converts 220V rms(meaning 311V amplitude) 50Hz AC to DC. Device is not a commercial and its adapter ...
0
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1answer
25 views

Unclear common-mode voltage specs in the DMS-20LCD volt-panel-meter

I would like to buy a small digital volt-panel-meter. My eye fell on this series from Murata: DMS-20LCD (https://www.murata-ps.com/data/meters/20lcd.pdf) This meter can be powerd by either a 5V ...
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0answers
185 views

Measuring impedance of wire/cable

I'm working on an audio project where I'd like to install common-mode filters consisting of a cm choke and 2 caps to chassis at each of the input connectors in order to shunt common-mode noise picked ...
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2answers
280 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 ...
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1answer
202 views

PCB layout for shunting RF common-mode current to chassis

I am laying out a pcb with a number of external connectors that will have RF common-mode filters at each connector, shunting any common-mode signals to the chassis. However, I'm not sure what is the ...
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1answer
96 views

RF Common-Mode Filter Topology

I am working on a project with two inputs and an output as shown. IN2/OUT are external connections, and IN1 is internal. Based on input from this forum and research I have come up with the following ...
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1answer
40 views

\$i_{b1}\$ in this simple bjt circuit

From Bartlett's bisection theorem. Common-node. So I don't get, shouldn't \$ i_{b1} \$ be: \$ \frac{V_{ic}}{r_\pi + 2 R_{ee} } \$ ? \$r_\pi\$ is in series with \$2 R_{ee}\$, so they have same ...