# Deep understanding of Differential Mode Voltage and Common Mode Voltage

Source: Internet
Author: User

What we need is a meaningful "input signal". We need to regard the two inputs as "whole ".

Just like in Junior High School, the plane coordinates must be expressed by X and Y, while in high school or university, only one "Number" V is used, but this V is composed of X, "vector" composed of two numbers of y "......

The common mode and differential mode are the attributes of the "input signal". The differential input can be expressedVi = (vi +, Vi -)
It can also be expressed
Vi = (Vic, VID)

C Indicates common mode, and D indicates differential mode. The two descriptions are as follows:Fully equivalent. It only changes the angle of understanding, just like Coordinate Transformation in the geometry. The coordinate values of the same point in different coordinate systems are different, but they are always the same point.

The common mode input range of the op amp: Device (op amp, amplifier ......) The range of common-mode signals allowed under the condition of maintaining normal amplification (maintaining a certain common-mode rejection ratio CMRR.

Obviously, there is no common-mode voltage problem on one end. However, "one end" also has an input voltage range problem. And this rangeEqualCommon Mode input voltage range.

The principle is very simple: when the Op Amplifier works normally, the two inputs are short, and the single-ended input voltage range is almost the same as the common-mode input voltage range.For other amplifiers, the common mode input voltage varies with the input voltage range of the single end. For example, for the instrument release, the differential input is not 0, and the common mode input voltage range during actual operation is smaller than the single-ended input voltage range.

It can be understood:
The two ships are standing on the water, standing two people, A and B respectively. A and B start each other. When the ship is fluctuating, A can feel the tension of B's changes. The height difference between the two ships is the difference model signal.
When the water level rises or falls, a cannot feel this tension. The absolute height of the two ships from the bottom is the common mode signal.

Therefore, we say that a and B only respond to the difference mode signal, but do not respond to the common mode signal. Of course, there is also a certain common mode range. If it is too low, it will sink to the bottom of the water, so that the ship will no longer fluctuate. If it is too high, it will cause the water to overflow and cause the water flow to make the ship unable to stay on the water.
Theoretically, A and B should only respond to the difference model.
But in fact, A and B are dizzy due to the ups and downs of the ship. Obviously, only the common mode is used, but the illusion is that the other side is moving relative to itself. This shows that the internal forces of A and B are weak, and the common mode rejection ratio is not good. I laughed, but that's basically what it means.Of course, the differential mode voltage cannot be too large, otherwise A and B will be opened.
The main sentence is "the common mode is the arithmetic average value of the two inputs, and the difference between the differential mode is the direct difference between the same phase end and the reverse phase end ".
The common mode voltage should be the common value added to the input end of the amplifier circuit when viewed from the source end, and the difference value added to the two inputs of the amplifier circuit.

Common mode voltage can be either DC or AC. DC is called dc common mode rejection (ratio), AC is called AC common mode rejection (ratio), collectively referred to as common mode rejection (ratio ). The general amplifier, especially the instrument amplifier, has good DC common-mode rejection, but the high frequency of the AC common-mode rejection often fails-sharp decline, that is, the frequency response is not good.

The average signal is an active impedance, which may damage the symmetry of the circuit to varying degrees. Therefore, be careful when using a differential amplifier. See related data volumes.It is not just in the operational amplifier circuit. As long as the electrical signal is transmitted, it can be divided into common mode and differential mode.
The difference mode is between two signal lines.
Common Mode is Signal-to-ground

Therefore, as long as there is signal transmission, there will be common mode interference.

Accurate description: the common mode and differential mode of a single line are combined and cannot be distinguished. Only dual-line transmission can be used.DifferentiationCommon Mode and Differential Mode
Let's first look at the origins of common mode and differential mode, that is, the value of this distinction.

1. Conduction Interference:
Assuming that the system's public reference point ("location") is affected, the potential fluctuates. In fact, the concept of potential has only relative significance. There is no "potential" in an isolated point, so the fluctuation must be relative to another reference point, such as the Earth, or the reference point of the device connected to your board or machine. At this time, the interference on the two signal lines between the two devices is similar.

2. Spatial coupling:
Electromagnetic waves have a certain degree of spatial continuity. in a very small space, it can be considered that the electromagnetic waves are even. If the two wires are very close, the interference between the two wires is similar. In general, the interference of any signal line relative to the ground is common mode interference. However, the distinction between common mode and differential mode is valuable only when two-line transmission is used. Moreover, a single line can have a common mode, but there is no difference mode.
Of course, the concept is also customized. Either according to accepted arguments (fact standards) or according to authoritative definitions, such as IEEE standards.
Let's take an example here:
Add 3 V at one end of the differential Op Amplifier and 2 V at one end
Equivalent to adding Vd = 0.5
VCs = 2.5 Add one end:
Vd =-1, 0.5
VCs = 2.5 Any signal is a combination of common mode and differential mode, but what determines the common mode and the difference mode is the reference signal. Simply speaking, a line is meaningless. The ground of reference is actually just a 0 signal.
If one end is VI, the ground end is equivalent to the common mode signal VI/2, the difference mode signal is-VI/2, and the combination is 0.
If any reference bit is V2, the common mode in VI should be (V1 + V2)/2, the difference modulus is (V1-V2)/2
The other end is equivalent to the common mode (V1 + V2)/2, the difference mode is-(V1-V2)/2, the difference mode and common mode only makes sense.
Easy to understand: After you select a place, the relative height of the two lines is the difference model. The average value of the absolute height of the two wires is the common mode. When the distance between the two wires is reduced to 0 and becomes a line, there is only one height, therefore, its absolute value is common mode.
In addition, some definitions made in public academic journals are the understandings of various authors for reference:

1. common Mode Interference refers to the communication signal between the interference voltage at one end of the instrument transmission terminal (positive end or negative end) and the ground, it can be measured between one end (positive end or negative end) and the Earth using a transistor voltmeter. Generally, the interference to the Earth is mostly within the range of several volts to dozens of volts.

2. common-mode interference refers to the interference produced by the simultaneous interaction of the two measurement points with the potential relative to the Earth, the difference mode JF disturbance is the interference produced by the relative change of the potential difference between the two measurement points in the circuit.

3. common-mode interference refers to the common interference voltage on the two input ends of the analog-to-digital converter. It may be DC or AC voltage. The voltage amplitude can reach several volts or even higher according to the application environment. common-mode interference, also known as common-mode interference. Common common-mode rejection ratio (CMRR) indicates the ability of the input circuit to suppress common-mode interference.

4. common-mode interference refers to interference in the loop formed by the power supply's phase line and ground line. differential Mode Interference refers to interference in the loop consisting of the power supply's phase line and phase line. conduction interference is mainly caused by high-frequency fluctuation caused by the interaction between voltage, current and stray parasitic parameters in the circuit during high-speed switching.

5. in fact, both common mode and differential mode are used for conducting interference. The so-called common mode interference refers to the interference signal between the ground and the phase. The phase between the lines is the same and the potential is equal, difference mode interference is the phase difference of the interference signal between the two phases: 180 (equal potential)

6. common Mode Interference refers to the interference (potential) formed between a point of all circuits or circuits of the protection device and the ground (or housing), as shown in VT in 1. it is an important reason for the abnormal operation of the protective device.

7. common Mode Interference means that the interference size is consistent with the direction, and its storage lies in the power supply between any relative earth or midline to the Earth. common-mode interference is also called longitudinal mode interference, asymmetric interference, or ground interference, which is the interference between the carrier fluid and the earth.

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