Six major questions help you get the common mode rejection ratio

Abstract: You may know that "the common mode rejection ratio is the ratio of differential mode gain to common mode gain", but do you know how much the common mode rejection ratio is different between 120dB and 60dB? Do you know why you want to suppress the common mode signal?

Six major questions help you get the common mode rejection ratio

First, what is the common mode rejection ratio?

The common mode rejection ratio is defined as the ratio of the differential mode gain to the common mode gain of the op amp when the op amp is operating in the linear region. The common mode rejection ratio is an extremely important indicator. It indicates the ability to reject common mode interference signals when the differential mode is input. It is a measure of the isolation capability of the op amp to the common signal of the input signal.

Six major questions help you get the common mode rejection ratio

Among them, KCMRR represents the common mode rejection ratio, AD represents the differential mode gain, and AC represents the common mode gain.

Second, what is the common mode signal and the differential mode signal?

The common mode signal is the voltage of the signal line to ground, and the differential mode signal is the voltage between the signal lines. The amplifier circuit is a two-port network with two terminals per port. When the input signals of the two input terminals are U1 and U2, respectively, the difference between the two signals is called a differential mode signal, and the arithmetic mean of the two signals is called a common mode signal.

Six major questions help you get the common mode rejection ratio

Third, why should we suppress the common mode signal?

Any signal can be decomposed into a common mode signal and a differential mode signal. The common mode signal is the same signal that acts on the two inputs of the differential amplifier or instrumentation amplifier. It is usually caused by line conduction and spatial magnetic field interference. It does not carry valid information and is an unwanted signal. The main performance is:

1. When transmitting single line, the common mode signal caused by the difference of ground potential will be superimposed on the signal to form common mode interference, which will cause the original signal to be distorted.

2. When transmitting in two lines, the effective signal is a differential mode signal and the common mode signal is an invalid signal. If the common mode signal is amplified a lot, it will affect the differential mode signal that really needs to be amplified.

. Six major questions help you get the common mode rejection ratio

4. How is common mode interference generated?

In practical applications, the influence of various environmental noises can be regarded as common mode interference, and there are many reasons for common mode interference. The main reasons are as follows:

1. The grid is connected to the common mode interference voltage.

2. Radiated interference (eg lightning, equipment arc, nearby radio, high power radiation source) induces common mode interference on the signal line.

3. The grounding voltage is different, that is, the ground potential difference introduces common mode interference.

4. The effect of the internal wires of the device on the power cord.

Large differential mode signal.

5. What is the role of high common mode rejection ratio in the test instrument?

1. It can ensure that common mode interference between channels is minimized.

2. You can measure any point at will, regardless of the ground potential.

3. It is guaranteed that each channel will be measured independently.

6. Is the common mode rejection ratio greater than 120dB and 60dB?

For example, the output differential mode signal is 1V, the differential mode gain is 1, and the theoretical test result is 1V. However, if there is a 100V common mode voltage, the 120dB common mode rejection ratio attenuation factor is 0.000001, the test error is 0.1mV, and the 60dB common mode rejection ratio is 0.001, and the test error is 100mV. In other words, the common mode rejection ratio of 60dB test error will be 1000 times the 120dB test error.

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