Why is the CMRR of high frequency and high voltage differential probe so important and what is the common mode rejection ratio

CMRR, Common Mode Rejection Ratio (Common Mode Reiection Ratio)CMRR refers to the ability to remove the input signal (noise component Vn) common to a differential amplifier with two input circuits. When the grid power signal (ln) etc. flows through the ground circuit, a noise signal () is generated and applied to the differential amplifier as an in-phase component. However, the in-phase noise component appearing at the output of the amplifier is very small (the in-phase gain is small, ideally 0). If the differential gain (the gain of Vs in the figure) is Adiff and the in-phase gain (the gain of Vn in the figure) is Acom, the definition of CMRR is as follows:

CMRR=Adiff/Acom=Adiff [dB]-Acom [dB]

As the system becomes larger,(1) and (2) will look the same, but because the potential and anode are different, the current of the ground circuit flows between (1) and (2) and becomes noise

When a differential amplifier is actually used, a signal line is required. In addition, the signal source has an output impedance. Therefore, the equivalent circuit when using a differential amplifier is simply shown in the figure below. Without these reactance, the noise source will be applied to the positive and input terminals of the differential amplifier with exactly the same voltage value, and the CMRR will not deteriorate. However, in practice, these values cannot be ignored, and when a noise source is applied to the positive and negative of the differential amplifier, a voltage difference will occur, and this difference will deteriorate the CMRR.

First of all, let's give a simple example, the signal source impedance RS:10 KQ, the differential amplifier Rin = Rin-: ignore the input resistance of 10 MQ, and other impedances such as Rc: for DC, the differential gain is 0.9990, the in-phase gain is 0.0009990,CMRR is 20log(0.9990/0.0009990), I .e. 60.0 dB. Next, if Rs:0 kΩ,Cc1 Cin = Cc2 Cin-:500 pF

At 10 kHz, the differential gain is 0.9532 and the in-phase gain is 0.2994, and the CMRR drops to about 0.1 dB.

Note that when traveling from the signal source to the positive input of the differential amplifier in this manner, the CMRR will deteriorate if there is an imbalance in the transmission path. In the case of the following figure, the same positive and negative input as Rs will be inserted to improve CMRR and it is difficult to achieve high CMRR above tens of hertz.

Under actual use conditions, it is difficult to achieve balance in a large range of input parts, and it is difficult to achieve high CMRR above tens of kHz.

RS: Signal Source Output Resistance

Rc1,Rc2: DC resistance of the cable

Lc1,Lc2: Signal cable inductance (1 uH/m for parallel cable, 250 nH/m for 50 Q coaxial cable)

Cc1,Cc2: Signal cable capacitance (150 pF/m for shielded cable, 100pF/m for 50 Q coaxial cable)