Attenuation & oscilloscope probes

China site

The attenuation device is used to attenuation a large input signal to ensure that the output of the amplifying circuit is not distorted. The main requirement for the attenuation is that the frequency band should be wide enough and the input impedance should be high enough. Since the input impedance of the amplifier circuit connected to the output of the attenuation device is capacitive, the attenuation device usually uses the RC attenuation device. Its principle is as follows:

 

It can be seen that the attenuation is irrelevant to the frequency and is called the optimal compensation state. The time of the oscilloscope's attenuation is composed of multiple above-mentioned RC divider.

 

 

Oscilloscope probe Structure

The input impedance at the input end of the oscilloscope is limited and can be equivalent to the parallel operation of the input resistor RI (such as 1 mohm) and the input capacitor Ci (such as dozens of PF. Connect the input end of the oscilloscope to the tested circuit through a cable. The input impedance of the oscilloscope and the distribution capacitance of the cable (up to several hundred PF) become the responsibility of the tested circuit, and connected to the test point, this will affect the tested circuit.

As shown in Figure A, a resistor, capacitor, and parallel circuit are installed in a metal shielded shell. one end of the parallel circuit is connected to the probe, and the other end is connected to the plug through the cable. Its Equivalent Circuit B is shown in. Ri is the input resistor, CI is the input capacitor, C0 is the distribution capacitor including the cable, and CX is the adjustment compensation capacitor. When the switch is disconnected, the circuit forms an attenuation. C2 = Ci + C0 + Cx. When r1c1 = ric2 is met, the partial pressure ratio is:

 

 

It can be seen that the size of the partial pressure ratio K is determined by R1 and RI, and is irrelevant to the frequency. The input resistor is r = R1 + Ri = Kri and the input capacitor is c = c2/K. that is, the input resistance will increase K times after the probe is connected, and the input capacitor will be reduced to 1/K, so the impact on the tested circuit is much smaller.

 

 

Actual reference circuit (reprinted http://www.ourdev.cn/bbs)

 

 

Is under compensation, is over compensation