Oscilloscope Impedance Selection

The oscilloscope itself also has input impedance, and the impedance of the probe should be consistent with the impedance of the oscilloscope. Generally, low-end oscilloscopes only have 1M Ohm input impedance, mid-range oscilloscopes can switch between 50 Ohm and 1M Ohm input impedance, and high-end oscilloscopes generally only have 50 Ohm input impedance. The specific input impedance to use depends on the specific signal to be measured.

The input impedance of the oscilloscope should be selected according to the signal being measured:

Here we must first distinguish the input impedance of the probe. First, a brief introduction to the probe is given. Usually, passive probes have two gears: 1X and 10X. The impedance of 1X is about tens of Ohms to hundreds of Ohms, and the impedance of 10X is 9M Ohms. When connected in series with the impedance of the oscilloscope, the total input impedance is 10M Ohms. The circuit of the probe is shown in the figure:

When using 1X, R1, C1 and C3 are not connected to the circuit because the load capacitance is relatively large due to the cable, and the bandwidth cannot be too high;

When using 10X, R1, C1 and C3 are connected to the circuit. C1 or C3 is adjustable to compensate for the cable and oscilloscope. The bandwidth can reach 500 or 600 MHz.

The input impedance of the oscilloscope is selected to be 1M Ohm, which is used to measure signals that are not particularly high frequency and signals with large voltage; 1M Ohm is equivalent to high impedance, just like the formula above: Gain=-(Rc||Rp)/Re. The closer the Gain=-Rc/Re is to that without connecting the measuring instrument (here refers to the oscilloscope), the larger the Rp, the better, and it can often be regarded as having no effect on the signal.

The input impedance of the oscilloscope is selected as 50 Ohm. The vertical index of 50 Ohm input impedance is better, the sensitivity is higher, and it cannot measure large voltage. It is easy to understand that it cannot measure large voltage, because the power consumption of large voltage on 50 Ohm is large, and the current input to the oscilloscope is also large, but how to understand that the vertical index of 50 Ohm input impedance is better and the sensitivity is higher! Look at the figure above and connect the oscilloscope to measure the signal. Gain1=-(Rc||Rp)/Re= Rc*Rp/(Rc+Rp)/Re. When Rp is 50 Ohm and Rc changes to a*Rc, the corresponding Gain2=-(a*Rc||Rp)/Re=-a*Rc*Rp/(a*Rc+Rp)/Re, where a*Rc*Rp/(a*Rc+Rp)

1. If it is a high-frequency circuit and there is no load, select a 50 ohm matched impedance input which can be equivalent to a load, and then observe the signal.

Some signals come from sources with 50Ω output impedance. In order to accurately measure these signals and avoid distortion, these signals must be accurately transmitted and terminated. In this case, a cable with 50Ω characteristic impedance should be used and terminated with a 50Ω load.

2. The impedance value of impedance matching is usually consistent with the characteristic impedance value of the transmission line used. For RF systems, 50Ω impedance is generally used.

3. For high impedance instruments, due to the existence of equivalent parallel capacitance, as the frequency increases, the parallel combination impedance gradually decreases, which will load the circuit being measured. For example, for a 1MΩ input impedance, when the frequency reaches 100MHz, the equivalent impedance is only about 100Ω. Therefore, high-bandwidth oscilloscopes generally use 50Ω input impedance, which can ensure the matching of the oscilloscope and the source end. However, when using 50Ω input impedance, it must be considered that the load effect of 50Ω input impedance is more obvious. At this time, it is best to use a low-capacitance active probe.

There are also output configurations for signal sources:

High impedance and 50Ω load. If the high impedance output is set to 1Vpp, the output of the signal source is Vs=1Vpp, and the internal resistance Rs=50Ω; if the oscilloscope is set to 1MΩ input impedance at this time, then the signal measured by the oscilloscope is 1MΩ/(1MΩ+50Ω)*1Vpp, which is approximately equal to 1Vpp; if the oscilloscope is set to 50Ω input impedance, then the signal measured by the oscilloscope is 50Ω/(50Ω+50Ω)*1Vpp, which is approximately equal to 0.5Vpp; Assume that when setting the 50Ω load output of 1Vpp, the output of the signal source is Vs=2Vpp, and the internal resistance Rs=50Ω; if the oscilloscope is set to an input impedance of 1MΩ at this time, then the signal measured by the oscilloscope is 1MΩ/(1MΩ+50Ω)*2Vpp, which is approximately equal to 2Vpp; if the oscilloscope is set to an input impedance of 50Ω, then the signal measured by the oscilloscope is 50Ω/(50Ω+50Ω)*2Vpp, which is approximately equal to 1Vpp;