What are the effects of oscilloscope active probes on measurements?
The connection part in front of the amplifier is a connecting line with uncontrolled impedance, which has a lot of equivalent capacitance and equivalent inductance. This part has a great impact on the system bandwidth, input impedance at high frequencies, and frequency response characteristics; behind the amplifier there is usually It is a 50Ω transmission line. This part is impedance controlled and has less impact on the system bandwidth.
The simplest way to reduce the impact of leads on system bandwidth is to shorten the length of the connecting wire between the probe and the device under test. The figure below is an example. A 2GHz single-ended active probe is used in the test. When using different connection accessories, the bandwidth of the system is different. The shorter the front-end accessory used, the higher the bandwidth of the system.
However, in some cases, for the convenience of use, the amplifier of the probe must be at a certain distance from the test point. This section of the connection line usually behaves inductively. If the inductance effect caused by this section of lead is not compensated, this long section of connection line will It is easy to cause signal oscillation. The following two pictures are the results of measuring the same 500MHz clock signal with a rise time of 100ps using a 4GHz single-ended active probe through a 2-inch long lead. In the picture on the left, the 2-inch-long lead has not been matched in any way, and the measured clock signal oscillates and deforms very seriously; in the picture on the right, the source end of the 2-inch-long lead is matched through a suitable resistor, and the signal oscillates and Deformation is significantly reduced.
Therefore, when the lead length of the probe and the device under test can no longer be shortened, using a suitable resistor to match the signal at one end close to the test point can improve the impact of the lead inductance. The specific size of the matching resistor used should be based on the size of the lead. Characteristics such as length are simulated and calculated. The picture below shows the differential welding and spot probes used by two differential probes. It can be seen that in the case of high frequencies, in order to improve the fidelity of signal measurement, even very short leads need to be properly matched. One thing to note about resistor matching is that this matching resistor only reduces the signal oscillation caused by long wires, and has limited improvement in bandwidth. If the length of the front-end lead is too long, the bandwidth of the system will still decrease.
As mentioned before, to increase the bandwidth of an active probe, in addition to using a high-bandwidth amplifier, you also need to minimize the length of the uncontrolled impedance transmission line from the test point to the probe amplifier and conduct the test at the front end of the connecting line. Resistor matching. However, usually high-bandwidth amplifiers require complex shielding, matching and power supply, and their size is not particularly small. If the amplifier is designed too close to the test point, it will be very inconvenient to use. In order to ensure ease of use and high measurement bandwidth at the same time, many high-bandwidth probes on the market now adopt a split structure.
This probe consists of two parts: the probe amplifier and the probe front end, which are connected through a 50Ω coaxial connector. The impedance of the front part of the usual probe amplifier is uncontrolled, so this length has a great impact on the signal. However, only a short section (about 5mm) of the front end of the InfiniiMax probe has uncontrolled impedance. This part The lead wire is very short to ensure high measurement bandwidth; and the part behind the front end of the probe (about 10cm) is a 50Ω coaxial transmission line, and the length of this part has little impact on the system bandwidth. Therefore, after adopting this structure, on the one hand, the probe bandwidth can be made wider, and on the other hand, the probe amplifier can be farther away from the test point, making the size of the probe front end smaller and easier to use. At the same time, this split structure facilitates users to replace different test front ends according to different testing needs, such as spot testing, welding, jack, etc.
