How to select a probe?

WisdomAugust

Below is a list you need to consider when you choose a probe for a given measurement.

Input resistance

Input impedance is used to describe the loading effects of a probe. At DC and low frequency ranges, the probe's resistive component is the main factor that loads down the circuit under test. But as the frequency goes up, the capacitance of the probe tip, in parallel with the input impedance of the probe, resulting in greater loading and a more adverse effect to the target.

WisdomAugust

The main advantage of the current probe is that it can measure current without

disconnecting the power line. Based on the Hall effect, it can  measure both DC

and AC. Typical applications of current probes are system power measurement,

power factor measurement, and switching machine impact current waveform

measurements. The main drawback of the current probe is that its small current

measurement capability is limited by the bottom noise of the oscilloscope, so the

small current measurement capability is limited. Generally less than 10mA of

current is difficult to measure.

WisdomAugust

There’s a Hall sensor inside magnetic ring, which can detect

the magnetic flux and its output voltage in proportion to magnetic

flux. Therefore, the current probe output voltage is in proportion

to the measured current flowing through the wire. The conversion

factor for a typical current probe is 0.1V / A or 0.01V / A.

WisdomAugust

Current probe is another kind of active probe. There’s a magnetic ring

on the front end of the current probe, when measure, the magnetic ring

cover on the measured power line. Because the generated magnetic

field when the current flowing through the wire, which was collected by

the magnetic ring, the magnetic flux in proportion to the current flowing

through the wire.

WisdomAugust

There is also a branch of the active probe, that is differential active probe.

The difference is that the front amplifier is a differential amplifier.
The advantage of the differential amplifier is that it can directly test the

high-speed differential signal, while its common mode rejection ratio is higher,

the common mode noise rejection ability is better.

WisdomAugust

However, the active probe is not no shortcomings, limiting the wide

usage of active probes besides price factors, the other reason is its

limited dynamic range. The input voltage range like this high bandwidth

amplifier is not very large, so its measurement range can not be as big

as the passive probe. Commonly used 10: 1 passive probe, its dynamic

range usually at hundreds of volts, and it’s just a few volts for common

active probes. So there will be some limiation for its application.

WisdomAugust

We can see all the excellent features of the active probe are

brought by the front amplifier, but this high bandwidth amplifier

is expensive and need to be placed in a limited space in the

front of the probe, resulting in high cost.
General passive probe prices are hundreds of dollars or so,

while the price of active probes usually are thousands of dollars.
So the active probe is generally used in the need for the

occasion pf high measurement bandwidth.

WisdomAugust

There is  a high-bandwidth amplifier on the front end of the active probe.

The amplifier needs power supply, and the name of active probe attribute

to it. The input impedance of the amplifier is relatively higher, so the

active probe can provide a relatively higher input impedance; while

the amplifier output drive capability is very strong, so it can directly

drive the rear 50 ohm load and transmission line. Because the 50-ohm

transmission line can provide a high transmission bandwidth, plus the

high bandwidth of the amplifier itself, so the entire probe system

compared to passive probes can provide higher bandwidth.

WisdomAugust

The high impedance passive probe input impedance is high,

but its bandwidth is not high, low resistance passive probe

bandwidth can be high but the input impedance is not high.

Then is there a probe with both high input impedance and

high bandwidth? Actually yes, it is active probe.

In fact, active probe is a broader understanding, indicates

the probe need to be powered.

The schematic diagram of porpular active voltage probes as

follows.

WisdomAugust

The low-impedance passive probe requires an oscilloscope's input impedance

of 50 ohms and its front-end equivalently series connect a divider resistor.

According to the different series resistance to achieve different partial voltage

ratio. For example, if series a 450 ohm resistor, the partial voltage is 10: 1.

As a result of 50 ohm transmission cable, and the oscilloscope is 50 ohm

as well, so the entire probe bandwidth is relatively high.