What Is a Passive Probe?
A passive probe is an electrical connection between the signal at a measurement point and the input terminal of an oscilloscope.
In the case of waveform observation with an oscilloscope, it is a device that transmits the signal to be measured from the measurement point to the oscilloscope.
There are two types of oscilloscope probes: active probes with built-in signal amplification circuitry and passive probes consisting only of passive components, but this paper will limit its discussion to the latter.
Uses of Passive Probes
A passive probe is used to detect the signal at a measurement point when observing a signal waveform with an oscilloscope.
The voltage at the applied point is transmitted to the oscilloscope. When observing current waveforms directly, current probes must be used.
Principle of Passive Probes
The use of a passive probe does not guarantee accurate waveform observation on an oscilloscope. Careful handling of the probe is required for accurate measurement.
1. Influence of the Passive Probe on the Circuit Under Test
When an oscilloscope is combined with a passive probe for waveform observation, the circuit under test is considered having the combined impedance of the oscilloscope’s input impedance and the probe’s impedance connected to it. That is, the probe has a built-in resistor Rp and a capacitor Cp for adjustment in parallel with it, and the cable of the probe has a stray capacitance Cs.
Since the input terminal of the oscilloscope consists of a parallel circuit of an input resistor Ri (1MΩ) and an input capacitor Ci, it is important to consider this combined impedance.
2. How to Adjust Cp
To obtain a constant attenuation rate regardless of frequency, Cp must be adjusted to satisfy the following equation.
Cp = (Cs + Ci) × Ri / Rp
The adjustment capacitor Cp is a semi-fixed type, so it should be set to the optimum value in combination with the oscilloscope itself. The oscilloscope has a square wave output terminal for setting the optimum value of the adjustment capacitor.
Connect the tip of the probe to this terminal and change the value of Cp so that a square wave is displayed on the oscilloscope display.
Other Information About Passive Probes
1. How to Use Passive Probes
A passive probe is used for measurement with an oscilloscope, and is selected from three types of attenuation ratios 1:1, 1:10, and 1:100 by the built-in resistor Rp, based on the characteristics of the circuit under test.
1:1 Probe
A probe without built-in resistor Rp and capacitor Cp for adjustment. Since the signal is applied directly to the oscilloscope’s input terminal, the oscilloscope’s input impedance of 1MΩ and input capacitance Ci are connected to the circuit under test.
Care must be taken if the circuit under test has a high impedance, as this will affect the measurement circuit. On the other hand, when dealing with small signals, the input sensitivity of the oscilloscope can be used as it is, which is more advantageous in terms of signal level than other types that pass through Rp.
10:1 Probe
This is a general probe that is commonly used, with a built-in resistor Rp of 9MΩ. The input impedance is 10 MΩ when combined with an oscilloscope, making it easy to use with minimal influence on the circuit under test.
100:1 Probe
With a built-in resistor Rp99MΩ and a capacitor Cp for adjustment, this probe is mainly used when the signal voltage exceeds 100V because the attenuation factor is 1/100. It is also characterized by its extremely high input impedance of 100MΩ, which has particularly little effect on the circuit under test.
Connection Points for Ground Leads
Particular attention should be paid to the connection of ground leads and how to handle them when using Passive Probe. When observing multiple points simultaneously on a multi-channel oscilloscope, it is fundamental that the ground leads of each channel probe be connected to a single common point (preferably a single ground point).
Connecting to different ground lines will adversely affect the measurement of small signals due to ground loops.
Length of Ground Leads
Longer ground leads are advantageous in terms of connection to the measurement point, but when observing high-frequency signals, faulty phenomena such as ringing and large fluctuations in signal amplitude can occur. This is due to the resonance between the inductive inductance component of the ground lead and the input terminal capacitance of the probe, resulting in an extremely large amplitude near the resonance frequency.
Therefore, when observing high-frequency signals with a frequency of 10 MHz or higher, consider using a ground spring or the like in place of the ground lead.
2. Frequency Characteristics of Probes
The main measurement performance is determined by the frequency bandwidth and rise time of the measurement system that combines the oscilloscope and the probe. Therefore, the frequency bandwidth and rise time specifications are published for each oscilloscope main unit and probe combined with it.
3. Cable Flotation Capacitance
A factor that greatly affects the frequency response of a probe is the cable’s floating capacitance. The higher the frequency, the smaller the capacitive reactance due to the floating capacitance, which in turn increases the load on the circuit under test.
As a result, the frequency bandwidth of the probe itself is narrowed, the rise time is slowed down, and other adverse effects occur. The stray capacitance of a probe also depends on the length of the cable, and the longer the cable, the larger the stray capacitance tends to be, so it is advantageous to keep the probe length as short as possible in terms of frequency response.