How the Minimized Probe Load Capacitance Affects Oscilloscope Measurements

Key Takeaways

• An ideal probe shows the ease of connection, noise immunity, signal fidelity, and zero source loading effects.

• Probe loading can be classified as resistive loading, inductive loading, or capacitive loading.

• Low input capacitance passive probes are used to overcome capacitive loading, which becomes critical above 500 MHz. 

Probes are a signal’s entry point to an oscilloscope

An oscilloscope is a display that shows the currents and voltages present in a circuit. The signal to be measured is given to an oscilloscope using probes (cables that run from the oscilloscope to the test circuit). These probes are the signal’s entry point to the oscilloscope.

An ideal probe shows the ease of connection, noise immunity, signal fidelity, and zero source loading effects. The probe loading capacitance is critical in oscilloscope measurements. It is important to achieve minimized probe load capacitance to increase the quality of oscilloscope measurements. Let’s look at why this is a little more closely.

The Ideal Oscilloscope Probe

For oscilloscope measurements, a physical, electrical connection between the oscilloscope and the test circuit is required. The oscilloscope is left stationary and the probe is connected at different points where the voltage or current needs to be measured. The length of the probe should not be too long, as this can limit the probe bandwidth.

An ideal probe is necessary for oscilloscope measurements; it will leave a minimal effect on the test signals and increase the accuracy and quality of oscilloscope measurements. An ideal probe exhibits the following attributes:

• Convenient physical connection - An easy and convenient physical connection from the oscilloscope to the test circuit using a probe is critical. There are a range of probe heads and probe tip adapters available for ease and convenience.

• High signal fidelity - In a probe with high signal fidelity, the signal at the probe tip and the signal reaching the oscilloscope are the same. The probes only give absolute signal fidelity when the circuit between the probe tip and head is linear for all frequencies. The other requirements of probes for high signal fidelity are infinite bandwidth and zero attenuation. 

• Excellent noise immunity - An ideal probe should not add any noise to the test signal. The noise immunity of the probe needs to be high so that there is no probe-induced noise present in the signal under measurement. 

• No signal source loading effects - Probes connected to test circuits sometimes act as a load. Assuming the test circuit is a signal source, the probe creates loading of a resistive, capacitive, inductive, or combined nature. Minimized probe loading is essential for good oscilloscope measurements. 

The Types of Signal Source Loading  

Oscilloscope probes should not alter the electrical characteristics of a test circuit. However, in most measurements, probe loading occurs, where the probe supplies a load to the test circuit and the electrical characteristics of the test circuit are different before and after the probe connection. The probe behaves like a passive element, such as a resistor, capacitor, inductor, or a combination of these elements to the test circuit and draws current. The impedance of the test circuit and probe loading collectively determines the signal reaching the oscilloscope.

Probe loading can be classified as:

1. Resistive loading - The test circuit output resistance and probe resistance form a potential divider that distorts the signal under measurement. 
2. Inductive loading - The probe inductance is created from its ground point. At low frequency and DC measurements, the inductive loading effect is less. As the frequency increases, inductive loading has a greater effect, causing a ringing effect in the waveform measurement. 
3. Capacitive loading - Capacitive loading is the greatest concern in any oscilloscope measurement. Capacitive loading becomes heavier as the frequency increases and affects the amplitude as well as the shape of the waveform measured. 

How to Achieve Minimized Probe Load Capacitance 

Probe capacitive loading is a major issue in most oscilloscope measurements. Minimized probe load capacitance is required to improve oscilloscope performance and accuracy. Some advances in probes can limit probe load capacitances. These advances include:

• Low input capacitance passive probes - Used to overcome capacitive loading, which becomes critical above 500 MHz. 

• Active probes - The active circuit in the tip of the probe helps reduce the probe load capacitance to one-tenth of the value compared to passive probes. 

• Higher-end probes - Utilizes digital signal processing to eliminate probe loading effects. 

The minimized probe load capacitance is important when making critical oscilloscope measurements. It would be beneficial to include an oscilloscope model when designing a circuit. Cadence software offers tools to simulate the effects of probe loading on a test circuit by including the oscilloscope model in it. 

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