Did you know that when you probe a circuit, you change the electrical characteristics of that circuit? Oscilloscope probes add resistive, capacitive, and inductive loads to your circuit. These loading affects can change the operation of your circuit under test. Understanding these loading impacts helps you avoid selecting the wrong probe for your specific circuit or system.
Figure 1 below shows a circuit under test and the electrical model of a probe connected to it. In a perfect world, Vin (voltage at the input of the probe) should be the same as the Vsource (voltage of your circuit before it is probed). But because of probe loading effects, the impedance of your circuit and probe determines the voltage at the input of the probe. It is a simple resistor divider circuit. Increases in frequency can also become a major source of loading because the probe’s capacitive reactance gets smaller. This loading alters not only the amplitude but also the shape of your original circuit waveform.
Figure 1: The probe’s electrical circuit
When the probe is connected to the circuit, the impedance matching of the circuit and probe determines the voltage at the input of the probe.
Capacitive loading can be the main culprit of your measurement errors. For general-purpose measurements less than 500 MHz, passive 1:1 and 10:1 high-impedance resistor divider probes are good choices. These passive probes begin to impose heavier capacitive loading as the frequency of the signal being measured increases. As the frequency of the signal goes up, the probe impedance drops and can load your circuit. High-impedance passive probes are a great choice for general-purpose debugging and troubleshooting on most analog or digital circuits below 500 MHz.
High-impedance active probes are the best selection below 500 MHz.
It is critical to remember that your probe’s impedance is not constant over frequency. Most of the inductance is created from the ground lead you chose for your probe. At DC and low frequency ranges, the probe’s impedance starts out at the rated impedance, but as the frequency goes up, inductance comes into play. The result is higher frequency ringing on your rising edge and across the top of your waveform. Figure 2 below shows the four different types of ground lead solutions’ stepped responses of a Keysight N2796A 2 GHz active probe. The three grounding solutions below decrease in inductance starting with the highest inductance in Case 1 to the lowest inductance solution shown in Case 4. Notice that the Case 4 black line solution has the least amount of overshoot and ringing.
Figure 2. An active probe’s stepped response with different accessories.
Resistive loading is the least likely to induce nonlinear or low amplitude behavior in your circuit. Your circuit’s output resistance and the probe’s own resistance form a voltage divider circuit. This divider circuit distorts the signal being measured because the probe is seen as a load to the circuit under test.
1:1 passive probes can cause resistive loading of you circuit under test above 500 MHz.
Passive and Active Probes
The higher the passive probe’s attenuation ratio, the lower the capacitive loading will be. 1:1 passive probes have capacitive loading around 100 pf, while a 10:1 probe is around 10 pf. But there is a tradeoff here. 1:1 probes transfer lower noise levels to the oscilloscope. 10:1 passive probes get both their signal and noise amplified by 10x because the oscilloscope accounts for the fact that the probe output is one tenth of the actual measured signal.
10:1 passive probes increase the noise level on the oscilloscope because both the signal and noise floor are amplified by the oscilloscope.
Active probes are another way to reduce probe loading. They have around one tenth the input capacitance of passive probes. Active probes can achieve this lower tip capacitance due to the active circuit at the tip of the probe. See these active vs. passive probe relationships below in Table 1 to aid you in your probe selection.
|Active probes||Passive probes|
|Low loading||Higher resistance|
|High bandwidth||High dynamic range|
|Least intrusive||Low cost|
|Higher cost||Bandwidth limited to 500 MHz|
|Limited input dynamic range||Heavy capacitive loading|
Table 1. Passive vs. active probe selection.
For faster frequency of rise time signals, use active probes with lower capacitive loading.
Higher-end oscilloscopes use digital signal processing to help compensate for probe loading but do not eliminate probe loading altogether. To minimize loading, you need to factor your design parameters with the impedance values of the probe you are using.
All probes have some type of impact on your circuit under test. It is up to you to determine what is most important for your tests. Understanding some of the common pitfalls helps you select the right probe. A probe draws a portion of the circuit energy and supplies this energy to the oscilloscope. All probes present a capacitive, resistive, and inductive loading element to your circuit. In order to avoid using a probe that adversely impacts your circuit and changes the signal from its original state, you need to factor in the probe’s resistive, capacitive, and inductive characteristics with the properties of your design.
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Probe impedance changes with frequency –
The bigger the probe resistance and smaller the probe capacitance, the less the loading your probe will have.