Don’t throw out that passive probe just yet

In today’s world of high-speed circuits, the active voltage probe has become the “go to” tool for connecting to signals. Active probes provide wider bandwidths and lower capacitive loading, making them a good choice for measuring high-frequency signals or high-impedance circuits. They are often the probe of choice when measuring frequencies in the lower frequency ranges, as well, because the passive probes that normally ship with mid-bandwidth oscilloscopes have lacked performance.

But technology never stands still. Such is the case with passive-probe technology. Circuitry within the oscilloscope, improvements in the probe’s bandwidth, input capacitance, and the development of automated probe compensation have combined to turn traditional passive-probing disadvantages into advantages.

Historically, general-purpose passive probes have favored ruggedness over performance. This trade-off has long sufficed because these probes have been used mainly to visualize low-speed signals. This trade-off has also been made because of the significant design challenges in creating a probe that is rugged, high performance, and capable of measuring hundreds of volts.

Active probes typically start at 1-GHz bandwidth, measure less than 20V, and lack the robustness of a passive probe. Passive probes have typically been 500 MHz or less, measure hundreds of volts, and are rugged. With the newer passive-probe technology bandwidths of up to 1 GHz are available while still maintaining a high-voltage measurement capability and the ruggedness required for day-to-day use.

A probe’s input capacitance and input resistance at the probe tip specification are important because they affect the circuit under test. When an external device, such as a probe, is attached to a test point, it will appear as an additional load on the signal source, drawing current from the circuit. This loading, or signal current draw, changes the operation of the circuitry behind the test point and changes the signal seen at the test point. The ideal probe would have infinite impedance, but this is not possible because a probe must draw some small amount of signal current.

The challenge is to keep loading as low as possible. The greatest concern is capacitance at the probe tip. For low frequencies, this capacitance has a reactance that is very high and has little or no effect on the circuit under test. As frequency increases, the capacitive reactance decreases, and at higher frequencies, the capacitive loading is higher. Capacitive loading affects the bandwidth and rise-time characteristics of the measurement system by reducing bandwidth and increasing rise time. New passive probes offer <4-pF input capacitance at the probe tip, compared with the ≥9.5 pF input capacitance typically offered by a standard passive probe. With these new passive probes, users can now attach longer ground leads without suffering signal degradation from probes with higher input capacitance.

Because the circuitry of the scope and probe are tied together, set-up time with the new passive probe can be reduced significantly. Previously, due to variations in the probe and oscilloscope input characteristics, general-purpose passive probes required low-frequency compensation. This step is often forgotten or skipped, leading to inaccurate signal acquisitions. A new passive probe can complete automated low-frequency and high-frequency compensation in less time than it takes to manually adjust a standard passive probe for low-frequency compensation. Generally, it’s as simple as choosing the compensation routine from the oscilloscopes set-up menu.

Before grabbing your “go to” active probe, it might be worth revisiting the latest passive probes. With lower input capacitance and wider bandwidths than previous models, the new passive probes bridge the gap between general-purpose passive probes and higher-cost active probes.