|Figure 1: Active oscilloscope probes|
sport high resistance and low
capacitance at their tips, but
terminate into a scope's 50Ω input.
So what's the main difference between a passive probe and an active probe? Basically, the passive probe contains no active components that must be powered, while the active probe contains an amplifier near the probe tip, most commonly based on a transistor or FET. Such probes typically provide higher overall impedance than passive types, presenting high resistance to DC voltages and low-frequency signals and low capacitance to high-frequency signals. Active probes have high resistance at the probe tip but terminate into the 50Ω input of the oscilloscope (Figure 1).
|Figure 2: An impedance vs. frequency plot for a high-|
impedance active probe
Meanwhile, active FET probes provide high impedance from DC to 20 kHz, maintaining that impedance out to about 1.5 GHz thanks to their low capacitance (Figure 2). FET probes, then, are truly general-purpose probes that can be used at nearly all frequencies. Their low capacitive loading makes them usable on high-impedance circuits that would suffer severely from loading with passive probes.
So based on all of the foregoing, where does it make sense to use a passive probe vs. an active probe and vice versa? Both types have their strengths and weaknesses, but knowing which to use where can go a long way toward avoiding misleading measurement results:
- Passive probes are an excellent choice for low-frequency measurements, especially if high voltages may be encountered
- Active FET probes are better suited for measurements requiring high bandwidths
- Active FET probes are a great general-purpose choice for all frequencies out to the multi-GHz range, but watch out for higher voltages, which could damage the probe amplifier.
In the next installment of this series, we'll look at differential probes.