Active scope probes use a small, usually FET-based amplifier mounted directly within the probe head, and a screened lead. By doing this they are able to obtain exceptionally low parasitic capacitance and high DC resistance (it is common to see capacitance of 1 picofarad or less with 1 megohm resistance). They are connected to the oscilloscope in the same fashion as passive Z0 probes (using 50 ohm coaxial cable terminated at the oscilloscope's input). Active probes have several disadvantages which have kept them from replacing passive probes for all applications:
They are several times more expensive than passive probes.
They require power (but this is usually supplied by the oscilloscope).
Their dynamic range is limited, sometimes as low as 3 to 5 volts.
They can be damaged by overvoltage, either from the signal or electrostatic discharge.
To overcome their often-limited dynamic range, many active probes allow the user to introduce an offset voltage. The total dynamic range is still limited, but the user may be able to adjust its centerpoint so that voltages in the range of, for example, zero to five volts may be measured rather than -2.5 to +2.5. Because of their inherent low voltage rating, there is little need to provide a large amount of insulation to ensure operator safety against electric shock. This allows the probe heads on active probes to be extremely small, making them very convenient for use with modern high-density electronic packaging. Because of their size and excellent electrical characteristics, they are strongly preferred for troubleshooting digital electronics. Before the advent of high-performance solid-state electronics a very few active probes were built using small vacuum tubes as the amplifiers. At extreme high frequencies a modern digital scope requires that the user solder a preamp to the DUT to get 50GS/s, 20 GHz performance.