Multimeters – Scope/Meters

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test probe (test leadtest prod, or scope probe) is a physical device used to connect electronic test equipment to the device under test (DUT). They range from very simple, rugged devices to complex probes that are sophisticated, expensive, and fragile.

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Voltage probes

Voltage probes are intended to measure or display voltages on the DUT. Ideally, the test instrument and its probe will not affect the voltage being measured. Practically, that translates into the test instrument and its probe presenting a high impedance that will not load the DUT. In many situations, an impedance with a resistive component of a megohm is adequate. For AC measurements, the reactive component of impedance may be more important than the resistive.

Simple test leads

Voltmeter probes usually consist of single wires that are equipped on one end with a connector that fits the user’s voltmeter and on the other end with a rigid plastic section (the probe itself) that allows the user to safely hold the probe while being protected from the danger of electric shock. Within the plastic body of the probe, the wire is connected to a rigid, pointed metal tip that makes the actual contact with the DUT.

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Oscilloscope probes

Oscilloscopes display the instantaneous waveform of varying electrical quantities, unlike other instruments which give numerical values of relatively stable quantities. Oscilloscopes, and other instruments which must acquire an accurate representation of a high-frequency signal use shielded cables. Open wire test leads (flying leads) are likely to pick up interference, so they are not suitable for low-level signals. Furthermore, the inductance of the leads is not negligible at high frequencies, making them unsuitable for this use. Using a shielded cable (i.e., coaxial cable) is better for low-level signals.

Active scope probes

Active scope probes use a high-impedance high-frequency amplifier mounted in the probe head, and a screened lead. The purpose of the amplifier is not gain, but isolation (buffering) between the circuit under test and the oscilloscope and cable, loading the circuit with only a low capacitance and high DC resistance, and matching the oscilloscope input. Active probes are commonly seen by the circuit under test as a capacitance of 1 picofarad or less in parallel with 1 megohm resistance. Probes are connected to the oscilloscope with a cable matching the characteristic impedance of the oscilloscope input. Tube based active probes were used before the advent of high-frequency solid-state electronics, using a small vacuum tube as cathode follower amplifier.

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, and they can be damaged by overvoltage, either from the signal or electrostatic discharge.

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Current probes

A current probe generates a voltage proportional to a current in the circuit being measured; as the proportionality constant is known, instruments that respond to voltage can be calibrated to indicate current. Current probes can be used both by measuring instruments and oscilloscopes.