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Probes work with digitizers as part of your measurement system. Signals travel from the tip of the probe to the input amplifier and are then digitized by the ADC. This signal path makes the probe an important electrical system component that can affect the accuracy of the measurement. A probe can potentially influence measured amplitude and phase, and the signal can pick up additional noise on its way to the input stage. Although NI high-speed digitizers do not ship with probes, several types of probes are available, including passive, active, and current probes.
The passive probe is the most widely used general-purpose probe. Passive probes are specified by bandwidth (or rise time), attenuation ratio, compensation range, and mechanical design aspects. Probes with attenuation, X10, X100, or X1000, have a tunable capacitor that can reduce capacitive effects at the input. The ability to cancel or minimize effective capacitance improves the probe's bandwidth and rise time. The following figure shows a typical X10 probe model.
Adjust the tunable capacitor, Cp, to obtain a flat frequency response. Cp is the probe capacitance, Rp is the probe resistance, Cin is the input capacitance, Rin is the input resistance of the digitizer.
Analytically, obtaining a flat frequency response means that
Rin/(Rin + Rp) = Cp/(Cp + Cin + Cc)
When tuned for flat response, it can be shown that
Rin(Cin + Cc) = CpRp
or the time constant of the probe equals the time constant of the digitizer input.
Active probes, such as differential and field-effect transistor (FET) probes, contain active circuitry in the probe itself to reject noise and amplify the signal. FET probes are useful for low-voltage measurements at high frequencies and differential probes are noted for their high common-mode rejection ratio (CMRR) and nongrounded reference.
Current probes magnetically measure AC and/or DC current flowing in a conductor instead of using a series resistance in the loop to measure current. This lack of series resistance causes very little interference in the circuit being tested.