Chapter3 DigitizerBasics
NI5102 User Manual 3-8 ni.com
Understanding the Probe and Its Effectson Your Waveform
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
electricalsystem component that can severelyaffect the accuracy of the
measurement.A p robecan potentially influence measured amplitude and
phase,an d the signal can pick up additional noise on its way to the input
stage.Several typesof probes are available including passive, active,
and current probes.

Passive Probe

The passive probe is the most widely used general-purpose oscilloscope
probe.Passive probes are specified by bandwidth (or rise t ime), attenuation
ratio,compensation range, and mechanical designaspects. 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
capacitanceimproves the probe’s bandwidt han drise tim e.F igure 3-6
showsa typical X10 probe model. You should ad justt he tunable capacitor,
Cp,t o obtain aflat frequency response. Cpis the probe capacitance, Rpis
theprobe resistance, Cin is the input capacitance, Rin is the input resistance.
Figure3-6. Typical X10 Probe
Probe Compensation
Adjustment
One-tenth of Signal
(1 V
pk-to-pk
) at Input
C
c
= Lumped Cable
Capacitance
X10 Probe Digitizer Input
20 pF
C
in
x
pF C
p
10 V
pk-to-pk
Signal