Choosing the right Pico Technology
active differential probe
Application Note
Pico Technology offers many active differential probes covering a wide range of voltages, category (CAT) ratings
and bandwidths. As the name suggests, these probes have two major features:
• Active: Active probes achieve low input capacitance and high sensitivity by buffering and amplifying the
signal close to the point of measurement.
• Differential: Differential probes present two non-grounded terminals for connection to the measurement
and can thus measure accurately without directly connecting one test node to ground or drawing signicant
current from either test node.
This ability to measure without a low-impedance connection improves safety when measuring high voltages or in
the presence of high voltages.
Before we look at choosing the correct probe for your application, let’s go back a step and explain why you would
want to use a differential probe.
Choosing the right Pico Technology
active differential probe
Application Note
1. The problem: signal with unwanted common-mode noise
Consider the circuit below. It consists of a signal source (purple) with AC and DC components producing a total
output of V
SIG
, which is the signal we wish to measure. In addition, the circuit contains an unwanted voltage
source (green) that also has AC and DC components adding up to V
CM
. V
CM
stands for common-mode voltage,
which means a signal that is added equally to both measurement terminals.
As the following diagram shows, viewing the output and ground terminals with a single-ended probe would
result in a distorted waveform (V
SUM
) on the display. We cannot solve this by connecting the probe ground to the
negative terminal of V
SIG
, as that would short-circuit V
CM
to ground through the oscilloscope and possibly cause a
damaging current to ow. We need a probe that can safely detect V
SIG
and ignore V
CM
.
0 V
V
CM
V
SIG
V
SIG
V
CM
V
SUM
0 V
V
SUM
Figure 1 - Using a single-ended oscilloscope probe