Datasheet

PicoScope 4444 differential oscilloscope
Why make differential measurements?
While you can make a wide variety of measurements with an ordinary ground-referenced scope, there are some circumstances where that just won’t work.
Common-mode voltages are unwanted signals that are applied equally to both measurement terminals in your probing system. The circuit above consists of a signal source (purple) with AC and DC
components producing a total output of V
SIG
, which we wish to measure. However, the circuit also contains an unwanted voltage source (green) that also has AC and DC components adding up to V
CM
,
a common-mode voltage. This situation is quite common, for example when probing high-side drivers in amplifiers and power supplies.
As the diagram above shows, probing this circuit with a single-ended scope results in a distorted waveform (V
SUM
) on the display. We cannot simply connect the probe ground to the negative terminal of
V
SIG
, as that would short-circuit V
CM
to ground through the oscilloscope, possibly causing a circuit malfunction or damage to the instrument. We need a measuring system that can safely detect V
SIG
and
ignore V
CM
.
The solution, as shown below, is to connect a differential scope input across the positive and negative terminals of the signal source. The differential input does not measure V
CM
, only V
SIG
, so V
SIG
is what
you see on the oscilloscope display.
Differential scopes can measure the AC or DC voltage between two points connected to the positive and negative leads, when neither of the points is grounded. This enables them to take measurements
where single-ended scopes can’t, for example at voltages that are much higher than ground potential. The resulting measurements focus exclusively on the potential difference between the probes.
0 V
V
CM
V
SIG
V
SIG
V
CM
V
SUM
0 V
V
SUM
V
CM MIN
0 V
V
CM MAX
V
CM
V
SIG
V
SUM
V
SIG
V
CM
V
SUM
0 V
V
DIF MAX
V
DIF MIN
V
DIF