Brochure

4
Differential or single-ended measurement
Differential probes make measurements possible when
neither of the two test points is connected to ground. An
example is voltage measurements on components without
a connection to ground, which is necessary when charac-
terizing switching power supplies. Differential probes are
also needed for low-noise measurements on differential
signals.
Differential probes can in fact also be used for single-
ended measurements. Single-ended probes often offer a
higher input impedance, a lower input capacitance and the
advantage of a wider dynamic range.
Bandwidth and rise time
Bandwidth is one of the most important parameters when
selecting a probe. It defines the cutoff frequency after
which a signal will be displayed more than 3dB (approx.
30 %) weaker than it actually is. For an accurate signal
representation, the cutoff frequency of the measurement
system (oscilloscope and probe) must be greater than the
highest frequency component to be displayed. When mea-
suring digital signals, the measurement bandwidth should
be 3 to 5 times greater than the clock rate (For debugging
a digital design, a bandwidth that is 3 times greater is suf-
ficient. For conformance tests on digital interfaces, the
bandwidth must be 5 times greater than the clock rate.).
When measuring fast slopes, such as when characterizing
switching power supplies, the critical parameter is the rise
time of the measurement system (oscilloscope and probe).
For precise measurements, the rise time of the measure-
ment system should be a factor of 3 to 5 times lower than
the rise time of the pulse being measured.
Dynamic range
The dynamic range of a probe is defined as the maximum
measurable input voltage. It is specified for DC voltage and
often decreases as the frequency increases. In the case
of differential probes, a distinction is also made between
common mode and differential mode dynamic range. The
common mode dynamic range determines the valid in-
put voltage range for a single differential input, measured
with reference to ground. The differential mode dynamic
range defines the maximum measurable input differential
voltage.
Selecting the right
probe
The first step in selecting the right probe is to
analyze the measurement task. Is a single-ended
or a differential measurement needed? Which
maximum frequency components need to be
transferred? What is the maximum input voltage
that can occur?
Probes_and_accessories_bro_en_3606-8866-12_v1400.indd 4 23.07.2019 13:47:42