App-3
IM 701923-01E
Appendix—Probing
Problems with Probing High-Speed Signals
Loading Effect
When a probe is connected to the circuit under test, the input impedance of the
probe itself has certain effects on the circuit. This is called the
loading effect
.
There can be cases where there are notable low pass filter effects caused by
the signal source impedance and the probe’s load capacitance, particularly
when observing frequency components of 100 MHz or more.
Let us take an example of a 50-
Ω
circuit shown below. In this example, the
equivalent signal source impedance appears as 25
Ω
(2 x 50
Ω
in parallel) when
observing the signal waveform at the load (terminal). The cutoff frequency at the
probe's input point is fc=1/2
π
RC, if the probe input capacitance is added. When
using a 14-pF input capacitance passive probe, fc equals 455 MHz, but when a
0.9-pF input capacitance active probe is used, fc equals 7 GHz.
We recommend that you use an active or FET probe with a smaller input
capacitance, because even more pronounced loading effects will emerge when
measuring a circuit with a high-output impedance.
R
S
= 50
Ω
Z
O
= 50
Ω
R
L
= 50
Ω
Probe
Signal
source
Resonance due to Inductance
Pins or wires of some sort are used to connect the probe to the circuit under
test. Inductance occurs when these pins or wires reach certain lengths, which
results in resonance with the probe’s input capacitance. This will not obscure
observation of the waveform if these resonant frequencies are outside of the
oscilloscope's frequency bandwidth. However, overshooting or ringing, which
was not originally present, will be introduced into the signal waveform being
observed if the inductance or capacitance is large.
The resonant frequency is surprisingly low when the resonant frequencies
of three different types of probes are compared as in Figure 1 with a given
connected inductance of 10 nH (a length of up to 1 to 2 cm). You must be aware
that effects of this inductance can be found not only on the probe input, but also
in the ground connection.
PB500
Passive Probe
PBA2500
Active Probe
PBL5000
Low Capacitance Probe
Input impedance
Approx. 14 pF
Approx. 0.9 pF Approx. 0.25 pF or 0.4 pF
Resonant frequency
(When L = 10 nH)
425 MHz
1.68 GHz
3.18 GHz
