Teledyne Lecroy Everywhereyoulook ZS4000, User Manual, Page: 16 / 32

ZS4000 High-Impedance Active Probe
14
Capacitive Loading
Capacitive loading is usually the most troublesome of the three loading effects.
It can affect the rise time, bandwidth and delay time measurements. At higher
frequencies the capacitive loading can affect the amplitude as well as the
waveshape of the measured signal by introducing an exponential response to
the waveform.
For a simple RC network the time constant of this exponential response is:
𝑡𝑡
𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
= 2.2 × 𝐿𝐿
𝑡𝑡𝑟𝑟𝑡𝑡𝑟𝑟𝑡𝑡
× 𝑅𝑅
𝑡𝑡𝑟𝑟𝑡𝑡𝑟𝑟𝑡𝑡
Where C
total
is the combined probe and circuit capacitance and R
total
is combined
circuit and probe resistance.
For a setup where Ct = 0.6 pF and a source resistance is 250
Ω, the measured
rise time will be 330 ps, which will correspond to a bandwidth of 909 MHz,
assuming no inductive loads.
𝑡𝑡
𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
= 2.2 × 0.6 × 10
−12
× 250 Ω = 330 ps
Parallel combination of 250 Ω and 1 M Ω is still 250 Ω
Probe input equivalent circuit
To illustrate the effect of capacitive loading at higher frequencies:
•
At frequency 750 MHz the reactance of the 0.6 pF capacitance is 354 Ω
• At frequency 1.0 GHz the reactance has been lowered to 265 Ω
If, at a given frequency, the source impedance is large with respect to the input
impedance, a measurable reduction in the output signal amplitude may occur.
𝑉𝑉
𝑟𝑟𝑜𝑜𝑡𝑡
=
𝑍𝑍
𝑝𝑝𝑟𝑟𝑟𝑟𝑝𝑝𝑟𝑟
𝑍𝑍
𝑝𝑝𝑟𝑟𝑟𝑟𝑝𝑝𝑟𝑟
+ 𝑍𝑍
𝑟𝑟𝑟𝑟𝑜𝑜𝑟𝑟𝑟𝑟𝑟𝑟
× 𝑉𝑉
𝑟𝑟𝑟𝑟