Section 5: Pulse measure and pulse generator units
Model 4200A-SCS Parameter Analyzer Reference Manual
5-42
4200A-901-01 Rev. C / February 2017
PMU capacitive charging/discharging effects
During pulse transitions, the measured current charges and discharges the capacitance in the system
(see next figure, red waveform). This system capacitance consists of the cable capacitance, PMU
(with RPM, if connected) capacitance, and device capacitance. The next figure shows the pulse
waveform showing capacitive charging and discharging current waveform in relation to the applied
voltage waveform of the PMU connected to the supplied 2 m (6.5 ft) SMA cable (no DUT connected).
Figure 179: Capacitive charging and discharging
The setup used to generate these waveforms is shown in the previous figure, and also shows the
capacitance and the charging effect (red arrows) seen during pulse transitions. This setup shows a
single channel of a PMU, with the supplied 2 m (6.5 ft) white SMA cable connected to the channel
output. Note that the other end of the SMA cable is open (no connection).
The current shown in the previous figure is measured by the PMU, but is not flowing through a device
under test (DUT). The measured current is the sum of this charging or discharging current, as well as
the current flowing through the DUT. This current is primarily caused by the capacitance in the cable
and is described by the following equation:
I = C * dV/dt
Where:
•
I is the measured current
•
C is the capacitance
•
dV/dt is the pulse voltage amplitude divided by the rise (or fall) time
The equation shows that this effect is a function of the capacitance as well as the dV/dt. Therefore,
minimizing the capacitance will reduce this measurement artifact. The cabling is typically the largest
contributor to the system capacitance. Slowing down the pulse transitions will also reduce the height
of the current charging effect.
