Capacitive Reverse Currents - Stanford Research Systems SIM954 Operation And Service Manual

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Figure 3.2: The SIM954 driving a 1 nF ce-
ramic capacitor with a 2 MHz square wave
to 20 Vpp

3.2.1 Capacitive Reverse Currents

When driving fast risetime pulses into small capacitive loads, cable
inductance can lead to resonant peaking, as shown in figure 3.2. If
flat frequency response below the RC-corner frequency is important,
cable lengths and impedances have to be carefully matched to the
application. For larger capacitors and electrically short connections,
these e ects are not important, and the waveforms are similar to
those of a pure RC low pass filter as seen in figure 3.3.
Every capacitor stores a charge equivalent to the product of the ap-
plied voltage and its capacitance. This charge can cause a reverse
current flow if the amplifier is turned o while it remains connected
to a charged capacitor. Since the SIM954 does not guarantee by de-
sign that this reverse current won't harm the amplifier or the SIM900
mainframe, caution should be used with circuits which drive large
capacitive loads or even electrochemical cells like batteries which can
store very large amounts of charge.
If a large reverse current (
powered SIM954 the user should consider adding a relays contact
between the module's output and the load. The relays coil can be
powered by the mainframe's 5, 15 or 24 V or the user supplied
voltage to close the circuit only when the SIM954 is under power.
SIM954 300 MHz Dual Inverting Driver Amplifier
TDS 3034 24 Jul 2008 14:44:12
Figure 3.3: The SIM954 driving a 100 nF ce-
ramic capacitor a with 100 kHz square wave
to 20 Vpp
10 mA for 1 s) may flow into an un-
Application notes