Keithley 6220 Reference Manual page 446

Model 6220/6221 Reference Manual
Shunt output with a capacitor
With little or no output capacitance, the output voltage of the Model 6221 can rise
at a rate of 5V/µs. This fast slew rate (dv/dt) is a primary cause of the overshoot.
Once the overshoot reaches the compliance setpoint, it takes approximately 1µs
for the compliance circuit to respond. This allows the output voltage to overshoot
(up to ~2V) before the compliance circuit can pull it down to its normal operating
level.
Slowing down the slew rate by adding a capacitor across the output will allow the
compliance circuit to respond fast enough to clamp the output voltage without any
significant overshoot into the compliance region.
The slew rate (dv/dt) is calculated as follows:
dv/dt = I / C
Where: I is the output current
For example, assume the output current is 100µA and a 1nF capacitor is placed
across the output. The slew rate is calculated as follows:
dv/dt = I / C
= 100µA / 1nF
= 0.1V/µs
This much slower slew rate (0.1V/µs versus 5V/µs with no capacitor) will allow the
compliance circuit to respond quickly enough to clamp the overshoot.
Always use the smallest possible capacitor that will meet your needs. Using a
capacitor that is too large may slow down the slew rate to an unacceptable level.
CAUTION
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C is capacitance placed across the output
A capacitor is an energy storage device. If the output is
shorted, a large current spike could be delivered to the load.
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Applications E-13