Keithley 2651A Reference Manual page 173

Section 3: Functions and features
Highest throughput is achieved by using normal operation. In high-capacitance mode, the speed of
the Model 2651A SMU is reduced to compensate for the larger load capacitance. Stability is achieved
by inserting an internal capacitance across the current measuring element of the SMU. This internal
capacitor limits the speed for the source and measurement ranges. Therefore, when optimizing the
speed of your test configuration in high-capacitance mode, you must consider the settling time,
voltage, and current ranges, measure delay, quality of the load capacitor, and load resistance.
High-capacitance mode settings apply when operating using the 100 nA through the 100 mA current
ranges. When operating using the 1 A, 5 A, 10 A, 20 A, or 50 A ranges, the high-capacitance mode
setting will not affect the instrument rise time or current measure settling time, because these ranges
are inherently immune to capacitive loads below 50 μF.
The Model 2651A is specified for operating into high Q inductances up to 3 μH on all ranges of
voltage and current.
Understanding source settling times
The Model 2651A source-measure unit (SMU) can drive up to 50 μF of a capacitance in
high-capacitance mode. In order to accomplish this, the speed of the Model 2651A SMU is reduced.
Source settling times increase when high-capacitance mode is enabled. The following table compares
the source settling times in normal and high-capacitance modes.
Models 2651A source settling times
Range
1 V
10 V
20 V
40 V
In high-capacitance mode, the frequency compensation capacitance across the measure range
resistors increases. This increase leads to longer settling times on some current measure ranges.
The same range elements that are used to measure current are used to source current. Therefore,
the current limit response times will respond in a similar manner.
When high-capacitance mode is enabled, the amount of time to change the current measure range
increases for each SMU. The current measure range and the current limit range are locked together.
Setting the current limit automatically updates the measure range.
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Model 2651A High Power System SourceMeter® Instrument Reference Manual
Normal mode High capacitance
(typical) mode
<50 μs 600 μs
<110 μs 1 ms
1.5 ms
<125 μs
<150 μs 7.5 ms
2651A-901-01 Rev. A / March 2011