Section 4: Sourcing and measuring Model 2601B-PULSE System SourceMeter Instrument Reference Manual
4-80 2601B-PULSE-901-01A April 2020
High-capacitance mode
The Keithley Instruments 2601B-PULSE has a high-capacitance mode.
Because the source-measure unit (SMU) can measure low current, issues can arise when driving a
capacitive load. The pole formed by the load capacitance and the current range resistor can cause a
phase shift in the SMU voltage control loop. This shift can lead to overshoot, ringing, and instability.
Due to the large dynamic range of current measurement and wide range of internal resistors, the
operating conditions for a given capacitive load can vary.
Based on the type, some test applications may require capacitors larger than 10 nF. While running
test scripts, it may not be possible to disconnect the capacitor from the IC (integrated circuit) and
extract accurate data. For this purpose, you can use the high-capacitance mode to minimize
overshoot, ringing, and instability.
This section provides the details that you need to estimate performance based on load capacitance
and measurement conditions.
High-capacitance mode is not available when the pulser is enabled.
Understanding high-capacitance mode
The source-measure unit (SMU) in the 2601B-PULSE drives 10 nF of capacitance in normal
operation. Typically, an internal capacitor across the current measuring element provides phase lead
to compensate for the phase lag caused by the load capacitance on the output. This internal
capacitance across the range resistance limits the speed for a specific measurement range.
The SMU in the 2601B-PULSE implements frequency compensation to achieve the highest
throughput possible for a 10 nF or less load. In addition, you must consider the settling time, voltage
range, measure delay, capacitor quality, the current measure range resistor, and the load resistor.
In normal operation, the SMU in the 2601B-PULSE can drive capacitive loads as large as 10 nF. In
high-capacitance mode, the SMU can drive a maximum of 50 μF of capacitance.
When high-capacitance mode is enabled, a minimum load capacitance of 100 nF is recommended.
In absence of this minimum load capacitance, overshoot and ringing may occur.
Highest throughput is achieved by using normal operation. In high-capacitance mode, the speed of
the 2601B-PULSE 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.
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