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5-2
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2600AS-901-01 Rev. B / September 2008
Section 5: High-Capacitance Mode
Series 2600A System SourceMeter® Instruments Reference Manual
Overview
The Keithley Instruments Series 2600A System SourceMeter
®
instrument features a high-
capacitance mode.
Because the source measure unit (SMU) has the ability to 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 10nF. 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.
Understanding high-capacitance mode
Each SMU in the Series 2600A drives 10nF 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.
It is important to note that each SMU in the Series 2600A implements frequency compensation to
achieve the highest throughput possible for a 10nF or less load. In addition you must consider the
settling time, voltage range, measure delay, the quality of the capacitor, the current measure range
resistor, and the load resistor.
In normal operation, each SMU in the Series 2600A can drive capacitive loads as large as 10nF. In
high-capacitance mode, each SMU can drive a maximum of 50
μ
F of capacitance.
NOTE NOTE: When high-capacitance mode is enabled, a minimum load
capacitance of 100nF is recommended. In absence of this minimum
load capacitance, overshoot and/or ringing may occur.
Highest throughput is achieved by using normal operation. In high capacitance mode, the speed of
the Series 2600A SMU is reduced in order 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.
Understanding source settling times
Each Series 2600A SMU can drive up to 50
µ
F of a capacitance in high-capacitance mode. In order
to accomplish this, the speed of the Series 2600A SMU is reduced. Source settling times increase
when high-capacitance mode is enabled.
compare the source settling
times for the Series 2602A, 2612A, and 2636A in normal and high-capacitance modes.