High-Capacitance Mode; Overview; Understanding High-Capacitance Mode - Keithley 2651A Reference Manual

Model 2651A High Power System SourceMeter® Instrument Reference Manual

High-capacitance mode

Overview

The Keithley Instruments Model 2651A High Power System SourceMeter
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 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.

Understanding high-capacitance mode

The source-measure unit (SMU) in the Model 2651A 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.
It is important to note that the SMU in the Model 2651A 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, the quality of the capacitor, the current measure range
resistor, and the load resistor.
In normal operation, the SMU in the Model 2651A 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.
2651A-901-01 Rev. A / March 2011
NOTE
Section 3: Functions and features
®
Instrument feature a high-
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