Understanding High-Capacitance Mode - Tektronix KEITHLEY System SourceMeter 2636B Reference Manual

Series 2600B System SourceMeter® Instrument Reference Manual
High-capacitance mode
The Series 2600B instrument 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.

Understanding high-capacitance mode

The source-measure unit (SMU) in the Series 2600B 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 Series 2600B 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 Series 2600B 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 Series 2600B 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.
2600BS-901-01 Rev. E / January 2019
Section 4: Sourcing and measuring
4-83