Keithley 2657A, Справочное руководство, Страница: 178 / 805

Section 3: Functions and features Model 2657A High Power System SourceMeter® Instrument Reference Manual
3-64 2657A-901-01 Rev. B/December 2012
Understanding high-capacitance mode
The source-measure unit (SMU) in the Model 2657A drives 100 nF of capacitance in normal
operation. Typically, an internal capacitor across the current measuring element provides a phase
lead that compensates for the phase lag caused by the output's load capacitance. 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 2657A implements frequency compensation to
achieve the highest throughput possible for a 100 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 2657A can drive capacitive loads as large as 100 nF. In
high-capacitance mode, the SMU can drive a maximum of 10
μ
F of capacitance.
When high-capacitance mode is enabled, a minimum load capacitance of 100 nF is recommended. In
the 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 Model 2657A SMU is reduced to compensate for the larger load capacitance. Stability is achieved
by placing 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 measure delay, quality of the load capacitor, and load resistance. High-capacitance
mode settings apply to all enabled current ranges.
Understanding source settling times
The Model 2657A source-measure unit (SMU) can drive up to 10
μ
F of capacitance in
high-capacitance mode. In order to accomplish this, the frequency compensation capacitance across
the measure range resistors increases. This increase causes 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.
Adjusting the voltage source
When driving large capacitive loads with high-capacitance mode enabled, the programmed current
limit affects the voltage step response time.
If a 1
μ
F capacitor charges to 100 V in 1 ms with a 100 mA limit and the limit is changed to 1
μ
A, the
charging time increases to 100 seconds (see the following equation).
NOTE
i C
Δ
v
Δ
t
------
=