Cable Considerations; Reduction In Gain-Bandwidth - Keithley 2651A Reference Manual

Section 4: Theory of operation

Cable considerations

The Model 2651A is supplied with a 1 m (3 foot) low inductance, 6 Ω cable. The supplied cable
addresses several important considerations:
•
Inductance
•
Power dissipation
•
Voltage drop at high DC current
This cable decreases the inductance as seen from the Model 2651A terminals to the load. This
reduction assures specified slew rates and minimal overshoot, which is especially true when
operating the Model 2651A in pulse mode at high levels of output current and voltage. The induction
equation: V = Ldi/dt illustrates the need for controlling inductance. In this equation, when operating
the Model 2651A in source I mode for any given inductance L, the voltage (V) represents the
overhead voltage required to be applied across the inductance of the cable and load connections in
order to maintain the specified di/dt. With a cable inductance of 5 μH, and a 50 A step in current in
30 μs:
V = Ldi/dt
= (5 μH)(50 A)/30 μs
= 8 V
Therefore 8 V of overhead (4 V for the Hi lead and 4 V for the Lo lead) is required in addition to the
device under test (DUT) DC load voltage at 50 A. This 8 V is too high for the remote sensing circuits
(3 V Max/lead). In addition, the 10 V range does not have the power supply overhead to provide this
8 V (though the 40 V range does). In this example, the load/cable inductance is therefore too high and
must be reduced. Using the supplied Model 2651A 6 Ω cable helps mitigate these problems since it
contributes less than 200 nA/meter.
The low resistance cable supplied will dissipate only 3.6 W when operating at 20 A DC; 3.6 W is not
excessive for a cable of this size.
The low resistance of the supplied cable will mitigate voltage drop due to operation at high DC
currents. This characteristic will leave more overhead voltage available for the actual connection to
the DUT.
Be careful to consider each issue outlined above when you make your own connections to your
device.

Reduction in gain-bandwidth

The settling time of the source-measure unit (SMU) can be influenced by the impedance of the device
under test (DUT) in several ways. One influence is caused by an interaction between the impedances
of the SMU current source feedback element and the DUT. This interaction can cause a reduction in
gain-bandwidth. When the SMU gain-bandwidth is reduced, the settling time of the current source
increases.
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Model 2651A High Power System SourceMeter® Instrument Reference Manual
2651A-901-01 Rev. A / March 2011