Keithley 6220 Reference Manual page 86

3-8 DC Current Source Operation
Model 6221 fast mode:
For peak-to-peak voltage swings <100Vpk-pk:
ST
Where:
ST is the settling time in microseconds.
Ifs is the full scale of range current in amps.
Rl is the load resistance in ohms.
For peak-to-peak voltage swings >100Vpk-pk (output slew rate limitation based
upon 5V/µs):
ST
Where:
ST is the settling time in microseconds.
Ifs is the full scale of range current in amps.
Rl is the load resistance in ohms.
Vpp is the peak-to-peak voltage step (100V < Vpp < 210V).
200nA, 20nA, and 2nA ranges (Models 6220 and 6221)
The step response for these ranges is largely determined by the impedance of the
load, which is primarily made up of the load resistance and parasitic load capaci-
tance. The construction of the DUT may also contribute capacitance and/or
inductance.
Cable capacitance can also contribute to settling time. Using a guarded triax cable
(Cable Guard) can significantly reduce the effects of cable capacitance but will not
eliminate it. For details, see
Another typical source of capacitance and leakage resistance is the test fixture.
These effects can be significantly reduced by using a guard plate in the test fix-
ture. For details, see
Even if a load has zero capacitance, there will still be approximately 10pF of
capacitance present for these low current source ranges. This is capacitance that
is inherent to the Model 622x and cannot be reduced or eliminated by guarding or
any other technique.
The following equation to calculate settling time assumes the use of short,
guarded cables. It also assumes that the shunt capacitance at the load is known.
ST
Where:
ST is the settling time in microseconds.
Rl is the load resistance in ohms.
C is the load capacitance plus 10pF (inherent to the 622x).
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176 - TestEquipmentDepot.com
( × ×
log 1 + 0.5 Ifs Rl
10
=
( × ×
log 1 + 0.5 Ifs Rl
10
=
"Triax Cable Guard," on page
"Guarded DUT mounting plate," on page
6
× × ×
= 5 10 Rl C
Return to
Model 6220/6221 Reference Manual
)
)
( ) 0.2 ×
Vpp 100
+ –
Section 3 topics
2-9.
2-11.