Agilent Technologies B1500A Training Manual page 105

ULTRA LOW CURRENT
Effect of Cable Charging Current
You can see an initial high current that drops off after a few measurements. This is the effect when
delay time is set to zero. Any residual capacitance due to un-guarded probes is the cause. This effect
is minimal in this case due to the use of fully guarded probes to within 2 mm of the probe tip. With
unguarded probes this initial charging current can be orders of magnitude larger, and dominate the
low end portion of the subthreshold curve.
The solution is to improve the guarding as much as practical. Then get the last bit of charging out of
the curve by using a delay time of at least 3 seconds in the Measurement Setup. This gives the cable
time to discharge prior to making the first measurement.
Not part of the subthreshold characteristic
6-33
Module 6
Low Current Measurement