Model 6517B Electrometer Reference Manual
Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015
4-35
Measurement times
Long measurement times may degrade charge measurement accuracy. See the Model 6517B
coulombs Specifications, available on the
Keithley Instruments support website
(
http://www.tektronix.com/keithley
).
Zero check hop and auto discharge hop
Using the zero check feature (going from the enabled state to the disabled state) causes a sudden
change in the charge reading and is known as zero check hop. This sudden change in charge also
occurs when the auto discharge feature resets the charge reading to zero. This hop in charge can be
eliminated by taking a reading the instant zero check is disabled or when an auto discharge occurs,
and subtracting it from all subsequent readings. A better way to deal with this hop in charge is to
enable REL immediately after zero check is disabled or when auto discharge resets the charge
reading. This action nulls out the charge reading caused by the hop.
Other measurement considerations
The following measurement considerations apply to all precision measurements. For comprehensive
information on all measurement considerations, refer to the
Low Level Measurements Handbook
,
which is available from Keithley Instruments.
Ground loops
Ground loops that occur in multiple-instrument test set-ups can create error signals that cause erratic
or erroneous measurements. The configuration shown in the following figure introduces errors in two
ways. Large ground currents flowing in one of the wires encounter small resistances, either in the
wires, or at the connecting points. This small resistance results in voltage drops that can affect the
measurement. Even if the ground loop currents are small, magnetic flux cutting across the large loops
formed by the ground leads can induce sufficient voltages to disturb sensitive measurements.
Figure 51: Multiple ground points create a ground loop
