Voltage Burden - Keithley 4200A-SCS Reference Manual

Section 12: Maintenance Model 4200A-SCS Parameter Analyzer Reference Manual
Contamination and humidity
Error currents also arise from electrochemical effects when ionic chemicals create weak batteries
between two conductors on a circuit board. For example, commonly-used epoxy-printed circuit
boards, when not thoroughly cleaned of etching solution, flux, or other contamination, can generate
currents of a few nanoamps between conductors.
Insulation resistance can be dramatically reduced by high humidity or ionic contamination. High-
humidity conditions occur with condensation or water absorption, while ionic contamination may be
the result of body oils, salts, or solder flux.
To avoid the effects of contamination and humidity, select insulators that resist water absorption
(such as Teflon), and keep humidity to <50% relative humidity. Also be sure that all insulators are
kept clean and free of contamination. If insulators become contaminated with, clean them thoroughly
with a pure solvent such as methanol. To clean ionic contamination, use a de-ionized (DI) water
wash.
Dielectric absorption
Dielectric absorption in an insulator can occur when a voltage across that insulator causes positive
and negative charges within the insulator to polarize. When the voltage is removed, the separated
charges generate a decaying current through circuits connected to the insulator as they recombine.
To minimize the effects of dielectric absorption on current measurements, avoid applying voltages
greater than a few volts to insulators being used for sensitive current measurements. In cases where
this practice is unavoidable, it may take minutes or even hours for the current caused by dielectric
absorption to dissipate.

Voltage burden

As shown in the next figure, the SMU or preamplifier ammeter may be represented by an ideal
ammeter (I ), with zero internal resistance, in series with a resistance (R ). When a current source is
M M
connected to the input of the ammeter, the current is reduced from what it would be with the ideal
= 0 Ω). This reduction is caused by R
resistance meter (R , which creates an additional voltage drop
M M
called the voltage burden (V ), which reduces the measured current from its theoretical value as
BURDEN
follows:
The percent error (E) in the measured reading due to voltage burden is:
If the voltage burden is 0 V, the percent error is zero.
Voltage burden for the SMUs is less than or equal to the offset specifications of the source voltage.
12-14 4200A-901-01 Rev. C / February 2017