Section 6: Optimizing SMU measurements
Model 4200A-SCS Source-Measure Unit (SMU) User's Manual
6-8
4200A-SMU-900-01 Rev. A December 2020
Contamination and humidity
Error currents can arise from electrochemical effects when ionic chemicals create weak batteries
between two conductors on a circuit board. For example, if epoxy-printed circuit boards are not
thoroughly cleaned of etching solution, flux, or other contamination, the contamination can generate
currents of a few nanoamps between conductors.
High humidity and ionic contamination can dramatically reduce insulation resistance. High-humidity
conditions occur with condensation or water absorption. 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 relative humidity to less than 50 percent. Also be sure that all insulators are kept
clean and free of contamination. If insulators become contaminated, clean them thoroughly with a
pure solvent such as methanol. To clean ionic contamination, use a deionized (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 that are 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
M
), with zero internal resistance, in series with a resistance (R
M
). When a current source is
connected to the input of the ammeter, the current is reduced from what it would be with the ideal
resistance meter (R
M
= 0 Ω). This reduction is caused by R
M
, which creates an additional voltage drop
called the voltage burden (V
BURDEN
), which reduces the measured current from its theoretical value as
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.
