Keithley 6430 Instruction Manual page 161

Measuring high resistance devices — When using offset-compensated ohms to measure
high resistance values, an appropriate source delay must be used to provide settled readings.
There is a rise time associated with high ohms measurements. For normal ohms measurements,
you can watch the reading change on the display. When it stops changing, you know you have
the final, settled reading. For offset-compensated ohms, this process is not as straight forward
since the source is constantly changing between two values. If measurements are performed
while the source is still rising (or falling), incorrect offset-compensated ohms readings will
result. Therefore, it is imperative that an adequate source delay be used to make sure that mea-
surements occur while the source is at its final, settled values.
Settling times are drastically different from one type of resistor to another. Another factor
that affects settling time is the test setup (i.e., cabling, fixturing, and guarding). These variables
make it necessary for the user to characterize his test system to assure that the source delay set-
ting is adequate.
NOTE Source delay is set from the source configuration menu (press CONFIG > select
SOURCE I (or V) > select DELAY). See "Source delay" in Section 3 for details.
Varistor alpha
This math formula is used to determine ALPHA (α), which is the logarithmic ratio of two
voltage measurement points on a non-linear V-I curve and is expressed as follows:
where: V1 is the voltage measurement at the first I-Source point.
V2 is the voltage measurement at the second I-Source point.
The log (x) function uses the absolute value of x.
When configuring this math function, you will be prompted to enter the two I-source values.
See Front panel math operations.
Voltage coefficient
High value or high-megohm resistors exhibit a change in resistance with a change in applied
voltage. This effect is known as voltage coefficient. The voltage coefficient is the percent
change in resistance per unit change in applied voltage and is defined as follows:
where: ∆R = R2 - R1
∆V = V2 - V1
R1 is the resistance measurement at the first source point.
R2 is the resistance measurement at the second source point.
V1 is the voltage measurement at the first source point.
V2 is the voltage measurement at the second source point.
If sourcing voltage, you will be prompted to enter the two V-source values. If sourcing cur-
rent, you will be prompted to enter the two I-source values. See Front panel math operations.
log I2 I1
( ⁄ )
= ------------------------------- -
α
log V2 V1
( ⁄ )
∆R
Coefficient% = --------------------- -
R2 ∆V
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Relative and Math
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