2400 Series SourceMeter
®
User’s Manual Relative and Math 8-5
This two-point measurement method is mathematically expressed as:
Offset-Compensated Ω = ∆V / ∆I where ∆V = V2 – V1 and ∆I = I2 – I1.
• V1 is the voltage measurement with the I-Source set to a specific level.
• V2 is the voltage measurement with the I-Source set to a different level (typically
zero).
• I1 is the current measurement with the I-Source set to a specific level.
• I2 is the current measurement with the I-Source set to a different level (typically
zero).
You will be prompted to enter the two I-Source values, and the results is displayed in ohms
with the “Ω” symbol in the units field of the display.
Varistor alpha
This math formula is used to determine varistor alpha (α), an important parameter that
defines varistor characteristics. Alpha is defined as 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 val-
ues. (See “Front panel math operations” later in this section.)
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 opera-
tions,” page 8-6.)
α
I2 I1⁄()log
V2 V1⁄()log
--------------------------------=
Coefficient%
∆R
R2 ∆V×
----------------------= 100%×
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