Range Resistor Voltage Coefficient - Keithley 261 Instruction Manual

X10.2 Compensating Equations
Since the picoammeter voltage drop can be found, the Model
261 output current can be corrected. Equation 6 gives the
output current value if the input voltage drop is the only
significant source of error and if the voltage drop is less than
l/l0 the source voltage.
la = I,(V, - vi1
Equation 6
where I8 is the actual output current from the Model 261;
Is is the range switch setting (reciprocal of range resistor
value);
V, is the Model 261 source voltage, read directly in volts
from the multiplier switch setting;
Vi is the picoammeter input voltage drop in volts.
Using this correction, accuracy is nominally f0.5% (Model
261 accuracy). This error results from the factory calibration
method used for the Model 261 that adjusts the source volt-
age from that indicated to match the range resistor used.
For greater accuracy, or if the input drop is more than 0.1
times the source voltage, use equation 8 to compensate for
the error. The following equation is based on the calibration
procedures.
v,, = v,IR,I)= V,R,,l,
Equation 7
l/Is
where V,, is the actual source voltage;
V, is the Model 261 source voltage, read directly in volts
from the multiplier switch setting;
I?,, is the actual source resistance from the calibration cer-
tificate supplied with the Model 261;
Is is the Model 261 range switch setting (reciprocal of range
resistor value).
Using equation 7 in equation 6 and substituting V,, for V,,
Ia= I,lI,V,R,, -vi1
Equation 8
where Ia is the actual output current from the Model 261;
Vi is the picoammeter input voltage drop in volts read from
the meter.
Using equation 8 completely compensates for the error due
to the input voltage drop.
2.11 COMPENSATION FOR INPUT VOLTAGE DROP OF
SHUNT-TYPE LOW-IMPEDANCE PICOAM-
METERS
For the most part, the voltage drop for this type picoammeter
is about the equivalent of that of the feedback-type
picoammeter-about 1mV or less. Following the directions in
paragraph 2.6 will ensure that Model 261 output currents are
within the specified accuracy.
To compensate for output current difference caused by input
voltage drops, use equation 8. This calculation will be
precise, since the voltage drop is read directly from the
picoammeter.
2.12 RANGE RESISTOR VOLTAGE COEFFICIENT
Ordinarily, the voltage coefficient of high-megohm resistors
can lead to significant errors. However, the range resistors
used in the Model 261 have an extremely low voltage coeffi-
cient of only 5ppmlV. This low value results in a worst-case
current change due to voltage coefficient of only 0.0055%
with the multiplier switches set for 1lV. Thus. the output
discrepancy because of voltage coefficient is one or two
orders of magnitude smaller than other sources of error, and
car? be ignored for all practical purposes.
2.13 COMPENSATION FOR TEMPERATURE COEFFI-
CIENT
When the Model 261 is used for range switch settings above
10~sor when the unit is operated at the temperature indicated
on the calibration certificate, no significant errors will occur in
the output. However, for range switch settings between
l@sA and lUJ2A. the change in range resistance because of
temperature coefficient can affect the output considerably.
The Model 261 is calibrated at a specified temperature
f0.5"C. which is stated on the calibration certificate, for the
various ranges. The certificate also lists the temperature coef-
ficients by range.
cause of Discrepancy in Output Current
Picoammeter input voltage drop more than
Table 2-2. Causes of Discrepancy in Model 261 Output Current and Correction Index
Equation to Compensate
for Discrepancy
5
6 or 8
all
I
2.11
8
Low-Impedance
Model 261 ambient temperature differs from all 10~8 to 10-14
2.13 9
that stated in Calibration Certificate
2-6