Calculating Resistance/Resistivity Accuracy And Repeatability Using The Alternating Polarity - Keithley 6517B Reference Manual

Appendix A: Accuracy calculations
Calculating resistance/resistivity accuracy and repeatability using the
alternating polarity method
From the specifications, accuracy and repeatability when using a Model 8009 test fixture are
calculated as follows:
Repeatability: I
Accuracy: (V
Where:

I
is a measured, typical background current noise from the sample and fixture over the
BG
measurement time programmed, under normal measurement conditions (1 PLC, same range,
filters off, etc.)

V is the alternating polarity voltage used
ALT

V Err is the accuracy (in volts) of the voltage source using V
SRC

I Err is the accuracy (in amps) of the ammeter using V
MEAS
The following example shows how to calculate accuracy and repeatability for a 10
measured in a Model 8009 test fixture, with 50 V stimulus and a background current of 4 pA. The
user-selected measurement time for the alternating polarity measurement is 15 seconds.
I
is measured with no voltage applied, under normal measurement conditions, and 15 seconds
BG
(according to the user's choice for the measurement) of readings are stored to the buffer. Normal
conditions require 1 PLC, the same amps range that is used for the alternating polarity measurement
and similar preconditioning to the normal measurements (if the samples are normally inserted into the
fixture and immediately measured, the stored readings should be performed similarly after lid
closure). After the storage is complete, the standard deviation (1 sigma) of these values is taken. This
can be done by retrieving the standard deviation of the buffer over the bus or by pressing RECALL
once and NEXT repeatedly, scrolling through the second-line displays to the standard deviation. This
is I
. Assume for example, that the result was 55 fA or 5.510
BG
Repeatability = 5.510
Note that with normal ohms methods, the 4 pA total background current contributes as much as 400
% error.
Err = 0.15 %  50 V + 10 mV = 0.085 V (from 100 V V-source range specs)
V
SRC
Err = 1 % (50 V / 10
I
MEAS
Accuracy = (0.085 V + 5.310
A-4
R/ V + 0.1 % (1 )
BG ALT
(instrument temp. constant ±1 °C)
Err R)/V
Err + I
SRC MEAS ALT
A  10  / 50 V + 0.1 % = 0.011 + 0.1 % or 1.2 %
-14 13
) + 30 counts  10
13
A 10
-14
Model 6517B Electrometer Reference Manual
ALT
-16 A / count = 5.310
) / 50 V = 0.0123 or 1.23 %
13
as the setting
ALT
/R as the reading
13
ohms sample
-14 A.
-14 A (from 20 pA spec)
6517B-901-01 Rev. C / August 2015