Keithley 6512 Instruction Manual page 30

Operation
7. Take the reading directly from the display. The exponent can be placed either in the alpha or numeric mode, as described
in paragraph 2.5.
Voltage measurement considerations
Two factors are of concern when making voltage measurements, especially for voltage sources with high output (source) resis-
tances. For one thing, the loading effects of the measuring instrument come into play at the high resistance levels involved. Sec-
ondly, the distributed capacitance of the source, the input cable, and the input circuit of the instrument itself are a factor when
making these measurements.
Figure 2-5 demonstrates how meter loading can affect measurement accuracy. Here, a voltage source with a value E
source resistance R is connected to the input of the electrometer. The input resistance of the electrometer is R
S
error due to loading can be calculated as follows:
Thus, to keep the error under 0.1%, the input resistance must be about 1000 times the value of the source resistance, R
that the Model 6512 input resistance is ≥ 200G Ω , but the cable resistance appears in parallel.
Figure 2-5
Meter loading considerations
OFF
V, Ω GUARD
V S
Figure 2-4
Connections for voltage measurements
2-10
ERROR
%
TRIAX CABLE
INPUT
ON
MODEL 6512
INPUT AMPLIFIER
HI
LO
PREAMP OUT
GND
COM
100R
S
= ----------------------- -
R R
+
S IN
SOURCE
R S
E S
At very high resistance levels, the very large time constants
created by even a minimal amount of capacitance can slow
down response time considerably. For example, measuring a
source with an internal resistance of 100GΩ would result in
A
an RC time constant of one second when measured through
a cable with a nominal capacitance of 10pF. If 1% accuracy
is required, a single measurement would require at least five
seconds. Note that typical input cables have unguarded ca-
pacitance (high to inner shield) of 120 to 150pF per meter.
Basically, there are two ways to minimize this problem: (1)
keep the input cable as short as possible, and (2) use guarding.
With the first method, there is a limit as to how short the cable
can be. Using guarding can reduce these effects and decrease
B
settling times by up to a factor of 1,000. The Model 6512 rear
and a
S
. The percent
IN
. Note
S
METER
R IN V