Applications
4-9
4.7
High-impedance voltmeter
The input resistance of the Model 6512 in the volts mode is
greater than 200T
Ω
. Because of this high value, the Model
6512 can be used to make voltage measurements in high-im-
pedance circuits with a minimum of loading effects on the
circuit.
Consider the situation where a circuit designer must measure
the gate-to-source voltage of a precision JFET amplifier that
has a gate impedance of 100M
Ω
. Further assume that the re-
quired accuracy of this measurement is 1%.
The setup for this measurement is shown in Figure 4-9. The
gate-source voltage is represented by V
GS
, while the effec-
tive gate impedance is represented as R
S
. The input resis-
tance of the voltmeter is given as R
IN
.
Figure 4-8
Measuring high-impedance gate-source voltage
The percent error due to voltmeter loading in this circuit can
be given as:
Suppose, for example, a typical DMM with a 10M
Ω
input
resistance were used to make this measurement. The error
because of meter loading would be:
Even if a DMM with an input resistance of 1G
Ω
(10
9
Ω
) were
used, the error would still be:
METER
HI
LO
RIN
VGS
METER
VGS
100M
Ω
RS
RIN
LO
HI
A. MEASUREMENT
CONFIGURATION
B. EQUIVALENT CIRCUIT
ERROR
R
S
R
S
R
IN
+
------------------------
100
×
=
%
%
ERROR
100M
Ω
100M
Ω
10M
Ω
+
----------------------------------------------
100
×
=
%
% = 91% error
ERROR
100M
Ω
100M
Ω
1
G
Ω
+
-----------------------------------------
100
×
=
%
% = 9.1% error
Such a large error would not be tolerable in this case because
of the 1% accuracy requirement. However, since the Model
6512 has an input resistance of 200T
Ω
, its error in this exam-
ple would be:
Note that this error term is so small as to be insignificant as it
would be dominated by the instruments’s accuracy specification.
Thus, the input impedance of the Model 6512 would be more
than adequate for this situation, because the error due to
meter loading is substantially better than the required 1%
value stated earlier. In addition, the 4
½
digit resolution of the
instrument allows the designer sufficient precision to make
use of the high input impedance.
4.8
Voltage coefficients of
high-megohm resistors
High-megohm resistors (above 10
9
Ω
) often exhibit a change
in resistance with applied voltage. This resistance change is
characterized as the voltage coefficient. The Model 6512 can
be used in conjunction with a Model 230 Voltage Source to
determine the voltage coefficient of such resistors.
The basic configuration for making voltage coefficient mea-
surements is shown in Figure 4-10. The voltage, V
S
, is ap-
plied to the resistor under test by the voltage source of the
instrument. The current is measured by the electrometer in-
put of the Model 6512. The resulting current can then be
used to calculate the resistance.
Two resistance readings at two different voltage values will
be required to calculate the voltage coefficient. The voltage
coefficient in %/V can then be calculated as follows:
where:
R
1
is the resistance with the first applied voltage.
R
2
is the resistance with the second applied voltage.
∆
V is the difference between the two applied voltages.
ERROR
100M
Ω
100M
Ω
200T
Ω
+
----------------------------------------------
100
×
=
%
% = 0.00005% error
100 R
1
R
1
–
(
)
R
1
V
∆
(
)
-----------------------------------
Voltage Coefficient (%/V) =
Summary of Contents for 6512
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