V/I Resistance Measurement Considerations: The main ad-
vantage of using the constant voltage method for resistance
measurements is that the effects of leakage resistance and
distributed capacitance are minimized. Because of these fac-
tors, the resistance range of the instrument can be greatly in-
creased, in the case of the Model 617, to 1016fl. However,
there are certain characteristics pertaining to high resistance
measurements that require discussion.
A primary consideration when using this mode is to match
the voltage and current ranges to optimize accuracy. In most
cases, it is best use the maximum voltage value possible (ex-
cept as indicated below) and set the current range according-
ly. As with other Model 617 measurements, the instrument
should be placed on the most sensitive range possible without
overranging the electrometer section. Doing so will optimize
the measurement for resolution and accuracy. Autoranging
can facilitate range selection.
At very high resistance values, the corresponding current, as
seen by the instrument, will be extremely low. Thus, any cur-
rent generated by the triaxial input cable will be reflected in
the final measurement. To minimize such problems, use low-
noise graphite hiaxial cable. (such as the Model 6011). Tie
down the cable to avoid any triboelectric currents that might
be generated by cabling flexing. In many situations, shielding
of the circuit under test will also be required to minimize noise
pickup.
Although V/I resistance measurements are much lms suscep-
tible to the effects of leakage resistance than resistance
measurements made with the constant current method, there
are sane cases where leakage resistance could affect V/I
resistance measurements. For example, test fixture leakage
paths may appear in parallel with the device beiig measured,
introducing errors in the measurement. As with other Model
617 high impedance measurements, these errors can be
minimized by using proper insulating material (such as
Teflon) in fixture terminal connections.
Any leakage current through cables and test fixtures can be
minimized if care is taken. To cancel these effects, set up the
measurement exactly as desired, but leave the resistor under
test disconnected. Program the voltage source to the desired
value and turn on its output. With the instrument in the amps
mode, enable suppress to null the leakage current. Turn off
the source, connect the resistor, and re-enable the voltage
source. Place the instrument in the V/I ohms mode and pro-
ceed with the measurement.
High megohm resistors are somewhat curious devices, often
exhibiting characteristics somewhere between those of an in-
sulator and a normal re&tor. Because of these unique traits,
the measured value of such a resistor will often vary with ap
plied voltage.
Such variation in resistance is known as rhe voltage coeffi-
cient. The Model 617 can be used to characterize such
resistance changes by measuring the resistance with a number
of different applied voltages. Once the variations are known,
the voltage coefficient of the resistor being tested can be
calculated. The method for determining the voltage coeffi-
cient of these resistors is discussed in Section 4.
2.9 ANALOG OUTPUTS
The Model 617 has two analog outputs on the rear panel. The
2V ANALOG OUTPUT provides a scaled O-2V output with a
value of 2V corresponding to full range input. The PREAMP
OUT is especially useful in situations requiring buffering.
These two analog outputs are discussed the the following
paragraphs.
WARNING
When floating Input Low above 30V from
earth ground. hazardous voltage will be
present at the analog outputs. Hazardous
voltage may also be present when msasur-
ing in ohms, or when the input voltage ex-
ceeds 30V in the volts mode.
CAUTION
Connecting PREAMP OUT, COM, or 2V ANALOG
OUTPUT to earth while floating input may
damage the instrument.
2.9.1 2V Analog Output
The 2V ANALOG OUTPUT provides a scaled Q2V output
that is inverting in the volts and ohms modes. Connections
for using this output are shown in Figure 2-15. For a full range
input, the output will be 2V; typical examples are listed in
Table 2-5. The 2V ANALOG OUTPUT is not corrected dur-
ing calibration. Gain errors of up to 3% may appear at this
output, depending on function and range selection.
Note that the output impedance is IOk% to minimize the ef-
fects of loading, the input impedance of the device connected
to the 2V ANALOG OUTPUT should be as high as possible.
For example, with a device with an input impedance of
lOM0, the error due to loading will be approximately 0.1%
2.9.2 Preamp Out
The PREAMP OUT of the Model 617 follows the signal
amplitude applied to the INPUT terminal. Some possible uses
for thepreamp output include buffering of the input signal, as
2-19
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