VTI Instruments Ametek EX1200A Series, Руководство пользователя

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VTI Instruments Corp.
94 EX1200A Triggering
Common Error Sources
The errors discussed previously for DC Voltage Measurements also apply to resistance
measurements. Additional error sources unique to resistance measurements are discussed below.
Self-Heating Error
When a current passes through a resistor, heat is generated in the resistance source. This is
unavoidable. When testing resistive devices, a
“self
-
heating”
error can occur. When the DUT is
heated, the temperature of the DUT increases,
changing the DUT’s resistance
value. The
magnitude of this error depends on the
DUT’s
temperature coefficient, and, thus, cannot be
specified. To minimize this error, it is best practice to minimize the time that the current is applied
to R
DUT
.
Settling Time Related Errors
In some cases, the test lead resistance combined with the R
DUT
capacitance is large enough to
where the RC time constant is significant. Settling due to RC time constant effects can be quite
long, particularly when resistances above 100
kΩ
are measured. For example, certain types of
precision resistors have a large, lumped capacitance tied across its terminals to arrest noise. In
these cases, taking a resistance reading without calculating the setting time results in an erroneous
reading. The same is true for when the input impedance of a power supply module is measured
where the input does not have a large amount of capacitance. In cases like this, using the Auto
Delay trigger setting may not help. To reduce this error, the settling time of the resistor should be
manually calculated/observed and this time should be programmed as the trigger delay (see
Trigger Settings in Section 7 for additional information on trigger delay).
Errors in High Resistance Measurements
When measuring large resistances
(MΩ
or larger), significant errors can occur due to insulation
resistance and surface cleanliness. To reduce errors of this type,
a “clean” high
-resistance system
should be maintained. Test cables and fixtures are susceptible to leakage though moisture
abs
orption in insulating materials and “dirty” surface films.
Selecting the proper cable insulation
can also reduce error. Nylon and PVC are relatively poor insulators (10
9
Ω
) when compared to
PTFE (Teflon
®
) (10
13
Ω
). Leakage from nylon or PVC insulators can easily contribute a 0.1%
error when measuring a 1
MΩ
resistance in humid conditions.
Physically touching the R
DUT
while the resistance is being measured should be avoided, as the
body acts as a leakage path for the test current. Also, another instrument should not be connected
in parallel with the test resistor, as the input impedance of that instruments will appear in parallel
to the test resistance resulting in the combined resistance values to be read by the DMM.
F
REQUENCY
/P
ERIOD
M
EASUREMENTS
The EX1200A DMM uses the reciprocal counting technique to perform frequency measurements
with a 24 MHz oscillator driving the counter circuitry. Frequency measurements are always
returned in hertz (Hz). Period measurements use the same technique, but the frequency
measurement is inverted to measure the period (T = 1/
f ). Period measurements are always returned
in seconds (s).
Configuring Frequency/Period Measurements
The EX1200A DMM can measure the frequency and time period of any time-continuous
waveform (sine wave, square wave, triangular wave, etc.) with oscillations that are approximately
0.1*V
RANGE
. Note that the DMM uses 10% of the selected range for threshold detection. For
example, if the DMM is set to the 10 V range, an input signal will only be detected if the
amplitude is greater than 1 V. For more information on voltage settings, please refer to the
AC
Voltage Measurements discussion in this section. The range of frequency measurements is 3 Hz to
1.5 MHz and, making the reciprocal period measurements 660 ns to 0.33 s.