Metek EX1200A SERIES, Руководство пользователя

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EX1200A Triggering 91
Low-Level AC Voltages and Shielding
When measuring ac voltages less than 100 mV, be aware that these measurements are especially
susceptible to errors introduced by external noise sources. Exposed (unshielded) cabling acts as an
antenna and the EX1200A DMM will measure the signals received. The entire measurement path,
including the power line, acts as a loop antenna. Circulating currents in the loop creates error
voltages across any impedances in series with the DMM’s input. For this reason, low-level ac
voltages should be input to the multimeter through shielded cables. The shield should be
connected to the input LO terminal.
Make sure the DMM and the ac signal source are connected to the same electrical outlet whenever
possible. This minimizes the difference in the earth potentials and avoids forming ground loops. If
ground loops cannot be avoided, the area of the loops should be minimized. Measurements of
high-impedance sources are more susceptible to noise pickup than measurements of low
impedance sources. To reduce noise pick-up, place a capacitor in parallel with the DMM’s input
terminals (HI and LO). The correct capacitor value can only be determined by experimentation, as
the capacitance also contributes some loading error, while helping reduce noise pick up.
Reverse Connections and AC Common Mode Offset
Ideally, a DMM reads the same voltage regardless of how the source is connected to the DMM (HI
and LO). In practice, errors are generated when the DMM’s input LO terminal is driven with an ac
voltage relative to earth. The most common situation where unnecessary turnover errors are
created is when the output of an ac calibrator is connected to the multimeter “backwards.”
Because of the capacitance between the LO input terminal and earth (approximately 200 pF for the
EX1200As), the source experiences a loading differential depending on how the input is applied.
The magnitude of this error is dependent upon the source’s response to this loading. The DMM’s
measurement circuitry, while extensively shielded, responds differently in the backward input case
due to slight differences in stray capacitance to earth. Because of this, the 100 V ac and 300 V ac
ranges may latch up for high-voltage, high-frequency “backward” inputs. Therefore, only drive the
high terminal when measuring ac voltages. The grounding techniques described for dc common
mode problems can be used to minimize ac common mode voltages (refer the
Common Mode
Rejection discussion earlier in this section).
DC
AND
AC C
URRENT
M
EASUREMENTS
The EX1200A DMM can directly measure ac and dc current (ACI and DCI) within its specified
limits. Note that the front panel terminals are different for connecting a current signal (I and LO).
DMM Front Panel
Sense HI
I
Input LO
Sense LO
Current Source
F
IGURE
4-11: DMM C
ONNECTIONS FOR
C
URRENT
M
EASUREMENTS
The current signal input is passed through an internal precision shunt resistor (4.1 Ω for 10 mA
and 100 mA ranges, 0.1 Ω for 1 A and 3 A ranges and 68.1 Ω for DCI 1 mA range) and the
resultant voltage drop is measured. Using this value and the shunt resistor value, the current input
level is calculated. In order to protect the shunt resistor (and the DMM) from the high-current
spikes, the input current is passed through a fuse (described earlier). When making current