Page 468 - Instrumentation Reference Book 3E

Digital voltmeters and digital multimeters 451

(6) Storing the maximum and minimum value of
High the measured variable.
(7) Generating statistical data from a given set of
measurements to provide the sample average,
standard deviation, variance, or rms vaiue.
- (8) Digital filtering of the measured variable to
V
Common-mode
- provide a continuous average, an average
-
voltage
over IZ readings, or a walking window average
Low High over n readings.
(al
20.3.2.5 Output
The visual display of DVMs and DMMs is
commonly provided by light-emitting diodes
(LEDs) or liquid crystal displays (LCDs).
The relative merits of each of these displays is
considered in Chapter 30 in Part 4. If the
results are to be communicated to further
currents digital systems the output may be provided as
(b)
either a parallel binary or binary-coded decimal
(BCD) output. Many DVMs and DMMs are
F:il ;inals fitted with the standard IEEE-488 or RS232
parallel or serial interfaces which allow data
and control to pass between the instrument
and a host control computer. The characteristics
Vi n
of IEEE-488 and RS232 interfaces are considered in
(cl Part 4.
TCi
cm
-
-L 20.3.3 DVM and DMM specifications
DVMs and DMMs cover a wide range of
High . instruments, fi-om hand-held, battery-operated
multimeters, through panel meters and bench
instruments to standards laboratory instru-
ments. These digital instruments are specified
Guard box
primarily by their resolution, accuracy, and
speed of reading. The resolution of the instru-
ment, which may be higher than its accuracy,
corresponds to the quantity indicated by a
change in the least significant digit of the dis-
play. Typically, digital instruments hzve dis-
Residual resistance plays which are between 3; and 8; digits.
and capacitance leakages The half digit indicates that the most signifi-
(d) cant digit can only take the value 1 or 0.
Figure 20.31 (a) Common-mode signalsin bridge Thus a 3; digit instrument has a resolution
measurements; (b) ground current-generated common- of 1 part in 2000 and an si-digit one has a
mode signals; (c) input equivalent circuit; (d) input guarding. resolution of 1 part in 2 x 10’. The accuracy
of the instrument is specified as i (x percent
of reading (R)+y per cent of scale (S)+n
instrument may enther display “hi-lo-pass” or digits). Table 20.7 gives condensed specifi-
may count the number of readings in each cations for comparison of a hand-held 35digit
category. DMM, a 5$-digit intelligent multimeter, and an
The calculation and display of the percentage 8 $-digit standards laboratory DVM. The
deviation from a given set point input by the accuracies quoted in Table 20.7 are only for
operetor. guidance, and for complete specifications the
Calculation of the ratio of the measured value reader should consult the specification pro-
to some value input by the operator. vided by the manufacturer.