Technical Information - Digital Multimeters0 pages

Analog to Digital Conversion
The A/D converts the analog input signal to a
digital output and is primarily responsible for
key instrument characteristics of reading speed,
linearity, resolution, normal mode rejection,
and precision. The digital output is shown or
obtained in several ways. One way is visually, via
the front panel with a display of digits and other
information. Another way is electronically, with
results sent via a port (GPIB, RS-232, USB, or
Ethernet) to a computer for further processing.
Resolution
Resolution is defined as the smallest detectable
change on any range referenced to full scale. For
example, if an instrument displays a maximum
of 19,999 on any range, and the smallest detectable change in the input signal is ±1 least significant digit (LSD), then the resolution is 1/19999
or 0.005%.

DIGITAL MULTIMETERS & SYSTEMS

Resolution is commonly expressed as a whole
number plus a fraction, e.g., 5½ digits. The
whole number represents the number of digits
that can display the numbers from 0 to 9. The
fraction indicates that the most significant digit
has one or more non-zero states, that is, it can
display 0, 1, or 2.
Sensitivity
Sensitivity is similar to resolution in that it deals
with the smallest change of the input signal the
instrument can detect. However, sensitivity is
not referenced to full scale, so it is expressed in
absolute terms and applies to the lowest range
on any function. The sensitivity of a 7½-digit
DMM is 10nV if its lowest measurement range is
200mV.
Accuracy
Accuracy is specified as a two-term specification:
±(% of reading + % of range) or as (ppm of
reading + ppm of range). The closer to zero on
the range that the percent of range term of the
specification is, the greater the weight it has in
the accuracy calculation. The closer to full scale
on the range the percent of reading term of the
specification is, the greater the weight it has in
the accuracy calculation. The best accuracy is
obtained near full scale.

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AC
Attenuator

AC
DC
HI
Amps

Digital
Display

AC
Converter
AC
DC

DC
Attenuator

Ohms

A/D
Converter

Ohms

INPUT

Precision
Reference

Ohms
Converter

Precision
Shunts

Digital
Output
Ports
(IEEE-488,
USB,
RS-232,
Ethernet)

LO

Figure 1: DMM Block Diagram

0.0075
0.0060

51⁄2

0.0045
0.0030

61⁄2

0.0015

Input as % of Full Scale
50%
100%
Accuracy ±(0.1% + 1 count)

Figure 2: Expected Reading Uncertainty: 5½- vs. 6½-Digit DMMs

Accuracy is also generally stated under several
conditions, including ±1°C, ±5°C operating temperature, and 24-hour, 90-day, and one-year calibration intervals. The expected accuracy can be
improved by controlling temperature variations
in the environment and by electing more frequent calibration intervals. Figure 2 illustrates
the effect on accuracy at various levels of input
signal within the measurement range. Accuracy
for both meters is specified at ±(0.1% + 1 count).
Loading and Input Impedance
Loading is the disturbance to the circuit being
measured caused by the finite input impedance
of the DMM. Input impedance is the equivalent
resistance and capacitance of the input terminals
of the DMM.
Loading error (Figure 3) is the difference
between the voltage measured by the meter (V M )
and the voltage of an ideal source (VS ).

A

G R E A T E R

Voltage burden error (Figure 4) is the difference between the expected current through the
load (R L ) and the measured current (IM) caused
by the finite voltage drop of the measuring
instrument.
Two-Wire vs. Four-Wire Ohms
Two-terminal DMMs source test current through
the measuring test leads, terminating at the
HI-LO inputs of the DMM. This two-wire ohms
system works fine for most resistance measurement applications. However, the I-R drop in
the test leads (R L ) can cause inaccuracies that
become apparent in lower resistance measurements (Figure 5).
Four-wire ohms or Kelvin measurements bypass
the voltage drop across R L by bringing two
high impedance voltage sense leads out to
the unknown R X. There is very little current
in the sense circuit because of the high input
impedance, so there's effectively no I-R drop
in the leads, and the voltage seen by the sense

M E A S U R E

O F

C O N F I D E N C E

Technical information: Digital multimeters

Technical information: Digital multimeters

Digital multimeters convert analog signals to
digital information. In general, DMMs have a
minimum of five typical functions. They are DC
voltage, AC voltage, DC current, AC current, and
resistance. While specifications vary, most DMMs
can be described with block diagrams similar to
Figure 1.

Digital Multimeters

± % Uncertainty

Technical
Information