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M e a s u r e m e n t P r i n c i p l e s a n d B a s i c s
Z
(V
)
in
Ideal function of
the a/d converter
0110
0101
0100
0011
0010
0001
Ue
Fig. 1: A/D converter offset error
Bild 32: A/D-Wandler differentielle Nichtlinearit‰t
Bild 32: A/D-Wandler differentielle Nichtlinearit‰t
Z
(V
)
in
Ideal function of
the a/d converter
(linear)
0110
0101
0100
0011
0010
Nonlinearity of the
a/d converter
0001
Fig. 3: A/D converter differential nonlinearity
Offset errors of the A/D converter
The input amplifier of the DMM is not properly adjusted and
shows an offset. This offset causes an offset error in the a/d
conversion. (Fig. 1)
Slope error (amplification factor error) of the A/D converter
The input amplifier's amplification factor is temperature-
dependent, or the amplification factor was maladjusted. Hence
the slope of the function differs from the ideal value. (Fig. 2).
Differential nonlinearity of the A/D converter
The quantizing steps of the a/d converter are unequal in size
and differ from the ideal theoretical value. The differential
nonlinearity indicates how much each voltage interval (actual)
differs from the ideal voltage interval (ideal, 1 LSB)) ΔV
3) when the analog voltage Vin is being converted.
Differential linearity error = k x ΔV
k= factor, describing the relationship ΔV
42
Subject to change without notice
Function of the a/d converter
is displaced by offset error
V
in
Actual from
interval V
in
at 0110
Ideal from interval V
at 0110
in
V
in
V
in
(Fig.
in
;
in
(actual) to ΔV
(ideal)
in
in
Z
(V
)
in
Ideal function of
the a/d converter
0110
0101
0100
0011
0010
0001
Fig. 2: A/D converter amplification error
Z
(V
)
in
Ideal function of
the a/d converter
(linear)
0110
0101
0100
0011
0010
0001
Fig. 4: A/D converter integral nonlinearity
Linearity error (integral nonlinearity) of the A/D converter
Due to the individual differential linearity errors and their sum
a maximum error between the ideal conversion characteristic
and the actual one will accrue. The linearity error specifies the
maximum distance between the two functions (Fig. 4).
A/D conversion methods
In the following, the Single Slope, the Dual Slope and the Multip-
le Slope methods will be described. These sawtooth converters
are based on the same principle: conversion of the input voltage
into a proportional time span.
The slope of the function of the
a/d converter is affected by
amplification error
V
in
Nonlinearity of the
a/d converter
Max. deviation of the nonlinear
slope curve of the a/d converter
to the ideal linear function
V
in
V
in
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