AD538
RE-EXAMINATION OF MULTIPLIER/DIVIDER
ACCURACY
Traditionally, the “accuracy” (actually the errors) of analog
multipliers and dividers have been specified in terms of percent
of full scale. Thus specified, a 1% multiplier error with a 10 V
full-scale output would mean a worst case error of +100 mV at
“any” level within its designated output range. While this type
of error specification is easy to test evaluate, and interpret, it can
leave the user guessing as to how useful the multiplier actually is
at low output levels, those approaching the specified error limit
(in this case) 100 mV.
The AD538’s error sources do not follow the percent of full-
scale approach to specification, thus it more optimally fits the
needs of the very wide dynamic range applications for which it is
best suited. Rather than as a percent of full scale, the AD538’s
error as a multiplier or divider for a 100:1 (100 mV to 10 V)
input range is specified as the sum of two error components: a
percent of reading (ideal output) term plus a fixed output offset.
Following this format the AD538AD, operating as a multiplier
or divider with inputs down to 100 mV, has a maximum error of
±
1% of reading
±
500
µV.
Some sample total error calculations
for both grades over the 100:1 input range are illustrated in the
chart below. This error specification format is a familiar one to
designers and users of digital voltmeters where error is specified
as a percent of reading
±
a certain number of digits on the meter
readout.
For operation as a multiplier or divider over a wider dynamic
range (>100:1), the AD538 has a more detailed error specifica-
tion that is the sum of three components: a percent of reading
term, an output offset term and an input offset term for the
V
Y
/V
X
log ratio section. A sample application of this specifica-
tion, taken from Table I, for the AD538AD with V
Y
= 1 V, V
Z
=
100 mV and V
X
= 10 mV would yield a maximum error of
±
2.0% of reading
±
500
µV ±
(1 V + 100 mV)/10 mV
×
250
µV
or
±
2.0% of reading
±500 µV ±
27.5 mV. This example illus-
trates that with very low level inputs the AD538’s incremental
gain (V
Y
+ V
Z
)/V
X
has increased to make the input offset contri-
bution to error substantial.
Table I. Sample Error Calculation Chart (Worst Case)
V
Y
Input
(in V)
100:1
INPUT
RANGE
Total Error =
±
% rdg
±
Output V
OS
10
V
Z
Input
(in V)
10
V
X
Input
(in V)
10
Ideal
Output
(in V)
10
Total Offset
Error Term
(in mV)
0.5
0.25
0.5
0.25
0.5
0.25
0.5
0.25
(AD)
(BD)
(AD)
(BD)
(AD)
(BD)
(AD)
(BD)
% of Reading
Error Term
(in mV)
100 (AD)
50 (BD)
100 (AD)
50 (BD)
10 (AD)
5 (BD)
1 (AD)
0.5 (BD)
200 (AD)
100 (BD)
5 (AD)
2.5 (BD)
100 (AD)
50 (BD)
20 (AD)
10 (BD)
Total Error
Summation
(in mV)
100.5 (AD)
50.25 (BD)
100.5 (AD)
50.25 (BD)
10.5
5.25
1.5
0.75
(AD)
(BD)
(AD)
(BD)
Total Error Summation
as a % of the Ideal
Output
1.0 (AD)
0.5 (BD)
1.0 (AD)
0.5 (BD)
1.05 (AD)
0.5 (BD)
1.5 (AD)
0.75 (BD)
2.28 (AD)
1.17 (BD)
2.7 (AD)
1.4 (BD)
4.52 (AD)
2.51 (BD)
4.55 (AD)
2.53 (BD)
10
0.1
0.1
10
1
1
1
1
0.1
WIDE
DYNAMIC
RANGE
Total Error =
±
% rdg
±
Output V
OS
±
Input V
OS
×
(V
Y
+ V
Z
)/V
X
1
0.1
0.10
0.1
0.01
0.1
10
28
(AD)
16.75 (BD)
1.76
1
(AD)
(BD)
228 (AD)
116.75 (BD)
6.76
3.5
(AD)
(BD)
10
0.05
2
0.25
5
0.01
0.01
5
125.75 (AD)
75.4 (BD)
25.53 (AD)
15.27 (BD)
225.75 (AD)
125.4 (BD)
45.53 (AD)
25.27 (BD)
10
0.01
0.1
1
REV. C
–3–
AD [ ANALOG DEVICES ]
