AD538 Analog Devices, AD538 Datasheet - Page 3

• Manufacturer Part Number: AD538
• Description: Real-Time Analog Computational Unit ACU
• Manufacturer: Analog Devices

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
Total Error =
Total Error =
REV. C
(V
DYNAMIC
Output V
Input V
Output V
Y
+ V
RANGE
RANGE
INPUT
WIDE
% rdg
% rdg
100:1
Z
)/V
OS
OS
OS
X
V
Input
(in V)
10
10
1
0.1
1
10
5
10
Y
V
Input
(in V)
10
1
0.1
0.10
0.01
0.01
0.1
0.05
Z
Input
0.01
V
(in V)
10
0.1
1
0.1
2
0.01
0.1
X
Table I. Sample Error Calculation Chart (Worst Case)
Ideal
Output
(in V)
10
10
1
0.1
10
0.25
5
1
Total Offset
Error Term
(in mV)
0.5
0.25
0.5
0.25
0.5
0.25
0.5
0.25
28
16.75 (BD)
1.76
1
125.75 (AD)
75.4
25.53 (AD)
15.27 (BD)
(AD)
(BD)
(AD)
(BD)
(AD)
(BD)
(AD)
(BD)
(AD)
(AD)
(BD)
(BD)
–3–
or divider with inputs down to 100 mV, has a maximum error of
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
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
tion, taken from Table I, for the AD538AD with V
100 mV and V
or 2.0% of reading 500 V
trates that with very low level inputs the AD538’s incremental
gain (V
bution to error substantial.
1% of reading 500 V. Some sample total error calculations
Y
2.0% of reading 500 V (1 V + 100 mV)/10 mV 250 V
/V
% of Reading
Error Term
(in mV)
100 (AD)
50 (BD)
100 (AD)
50 (BD)
10 (AD)
5
1
0.5 (BD)
200 (AD)
100 (BD)
5
2.5 (BD)
100 (AD)
50 (BD)
20 (AD)
10 (BD)
X
log ratio section. A sample application of this specifica-
Y
+ V
(BD)
(AD)
(AD)
Z
)/V
X
X
= 10 mV would yield a maximum error of
has increased to make the input offset contri-
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
228
116.75 (BD)
6.76
3.5
225.75 (AD)
125.4 (BD)
45.53 (AD)
25.27 (BD)
a certain number of digits on the meter
(AD)
(BD)
(AD)
(BD)
(AD)
(AD)
(BD)
27.5 mV. This example illus-
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)
AD538
Y
= 1 V, V
Z
=