AD538

–3–

REV. C

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

±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

bution to error substantial.

Table I. Sample Error Calculation Chart (Worst Case)

Ideal

Total Offset

% of Reading

Total Error

Total Error Summation

Input

Input

Input

Output

Error Term

Error Term

Summation

as a % of the Ideal

(in V)

(in V)

(in V)

(in V)

(in mV)

(in mV)

(in mV)

Output

100:1

10

10

10

10

0.5

(AD)

100 (AD)

100.5 (AD)

1.0

(AD)

INPUT

0.25

(BD)

50

(BD)

50.25 (BD)

0.5

(BD)

RANGE

Total Error =

10

0.1

0.1

10

0.5

(AD)

100 (AD)

100.5 (AD)

1.0

(AD)

±% rdg

0.25

(BD)

50

(BD)

50.25 (BD)

0.5

(BD)

±Output V

OS

1

1

1

1

0.5

(AD)

10

(AD)

10.5

(AD)

1.05 (AD)

0.25

(BD)

5

(BD)

5.25

(BD)

0.5

(BD)

0.1

0.1

0.1

0.1

0.5

(AD)

1

(AD)

1.5

(AD)

1.5

(AD)

0.25

(BD)

0.5 (BD)

0.75

(BD)

0.75 (BD)

WIDE

1

0.10

0.01

10

28

(AD)

200 (AD)

228

(AD)

2.28 (AD)

DYNAMIC

16.75 (BD)

100 (BD)

116.75 (BD)

1.17 (BD)

RANGE

Total Error =

10

0.05

2

0.25

1.76

(AD)

5

(AD)

6.76

(AD)

2.7

(AD)

±% rdg

1

(BD)

2.5 (BD)

3.5

(BD)

1.4

(BD)

±Output V

OS

±Input V

OS

×

5

0.01

0.01

5

125.75 (AD)

100 (AD)

225.75 (AD)

4.52 (AD)

75.4

(BD)

50

(BD)

125.4 (BD)

2.51 (BD)

10

0.01

0.1

1

25.53 (AD)

20

(AD)

45.53 (AD)

4.55 (AD)

15.27 (BD)

10

(BD)

25.27 (BD)

2.53 (BD)