AD538

Rev. E | Page 9 of 16

THEORY OF OPERATION

RE-EXAMINATION OF MULTIPLIER/DIVIDER ACCURACY

Traditionally, the accuracy (actually the errors) of analog

multipliers and dividers has 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 error sources of the AD538 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 Table 4. 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 specification

that is the sum of three components: a percent of reading term,

ratio section. A sample application of this specification, taken

reading ±500 μV ± (1 V + 100 mV)/10 mV × 250 μV or ±2.0%

of reading ±500 μV ± 27.5 mV. This example illustrates that

error substantial.

Table 4. Sample Error Calculation Chart (Worst Case)

Input

(V)

Input

(V)

Input

(V)

Ideal

Output

(V)

Total Offset Error

Term (mV)

% of Reading

Error Term

(mV)

Total Error

Summation

(mV)

Total Error

Summation as a

% of the Ideal

Output

100:1 INPUT

RANGE

Total Error =

±% rdg

10

10

10

10

0.5

(AD)

100

(AD)

100.5

(AD)

1.0

(AD)

0.25

(BD)

50

(BD)

50.25

(BD)

0.5

(BD)

10

0.1

0.1

10

0.5

(AD)

100

(AD)

100.5

(AD)

1.0

(AD)

0.25

(BD)

50

(BD)

50.25

(BD)

0.5

(BD)

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

DYNAMIC

RANGE

Total Error =

±% rdg ±

1

0.10

0.01

10

28

(AD)

200

(AD)

228

(AD)

2.28

(AD)

16.75

(BD)

100

(BD)

116.75

(BD)

1.17

(BD)

10

0.05

2

0.25

1.76

(AD)

5

(AD)

6.76

(AD)

2.7

(AD)

1

(BD)

2.5

(BD)

3.5

(BD)

1.4

(BD)

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)