ADC08034CIWM datasheet(15/24 Pages) NSC

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Functional Description (Continued)

put 0000 0000 digital code for this minimum input voltage by

biasing any V

IN (−) input at this VIN(MIN) value. This utilizes

the differential mode operation of the A/D.

The zero error of the A/D converter relates to the location of

the first riser of the transfer function and can be measured by

grounding the V

IN (−) input and applying a small magnitude

positive voltage to the V

IN (+) input. Zero error is the differ-

ence between the actual DC input voltage which is neces-

sary to just cause an output digital code transition from 0000

0000 to 0000 0001 and the ideal 1⁄2 LSB value (1⁄2 LSB =

9.8mV for V

REF = 5.000VDC).

5.2 Full Scale

The full-scale adjustment can be made by applying a differ-

ential input voltage which is 11⁄2 LSB down from the desired

analog full-scale voltage range and then adjusting the mag-

nitude of the V

REFIN input (or VCC for the ADC08032) for a

digital output code which is just changing from 1111 1110 to

1111 1111.

5.3 Adjusting for an Arbitrary Analog Input

Voltage Range

If the analog zero voltage of the A/D is shifted away from

ground (for example, to accommodate an analog input signal

which does not go to ground), this new zero reference

should be properly adjusted first. A V

IN (+) voltage which

equals this desired zero reference plus 1⁄2 LSB (where the

LSB is calculated for the desired analog span, using 1 LSB =

analog span/256) is applied to selected “+” input and the

zero reference voltage at the corresponding “−” input should

then be adjusted to just obtain the 00

HEX to 01HEX code tran-

sition.

The full-scale adjustment should be made [with the proper

IN (−) voltage applied] by forcing a voltage to the VIN (+) in-

put which is given by:

where:

MAX = the high end of the analog input range

and

MIN = the low end (the offset zero) of the analog range.

(Both are ground referenced.)

The V

REFIN (or VCC) voltage is then adjusted to provide a

code change from FE

HEX to FFHEX. This completes the ad-

justment procedure.

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