ADC0832CIWM datasheet(18/33 Pages) NSC

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Functional Description (Continued)

resistance of the analog signal source. Bypass capacitors

should not be used if the source resistance is greater than 1

Ω.

This source resistance limitation is important with regard to

the DC leakage currents of input multiplexer as well. The

worst-case leakage current of ±1 µA over temperature will

createa1mV input error witha1k

Ω source resistance. An

op amp RC active low pass filter can provide both imped-

ance buffering and noise filtering should a high impedance

signal source be required.

5.0 Optional Adjustments

5.1 Zero Error

The zero of the A/D does not require adjustment. If the

minimum analog input voltage value, V

IN(MIN), is not ground

a zero offset can be done. The converter can be made to

output 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=9.8

mV for V

REF=5.000 VDC).

5.2 Full-Scale

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

ential input voltage which is 1 1⁄2 LSB down from the desired

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

nitude of the V

REF input (or VCC for the ADC0832) 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

transition.

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

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

input 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

REF (or VCC) voltage is then adjusted to provide a code

change from FE

HEX to FFHEX. This completes the adjust-

ment procedure.

6.0 Power Supply

A unique feature of the ADC0838 and ADC0834 is the inclu-

sion of a zener diode connected from the V

+ terminal to

ground which also connects to the V

CC terminal (which is the

actual converter supply) through a silicon diode, as shown in

Figure 3. (Note 3)

This zener is intended for use as a shunt voltage regulator to

eliminate the need for any additional regulating components.

This is most desirable if the converter is to be remotely

located from the system power source.Figure 4 and Figure 5

illustrate two useful applications of this on-board zener when

an external transistor can be afforded.

An important use of the interconnecting diode between V

and V

CC is shown in Figure 6 and Figure 7. Here, this diode

is used as a rectifier to allow the V

CC supply for the converter

to be derived from the clock. The low current requirements of

the A/D and the relatively high clock frequencies used (typi-

cally in the range of 10k–400 kHz) allows using the small

value filter capacitor shown to keep the ripple on the V

CC line

to well under 1⁄4 of an LSB. The shunt zener regulator can

also be used in this mode. This requires a clock voltage

swing which is in excess of V

Z. A current limit for the zener is

needed, either built into the clock generator or a resistor can

be used from the CLK pin to the V

+ pin.

00558311

FIGURE 3. An On-Chip Shunt Regulator Diode

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