ADC0803 datasheet(21/41 Pages) NSC

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Functional Description (Continued)

100

Ω series resistor can be used to isolate this

capacitor — both the R and C are placed outside the feed-

back loop — from the output of an op amp, if used.

2.3.4 Noise

The leads to the analog inputs (pins 6 and 7) should be kept

as short as possible to minimize input noise coupling. Both

noise and undesired digital clock coupling to these inputs

can cause system errors. The source resistance for these in-

puts should, in general, be kept below 5 k

Ω. Larger values of

source resistance can cause undesired system noise

pickup. Input bypass capacitors, placed from the analog in-

puts to ground, will eliminate system noise pickup but can

create analog scale errors as these capacitors will average

the transient input switching currents of the A/D (see section

2.3.1.). This scale error depends on both a large source re-

sistance and the use of an input bypass capacitor. This error

can be eliminated by doing a full-scale adjustment of the A/D

(adjust V

REF/2 for a proper full-scale reading — see section

2.5.2 on Full-Scale Adjustment) with the source resistance

and input bypass capacitor in place.

2.4 Reference Voltage

2.4.1 Span Adjust

For maximum applications flexibility, these A/Ds have been

designed to accommodatea5V

DC, 2.5 VDC or an adjusted

voltage reference. This has been achieved in the design of

the IC as shown in

Figure 6.

Notice that the reference voltage for the IC is either 1⁄2 of the

voltage applied to the V

CC supply pin, or is equal to the volt-

age that is externally forced at the V

REF/2 pin. This allows for

a ratiometric voltage reference using the V

CC supply, a 5

DC reference voltage can be used for the VCC supply or a

voltage less than 2.5 V

DC can be applied to the VREF/2 input

for increased application flexibility. The internal gain to the

REF/2 input is 2, making the full-scale differential input volt-

age twice the voltage at pin 9.

An example of the use of an adjusted reference voltage is to

accommodate a reduced span — or dynamic voltage range

of the analog input voltage. If the analog input voltage were

to range from 0.5 V

DC to 3.5 VDC, instead of 0V to 5 VDC, the

span would be 3V as shown in

Figure 7. With 0.5 V

DC ap-

plied to the V

IN(−) pin to absorb the offset, the reference volt-

age can be made equal to 1⁄2 of the 3V span or 1.5 V

DC. The

A/D now will encode the V

IN(+) signal from 0.5V to 3.5 V with

the 0.5V input corresponding to zero and the 3.5 V

DC input

corresponding to full-scale. The full 8 bits of resolution are

therefore applied over this reduced analog input voltage

range.

2.4.2 Reference Accuracy Requirements

The converter can be operated in a ratiometric mode or an

absolute mode. In ratiometric converter applications, the

magnitude of the reference voltage is a factor in both the out-

put of the source transducer and the output of the A/D con-

verter and therefore cancels out in the final digital output

code. The ADC0805 is specified particularly for use in ratio-

metric applications with no adjustments required. In absolute

conversion applications, both the initial value and the tem-

perature stability of the reference voltage are important fac-

tors in the accuracy of the A/D converter. For V

REF/2 volt-

ages of 2.4 V

DC nominal value, initial errors of ±10 mVDC will

cause conversion errors of ±1 LSB due to the gain of 2 of the

REF/2 input. In reduced span applications, the initial value

and the stability of the V

REF/2 input voltage become even

more important. For example, if the span is reduced to 2.5V,

the analog input LSB voltage value is correspondingly re-

duced from 20 mV (5V span) to 10 mV and 1 LSB at the

REF/2 input becomes 5 mV. As can be seen, this reduces

the allowed initial tolerance of the reference voltage and re-

quires correspondingly less absolute change with tempera-

ture variations. Note that spans smaller than 2.5V place

even tighter requirements on the initial accuracy and stability

of the reference source.

In general, the magnitude of the reference voltage will re-

quire an initial adjustment. Errors due to an improper value

of reference voltage appear as full-scale errors in the A/D

transfer function. IC voltage regulators may be used for ref-

erences if the ambient temperature changes are not exces-

sive. The LM336B 2.5V IC reference diode (from National

Semiconductor) has a temperature stability of 1.8 mV typ

(6 mV max) over 0˚C

≤T

A≤+70˚C. Other temperature range

parts are also available.

DS005671-15

FIGURE 6. The V

REFERENCE Design on the IC

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