ADC08031 datasheet(14/24 Pages) NSC

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Functional Description (Continued)

3.0 REFERENCE CONSIDERATIONS

The voltage applied to the reference input on these convert-

ers, V

REFIN, defines the voltage span of the analog input

(the difference between V

IN(MAX) and VIN(MIN) over which the

256 possible output codes apply. The devices can be used

either in ratiometric applications or in systems requiring ab-

solute accuracy. The reference pin must be connected to a

voltage source capable of driving the reference input resis-

tance which can be as low as 1.3k

Ω. This pin is the top of a

resistor divider string and capacitor array used for the suc-

cessive approximation conversion.

In a ratiometric system the analog input voltage is propor-

tional to the voltage used for the A/D reference. This voltage

is typically the system power supply, so the V

REFIN pin can

be tied to V

CC (done internally on the ADC08032). This tech-

nique relaxes the stability requirements of the system refer-

ence as the analog input and A/D reference move together

maintaining the same output code for a given input condition.

For absolute accuracy, where the analog input varies be-

tween very specific voltage limits, the reference pin can be

biased with a time and temperature stable voltage source.

For the ADC08034 and the ADC08038 a band-gap derived

reference voltage of 2.6V (Note 8) is tied to V

REFOUT. This

can be tied back to V

REFIN. Bypassing VREFOUT with a

100µF capacitor is recommended. The LM385 and LM336

reference diodes are good low current devices to use with

these converters.

The maximum value of the reference is limited to the V

supply voltage. The minimum value, however, can be quite

small (see Typical Performance Characteristics) to allow di-

rect conversions of transducer outputs providing less than a

5V output span. Particular care must be taken with regard to

noise pickup, circuit layout and system error voltage sources

when operating with a reduced span due to the increased

sensitivity of the converter (1 LSB equals V

REF/256).

4.0 THE ANALOG INPUTS

The most important feature of these converters is that they

can be located right at the analog signal source and through

just a few wires can communicate with a controlling proces-

sor with a highly noise immune serial bit stream. This in itself

greatly minimizes circuitry to maintain analog signal accu-

racy which otherwise is most susceptible to noise pickup.

However, a few words are in order with regard to the analog

inputs should the input be noisy to begin with or possibly

riding on a large common-mode voltage.

The differential input of these converters actually reduces

the effects of common-mode input noise, a signal common

to both selected “+” and “−” inputs for a conversion (60 Hz is

most typical). The time interval between sampling the “+” in-

put and then the “−” input is 1⁄2 of a clock period. The change

in the common-mode voltage during this short time interval

can

cause

conversion

errors.

For

sinusoidal

common-mode signal this error is:

where f

CM is the frequency of the common-mode signal,

PEAK is its peak voltage value

and f

CLK is the A/D clock frequency.

For a 60Hz common-mode signal to generate a 1⁄4 LSB error

(

≈5mV) with the converter running at 250kHz, its peak value

would have to be 6.63V which would be larger than allowed

as it exceeds the maximum analog input limits.

Source resistance limitation is important with regard to the

DC leakage currents of the input multiplexer. Bypass capaci-

tors should not be used if the source resistance is greater

than 1k

Ω. The worst-case leakage current of ±1µA over tem-

perature will create a 1mV input error with a 1k

Ω source re-

sistance. An op amp RC active low pass filter can provide

both impedance 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 mini-

mum analog input voltage value, V

IN(MIN), is not ground a

zero offset can be done. The converter can be made to out-

DS010555-52

a) Ratiometric

DS010555-53

b) Absolute with a Reduced Span

FIGURE 2. Reference Examples

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