Datasheet

ADC08831, ADC08832

SNAS015C –SEPTEMBER 1999–REVISED MARCH 2013

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REFERENCE CONSIDERATIONS

The voltage applied to the reference input on these converters, V

REF

, defines the voltage span of the analog

input (the difference between V

IN(MAX)

and V

IN(MIN)

over which the 256 possible output codes apply. The devices

can be used either in ratiometric applications or in systems requiring absolute accuracy. The reference pin must

be connected to a voltage source capable of driving the reference input resistance which can be as low as

2.8kΩ. This pin is the top of a resistor divider string and capacitor array used for the successive approximation

conversion.

In a ratiometric system the analog input voltage is proportional to the voltage used for the A/D reference. This

voltage is typically the system power supply, so the V

REF

pin can be tied to V

(done internally on the

ADC08832). This technique relaxes the stability requirements of the system reference 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 between very specific voltage limits, the reference pin can

be biased with a time and temperature stable voltage source. The LM385, LM336 and LM4040 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 direct 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).

Left) Ratiometric

Right) Absolute with a Reduced Span

Figure 22. Reference Examples

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 processor with a highly noise immune serial bit

stream. This in itself greatly minimizes circuitry to maintain analog signal accuracy 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.

Product Folder Links: ADC08831 ADC08832