ADC0848BCN datasheet(10/20 Pages) NSC

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Applications Information (Continued)

2.0 REFERENCE CONSIDERATIONS

The voltage applied to the reference input of these convert-

ers defines the voltage span of the analog input (the differ-

ence between V

IN(MAX) and VIN(MIN)) over which the 256

possible output codes apply. The devices can be used in ei-

ther ratiometric applications or in systems requiring absolute

accuracy. The reference pin must be connected to a voltage

source capable of driving the minimum reference input resis-

tance of 1.1 k

Ω. This pin is the top of a resistor divider string

used for the successive approximation conversion.

In a ratiometric system (

Figure 2a), 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

CC. This technique relaxes the stability re-

quirements 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 (

Figure 2b), 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 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).

3.0 THE ANALOG INPUTS

3.1 Analog Differential Voltage Inputs and

Common-Mode Rejection

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 “−” inputs is 1⁄2 of a clock period. The

change in the common-mode voltage during this short time

interval can cause conversion errors. For a 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 tC is the conversion time.

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

(

≈5 mV) with the converter running at 40 µS, its peak value

would have to be 5.43V. This large a common-mode signal is

much greater than that generally found in a well designed

data acquisition system.

4 Single-Ended

DS005016-12

2 Differential

DS005016-13

3 Pseudo-Differential

DS005016-14

Combined

DS005016-15

FIGURE 1. Analog Input Multiplexer Options

DS005016-38

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