CS5571

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DS768A5

6/25/07

14:12

3.5 Analog Input

in Figure 5 and Figure 6. These diagrams also illustrate a differential buffer amplifier configuration for driv-

ing the CS5571.

The capacitors at the outputs of the amplifiers provide a charge reservoir for the dynamic current from the

A/D inputs while the resistors isolate the dynamic current from the amplifier. The amplifiers can be pow-

ered from higher supplies than those used by the A/D but precautions should be taken to ensure that the

op-amp output voltage remains within the power supply limits of the A/D, especially under start-up condi-

tions.

3.6 Output Coding Format

The reference voltage directly defines the input voltage range in both the unipolar and bipolar configura-

tions. In the unipolar configuration (BP/UP low), the first code transition occurs 0.5 LSB above zero, and

the final code transition occurs 1.5 LSBs below VREF. In the bipolar configuration (BP/UP high), the first

code transition occurs 0.5 LSB above -VREF and the last transition occurs 1.5 LSBs below +VREF. See

Table 1 for the output coding of the converter.

NOTE: VREF = [(VREF+) - (VREF-)] / 2

Table 1. Output Coding, Two’s Complement

Bipolar Input Voltage

Two’s

Complement

>(VREF-1.5 LSB)

7F FF

VREF-1.5 LSB

7F FF

7F FE

-0.5 LSB

00 00

FF FF

-VREF+0.5 LSB

80 01

80 00

<(-VREF+0.5 LSB)

80 00

NOTE: VREF = [(VREF+) - (VREF-)] / 2

Table 2. Output Coding, Offset Binary

Unipolar Input Voltage

Offset

Binary

>(VREF-1.5 LSB)

FF FF

VREF-1.5 LSB

FF FF

FF FE

(VREF/2)-0.5 LSB

80 00

7F FF

+0.5 LSB

00 01

00 00

<(+0.5 LSB)

00 00