_____________Analog Considerations

Reference and Input

zero and the full-scale of the ADC. In other words, the

duces an output code of all zeros, and the voltage at

code of all ones (Figure 7).

Figure 8 shows some possible reference configura-

tions. A 0.01µF bypass capacitor to GND should be

used to reduce the high-frequency output impedance

of the internal reference. Larger capacitors should not

be used, as this degrades the stability of the reference

buffer. The 2.5V reference output is with respect to the

GND pin.

Bypassing

A 47µF electrolytic and 0.1µF ceramic capacitor should

tors must have minimum lead length, since excess lead

length may contribute to conversion errors and insta-

bility. If the reference inputs are driven by long lines,

they should be bypassed to GND with 0.1µF capac-

itors at the reference input pins.

CMOS High-Speed 8-Bit ADCs with

Multiplexer and Reference

8

_______________________________________________________________________________________

DATA

NEW

DATA

ADDR

VALID

INT

RDY

RD

ANALOG

CHANNEL

ADDRESS

CS

ADDR

VALID

OLD

DATA

Figure 6. Mode 1 Timing Diagram

11111111

11111110

11111101

00000011

00000010

00000001

00000000

1

23

FS

FS–1LSB

OUTPUT

CODE

FULL-SCALE

TRANSITION

256

256

AIN INPUT VOLTAGE

(IN TERMS OF LSBs)

Figure 7. Transfer Function