LTC1288CS8 datasheet(15/24 Pages) LINER

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LTC1285/LTC1288

APPLICATION INFORMATION

Figure 7. LTC1288 “+” and “–” Input Settling Windows

BOARD LAYOUT CONSIDERATIONS

Grounding and Bypassing

The LTC1285/LTC1288 are easy to use if some care is

taken. They should be used with an analog ground plane

and single point grounding techniques. The GND pin

should be tied directly to the ground plane.

The VCC pin should be bypassed to the ground plane with

a 10

µF tantalum capacitor with leads as short as possible.

If the power supply is clean, the LTC1285/LTC1288 can

also operate with smaller 1

µF or less surface mount or

ceramic bypass capacitors. All analog inputs should be

referenced directly to the single point ground. Digital

inputs and outputs should be shielded from and/or routed

away from the reference and analog circuitry.

SAMPLE-AND-HOLD

Both the LTC1285 and the LTC1288 provide a built-in

sample-and-hold (S&H) function to acquire signals. The

S&H of the LTC1285 acquires input signals from “+” input

relative to “–” input during the tSMPL time (see Figure 1).

However, the S&H of the LTC1288 can sample input

signals in the single-ended mode or in the differential

inputs during the tSMPL time (see Figure 7).

CLK

DIN

DOUT

"+" INPUT

"–" INPUT

SAMPLE

HOLD

"+" INPUT MUST

SETTLE DURING

THIS TIME

tSMPL

tCONV

SGL/DIFF

START

MSBF

DON’T CARE

1ST BIT TEST "–" INPUT MUST

SETTLE DURING THIS TIME

B11

LTC1285/88 • F07

Figure 6. Interfacing a 3V Powered LTC1285 to a 5V System

+IN

–IN

GND

VCC

CLK

DOUT

VREF

4.7

µF

MPU

(e.g. 8051)

P1.4

P1.3

P1.2

LTC1285/88 • F06

DIFFERENTIAL INPUTS

COMMON-MODE RANGE

0V TO 3V

LTC1285

of Maximum Clock Rate vs Supply Voltage). If the maxi-

mum clock frequency is used, care must be taken to

ensure that the device converts correctly.

Mixed Supplies

It is possible to have a microprocessor running off a 5V

supply and communicate with the LTC1285/LTC1288

operating on a 3V supply. The inputs of CS, CLK and DIN

of the LTC1285/LTC1288 have no problem to take a

voltage swing from 0V to 5V. With the LTC1285 operating

on a 3V supply, the output of DOUT may only go between

0V and 3V. The 3V output level is higher enough to trip a

TTL input of the MPU. Figure 6 shows a 3V powered

LTC1285 interfacing a 5V system.

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