LTC1289BCSW datasheet(15/28 Pages) LINER

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LTC1289

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APPLICATI

I FOR ATIO

Logic Levels

The logic level standards for this supply range have not

been well defined. What standards that do exist are not

universally accepted. The trip point on the logic inputs of

the LTC1289 is 0.28

× VCC. This makes the logic inputs

compatible with HC type logic levels and processors that

are specified at 3.3V. The output DOUT is also compatible

with the above standards. The following summarizes such

levels.

The LTC1289 can be driven with 5V logic even when VCC

is at 3.3V. This is due to a unique input protection device

that is found on the LTC1289.

Microprocessor Interfaces

The LTC1289 can interface directly (without external hard-

ware) to most popular microprocessor (MPU) synchro-

nous serial formats. If an MPU without a serial interface is

used, then four of the MPU’s parallel port lines can be

programmed to form the serial link to the LTC1289. Many

of the popular MPU's can operate with 3V supplies. For

example the MC68HC11 is an MPU with a serial format

(SPI). Likewise parallel MPU’s that have the 8051 type

architecture are also capable of operating at this voltage

range. The code for these processors remains the same

and can be found in the LTC1290 datasheet or application

notes AN36A and AN36B.

Sharing the Serial Interface

The LTC1289 can share 3-wire serial interface with other

peripheral components or other LTC1289s (See Figure 5).

In this case, the CS signals decide which LTC1289 is being

addressed by the MPU.

ANALOG CONSIDERATIONS

1. Grounding

The LTC1289 should be used with an analog ground plane

and single point grounding techniques.

Pin 11 (AGND) should be tied directly to this ground plane.

Pin 10 (DGND) can also be tied directly to this ground

plane because minimal digital noise is generated within

the chip itself.

Pin 20(VCC) should be bypassed to the ground plane with a

µF tantalum with leads as short as possible. Pin 12 (V–)

should be bypassed with a 0.1

µF ceramic disk. For single

supply applications, V– can be tied to the ground plane.

It is also recommended that pin 13 (REF –) and pin 9 (COM)

be tied directly to the ground plane. 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.

Figure 5. Several LTC1289s Sharing One 3-Wire Serial Interface

VOH (no load)

VCC - 0.1V

VOL (no load)

0.1V

VOH

0.9

× VCC

VOL

0.1

× VCC

VIH

0.7

× VCC

VIL

0.2

× VCC

8 CHANNELS

3-WIRE SERIAL

INTERFACE TO OTHER

PERIPHERALS OR LTC1289s

OUTPUT PORT

SERIAL DATA

MPU

LTC1289 AIF05

LTC1289

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