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LTC1287C

3V Single Chip 12-Bit Data Acquisition System

Linear Technology 

LTC1287

APPLICATI S I FOR ATIO

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

the above standards. The following summarizes such

levels.

VOH (no load)

VOL (no load)

VOH

VOL

VIH

VIL

VCC – 0.1V

0.1V

0.9 × VCC

0.1 × VCC

0.7 × VCC

0.2 × VCC

The LTC1287 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 LTC1287.

Microprocessor Interfaces

The LTC1287 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 three of the MPU’s parallel port lines can be

programmed to form the serial link to the LTC1287. Many

of the popular MPUs can operate with 3V supplies. For

example the MC68HC11 is an MPU with a serial format

(SPI). Likewise parallel MPUs 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 LTC1292 data sheet.

Sharing the Serial Interface

The LTC1287 can share the same two-wire serial interface

with other peripheral components or other LTC1287s

(Figure 2). In this case, the CS signals decide which

LTC1287 is being addressed by the MPU.

ANALOG CONSIDERATIONS

Grounding

The LTC1287 should be used with an analog ground plane

and single point grounding techniques. Do not use wire

wrapping techniques to breadboard and evaluate the device.

To achieve the optimum performance use a PC board. The

ground pin (Pin 4) should be tied directly to the ground

plane with minimum lead length (a low profile socket is

fine). Pin 7 (VCC) should be bypassed to the ground plane

with a 22µF (minimum value) tantalum with leads as short

as possible and as close as possible to the pin. A 0.1µF

ceramic disk also should be placed in parallel with the

22µF and again with leads as short as possible and as close

to VCC as possible. Figure 3 shows an example of an ideal

LTC1287 ground plane design for a two-sided board. Of

course this much ground plane will not always be possible,

but users should strive to get as close to this ideal as

possible.

Bypassing

For good performance, VCC must be free of noise and

ripple. Any changes in the VCC voltage with respect to

ground during a conversion cycle can induce errors or

noise in the output code. VCC noise and ripple can be kept

below 0.5mV by bypassing the VCC pin directly to the

analog plane with a minimum of 22µF tantalum capacitor

and with leads as short as possible. The lead from the

device to the VCC supply also should be kept to a minimum

and the VCC supply should have a low output impedance

0.1µF VCC

21 0

OUTPUT PORT

SERIAL DATA

MPU

2

2

CS

LTC1287

2

CS

LTC1287

2

CS

LTC1287

2 CHANNELS 2 CHANNELS 2 CHANNELS

2-WIRE SERIAL

INTERFACE TO OTHER

PERIPHERALS OR LTC1287s

LTC1287 F02

22µF TANTALUM

1

8

2

7

LTC1287

3

6

4

5

LTC1287 F03

Figure 2. Several LTC1287s Sharing One 2-Wire Serial Interface

8

Figure 3. Example Ground Plane for the LTC1287

1287fa

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