LTC1090CSW datasheet(18/28 Pages) LINER

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LTC1090

1090fc

6. Sharing the Serial Interface

The LTC1090 can share the same 3-wire serial interface

with other peripheral components or other LTC1090s (see

Figure 5). In this case, the CS signals decide which

LTC1090 is being addressed by the MPU.

ANALOG CONSIDERATIONS

1. Grounding

The LTC1090 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 4.7µF tantalum with leads as short as possible. Pin 12

(V–) should be bypassed with a 0.1µF 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 6 shows an example of an ideal 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.

2. Bypassing

For good performance, VCC must be free of noise and

ripple. Any changes in the VCC voltage with respect to

analog ground during a conversion cycle can induce

errors or noise in the output code. VCC noise and ripple can

be kept below 1mV by bypassing the VCC pin directly to the

analog ground plane with a 4.7µF tantalum with leads as

short as possible. Figures 7 and 8 show the effects of good

and poor VCC bypassing.

APPLICATIO S I FOR ATIO

Figure 7. Poor VCC Bypassing. Noise and Ripple

can Cause A/D Errors

Figure 6. Example Ground Plane for the LTC1090

HORIZONTAL: 10µs/DIV

Figure 8. Good VCC Bypassing Keeps Noise and Ripple

on VCC Below 1mV

HORIZONTAL: 10µs/DIV

VCC

V–

LTC1090 • AI15

ANALOG

GROUND

PLANE

0.1µF CERAMIC DISK

4.7µF TANTALUM

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