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LTC1091AC

1-, 2-, 6- and 8-Channel, 10-Bit Serial I/O Data Acquisition Systems

Linear Technology 

LTC1091/LTC1092

LTC1093/LTC1094

APPLICATI S I FOR ATIO

21 0

OUTPUT PORT

3

SERIAL DATA

3

CS

MPU

LTC1094

3

CS

LTC1094

3

CS

LTC1094

3-WIRE SERIAL

INTERFACE TO OTHER

PERIPHERALS OR LTC1094s

LTC1091-4 F03

8 CHANNELS 8 CHANNELS 8 CHANNELS

Figure 3. Several LTC1094s Sharing One 3-Wire Serial Interface

Sharing the Serial Interface

The LTC1094 can share the same 2- or 3-wire serial

interface with other peripheral components or other

LTC1094s (see Figure 3). In this case, the CS signals

decide which LTC1094 is being addressed by the MPU.

ANALOG CONSIDERATIONS

1. Grounding

The LTC1091/LTC1092/LTC1093/LTC1094 should be used

with an analog ground plane and single point grounding

techniques.

The AGND pin (GND on the LTC1091/LTC1092) should be

tied directly to this ground plane.

The DGND pin of the LTC1093/LTC1094 can also be tied

directly to this ground plane because minimal digital noise

is generated within the chip itself.

The VCC pin should be bypassed to the ground plane with

a 4.7µF tantalum with leads as short as possible. AVCC and

DVCC should be tied together on the LTC1094. The V– pin

(LTC1093/LTC1094) 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 the REF – pin and the COM pin

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 4 shows an example of an ideal LTC1091 ground

plane design for a 2-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. Because the VCC (VREF)

pin of the LTC1091 defines the voltage span of the A/D

converter, its bypassing is especially important. 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. AVCC and DVCC should be

tied together on the LTC1094. Figures 5 and 6 show the

effects of good and poor VCC bypassing.

4.7µF VCC

TANTALUM

S

1

8S

2

7

3

6

4

5

LTC1091-4 F04

Figure 4. Example Ground Plane for the LTC1091

20

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