ADC12762 datasheet(16/20 Pages) NSC

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Applications Information (Continued)

POWER SUPPLY CONSIDERATIONS

The ADC12762 is designed to operate from a single +5V

power supply. There are two analog supply pins (AV

CC) and

one digital supply pin (DV

CC). These pins allow separate ex-

ternal bypass capacitors for the analog and digital portions of

the circuit. To guarantee proper operation of the converter,

all three supply pins should be connected to the same volt-

age source. In systems with separate analog and digital sup-

plies, the converter should be powered from the analog sup-

ply.

The ground pins are AGND (analog ground), DGND1 (digital

input ground), and DGND2 (digital output ground). These

pins allow for three separate ground planes for these sec-

tions of the chip. Isolating the analog section from the two

digital sections reduces digital interference in the analog cir-

cuitry, improving the dynamic performance of the converter.

Separating the digital outputs from the digital inputs (particu-

larly the S/H input) reduces the possibility of ground bounce

from the 12 data lines causing jitter on the S/H input. The

analog ground plane should be connected to the Digital2

ground plane at the ground return for the power supply. The

Digital1 ground plane should be tied to the Digital2 ground

plane at the DGND1 and DGND2 pins.

Both AV

CC pins should be bypassed to the AGND ground

plane with 0.1 µF ceramic capacitors. One of the two AV

pins should also be bypassed with a 10 µF tantalum capaci-

tor. DV

CC should be bypassed to the DGND2 ground pIane

with a 0.1 µF capacitor in parallel with a 10 µF tantalum ca-

pacitor.

LAYOUT AND GROUNDING

In order to ensure fast, accurate conversions from the

ADC12762, it is necessary to use appropriate circuit board

layout techniques. Separate analog and digital ground

planes are required to meet datasheet AC and DC limits. The

analog ground plane should be low-impedance and free of

noise from other parts of the system.

All bypass capacitors should be located as close to the con-

verter as possible and should connect to the converter and

to ground with short traces. The analog input should be iso-

lated from noisy signal traces to avoid having spurious sig-

nals couple to the input. Any external component (e.g., a fil-

ter capacitor) connected across the converter’s input should

be connected to a very clean analog ground return point.

Grounding the component at the wrong point will result in in-

creased noise and reduced conversion accuracy.

Figure 12 gives an example of a suitable layout, including

power supply routing, ground plane separation, and bypass

capacitor placement. All analog circuitry (input amplifiers, fil-

ters, reference components, etc.) should be placed on the

analog ground plane. All digital circuitry and I/O lines (ex-

DS012811-34

FIGURE 11. Using the V

REF Force Pins Only

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