MCP3204-CISLASHP datasheet(20/34 Pages) MICROCHIP

20 / 34 page

MCP3204/3208

DS21298D-page 20

6.4

Layout Considerations

When laying out a printed circuit board for use with

analog components, care should be taken to reduce

noise wherever possible. A bypass capacitor should

always be used with this device, placed as close as

possible to the device pin. A bypass capacitor value of

1 µF is recommended.

Digital and analog traces should be separated as much

as possible on the board, with no traces running under-

neath the device or the bypass capacitor. Extra precau-

tions should be taken to keep traces with high

frequency signals (such as clock lines) as far as

possible from analog traces.

Use of an analog ground plane is recommended in

order to keep the ground potential the same for all

devices on the board. Providing V

DD connections to

devices in a “star” configuration can also reduce noise

by eliminating return current paths and associated

errors (see Figure 6-4). For more information on layout

tips when using A/D converters, refer to AN688,

“Layout Tips for 12-Bit A/D converter Applications”.

FIGURE 6-4:

DD traces arranged in a

‘Star’ configuration in order to reduce errors

caused by current return paths.

6.5

Utilizing the Digital and Analog

Ground Pins

The MCP3204/3208 devices provide both digital and

analog ground connections to provide another means

of noise reduction. As shown in Figure 6-5, the analog

and digital circuitry is separated internal to the device.

This reduces noise from the digital portion of the device

being coupled into the analog portion of the device. The

two grounds are connected internally through the sub-

strate, which has a resistance of 5 -10

Ω.

If no ground plane is utilized, then both grounds must

be connected to VSS on the board. If a ground plane is

available, both digital and analog ground pins should

be connected to the analog ground plane. If both an

analog and a digital ground plane are available, both

the digital and the analog ground pins should be con-

nected to the analog ground plane. Following these

steps will reduce the amount of digital noise from the

rest of the board being coupled into the A/D converter.

FIGURE 6-5:

Separation of Analog and

Digital Ground Pins.

Connection

Device 1

Device 2

Device 3

Device 4

MCP3204/08

Analog Ground Plane

DGND

AGND

0.1 µF

Substrate

5 - 10

Digital Side

-SPI Interface

-Shift Register

-Control Logic

Analog Side

-Sample Cap

-Capacitor Array

-Comparator

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