MCP3918 datasheet(60/88 Pages) MICROCHIP

60 / 88 page

MCP3918

DS20005287A-page 60

 2014 Microchip Technology Inc.

In any system, the analog ICs (such as references or

operational amplifiers) are always connected to the

analog ground plane. The MCP3918 should also be

considered as a sensitive analog component, and

should be connected to the analog ground plane. The

ADC features two pairs of pins: AGND, AVDD, DGND and

DVDD. For best performance, it is recommended to

keep the two pairs connected to two different networks

(Figure 8-2). This way, the design will feature two

ground traces and two power supplies (Figure 8-3).

This means the analog circuitry (including MCP3918)

and the digital circuitry (MCU) should have separate

power supplies and return paths to the external ground

reference, as described in Figure 8-2.

An example of a typical power supply circuit, with

different lines for analog and digital power, is shown in

Figure 8-3. A possible split example is shown in

Figure 8-4, where the ground star connection can be

done at the bottom of the device with the exposed pad.

The split between analog and digital can be done under

the device, and AVDD and DVDD can be connected

together with lines coming under the ground plane.

Another possibility, sometimes easier to implement in

terms of PCB layout, is to consider the MCP3918 as an

analog component and, therefore, to connect both

AVDD and DVDD together, and AGND and DGND

together, with a star connection. In this scheme, the

decoupling capacitors may be larger, due to the ripple

on the digital power supply (caused by the digital filters

and the SPI interface of the MCP3918) now causing

glitches on the analog power supply.

FIGURE 8-2:

All Analog and Digital

Return Paths Need to Stay Separate with Proper

Bypass Capacitors.

FIGURE 8-3:

Power Supply with Separate Lines for Analog and Digital Sections.

MCU

“Star” Point

AVDD DVDD

AGND DGND

D-=

A-=

0.1 μF

MCP39XX

0.1 μF

Note:

The “Net Tie” Object NT2 represents the start ground connection.

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