MCP3461RT-ESLASHST datasheet(87/118 Pages) MICROCHIP

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DS20006404C-page 87

MCP3461/2/4R

7.3

Power Supply Design and

Bypassing

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

operational amplifiers) are always connected to the

analog ground plane. The MCP3461/2/4R devices

should also be considered sensitive analog components

and connected to the analog ground plane. The ADC

features two pairs of power supply voltage pins: AGND

and AVDD, DGND and DVDD. For best performance, it is

recommended to keep the two pairs of pins connected to

two different networks (see Figure 7-5), so that the

design will feature two ground traces and two power

supplies (see Figure 7-6).

The analog circuitry (including MCP3461/2/4R) and the

digital circuitry (MCU) should have separate power

supplies and return paths to the external ground refer-

ence, as described in Figure 7-5. An example of a

typical power supply circuit, with different paths for

analog and digital return currents, is shown in

Figure 7-6. A possible split example is shown in

Figure 7-7, where the ground star connection can be

located underneath 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 with

lines coming under the ground plane. The two separate

return paths will eventually share a unique connection

point (star connection) in order to minimize coupling

between the two power supply domains.

Another possibility, sometimes easier to implement in

terms of PCB layout, is to consider the MCP3461/2/4R

as an analog component, and therefore, connect AVDD

to DVDD and AGND to DGND 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

MCP3461/2/4R) now causing glitches on the analog

power supply.

Figure 7-6 shows an example of a power supply

schematic with separate DVDD and AVDD. A high-current

LDO (MCP1825) was used for the DVDD line to be able

to power the MCU and other peripherals attached to the

MCU. A high PSRR LDO (MCP1754) is used for the

AVDD that goes to the ADC and a few other components

sensitive to noise. The Net tie is used to separate DGND

from AGND.

FIGURE 7-5:

Separating Digital and

Analog Ground by Using a Star Connection.

FIGURE 7-6:

Power Supply with Separate Lines for Analog and Digital Sections (the “Net Tie”

Object Represents the Star Ground Connection).

MCU

“Star” Point

AVDD DVDD

AGND

GND

D-=

A-=

0.1 F

MCP3 6

0.1 F

GND

+5V USB

GNDA

+9V IN

Power Jack 2.5 mm

J10

MRA4005

0.1 μF

0603

C11

0.1 μF

0603

C10

GND

3.3A

3.3D

GNDA

0.1 μF

0603

C12

MCP1754-3.3V

VOUT 3

VIN

GND

LM1117-5V

VIN

VOUT 2

MCP1825S-3.3V

VOUT 3

VIN

GND

Net Tie

GND

GNDA

10 μF

TANT-B

C15

10 μF

TANT-B

C44

10 μF

TANT-B

C13

10 μF

TANT-B

C45

GND

5V_USB

0.1 μF

0603

C14

GNDA

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