MCP3912 datasheet(53/82 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005348A-page 53

MCP3912

7.2

Power Supply Design and

Bypassing

The MCP3912 device was designed to measure posi-

tive and negative voltages that might be generated by

a current sensing device. This current sensing device,

with a common mode voltage close to 0V, is referred to

as AGND, which is a shunt or current transformer (CT)

with burden resistors attached to ground. The high per-

formance and good flexibility that characterize this

ADC enables them to be used in other applications, as

long as the absolute voltage on each pin, referred to

AGND, stays in the -1V to +1V interval.

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

operational amplifiers) are always connected to the

analog ground plane. The MCP3912 should also be

considered as a sensitive analog component, and con-

nected 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 7-2).

This way, the design will feature two ground traces and

two power supplies (Figure 7-3).

This means the analog circuitry (including MCP3912)

and the digital circuitry (MCU) should have separate

power supplies and return paths to the external ground

reference, as described in Figure 7-2. An example of a

typical power supply circuit, with different lines for ana-

log and digital power, is shown in Figure 7-3. A possible

split example is shown in Figure 7-4, where the ground

star connection can be done at the bottom of the device

with the exposed pad. The split here 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 MCP3912 as an

analog component and, therefore, 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 MCP3912) now causing glitches on the

analog power supply.

FIGURE 7-2:

All Analog and Digital

Return Paths Need to Stay Separate with Proper

Bypass Capacitors.

FIGURE 7-3:

Power Supply with Separate Lines for Analog and Digital Sections. Note the "Net Tie"

Object NT2 that Represents the Start Ground Connection.

MCU

“Star” Point

AVDD DVDD

AGND DGND

D-=

A-=

0.1 μF

MCP39XX

0.1 μF

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