MCP3918 datasheet(61/88 Pages) MICROCHIP

61 / 88 page

 2014 Microchip Technology Inc.

DS20005287A-page 61

MCP3918

FIGURE 8-4:

Separation of Analog and

Digital Circuits on Layout.

Figure 7-5 shows a more detailed example with a direct

connection to a high-voltage line (e.g., a two-wire 120V

or 220V system). A current-sensing shunt is used for

current measurement on the high/line side that also

supplies the ground for the system. This is necessary

as the shunt is directly connected to the channel input

pins of the MCP3918. To reduce sensitivity to external

influences, such as EMI, these two wires should form a

twisted pair, as noted in Figure 8-5. The power supply

and MCU are separated on the right side of the PCB,

surrounded by the digital ground plane. The MCP3918

is kept on the left side, surrounded by the analog

ground plane. There are two separate power supplies

going to the digital section of the system and the analog

section, including the MCP3918. With this placement,

there are two separate current supply paths and

current return paths, IA and ID.

FIGURE 8-5:

Connection Diagram.

The ferrite bead between the digital and analog ground

planes helps keep high-frequency noise from entering

the device. This ferrite bead is recommended to be low

resistance; most often it is a THT component. Ferrite

beads are typically placed on the shunt inputs and into

the power supply circuit for additional protection.

8.3

SPI Interface Digital Crosstalk

The MCP3918 incorporates a high-speed 20-MHz SPI

digital interface. This interface can induce a crosstalk,

if it is running at its full speed without any precautions.

The crosstalk is caused by the switching noise created

by the digital SPI signals (also called ground bouncing).

This crosstalk would negatively impact the SNR in this

case. The noise is attenuated if a proper separation

between the analog and digital power supplies is put in

place (see Section 8.2 “Power Supply Design and

Bypassing”

). In order to further remove the influence

of the SPI communication on measurement accuracy,

it is recommended to add series resistors on the SPI

lines to reduce the current spikes caused by the digital

switching noise (see Figure 8-1 where these resistors

have been implemented). The resistors also help to

keep the level of electromagnetic emissions low. The

measurement graphs provided in this data sheet have

been performed with 100

series resistors connected

on each SPI I/O pin. Measurement accuracy distur-

bances have not been observed even at the full speed

of 20 MHz interfacing. The crosstalk performance is

dependent on the package choice due to the difference

in the pin arrangement (dual in-line or quad), and is

improved in the QFN-20 package.

MCU

Power Supply

Circuitry

LINE

NEUTRAL

SHUNT

Twisted

Pair

“Star” Point

Analog Ground Plane

Digital Ground Plane

MCP3918

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