MCP3912 datasheet(54/82 Pages) MICROCHIP

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MCP3912

DS20005348A-page 54

 2014 Microchip Technology Inc.

FIGURE 7-4:

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 side/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 MCP3912. To reduce

sensitivity to external influences such as EMI, these

two wires should form a twisted pair, as noted in

Figure 7-5. The power supply and MCU are separated

on the right side of the PCB, surrounded by the digital

ground plane. The MCP3912 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

MCP3912. With this placement, there are two separate

current supply paths and current return paths, IA and ID.

FIGURE 7-5:

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.

7.3

SPI Interface Digital Crosstalk

The MCP3912 incorporates a high-speed 20 MHz SPI

digital interface. This interface can induce a crosstalk,

especially with the outer channels (CH0, for example),

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 7.2 “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 7-5 where these resistors

have been implemented). The resistors also help to

keep the level of electromagnetic emissions low.

The measurement graphs provided in this MCP3912

data sheet have been performed with 100

 series

resistors connected on each SPI I/O pin. Measurement

accuracy disturbances 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-28 package.

7.4

Sampling Speed and Bandwidth

If ADC power consumption is not a concern in the

design, the boost settings can be increased for best

performance so that the OSR is always kept at the

maximum settings to improve the SINAD performance

(see Table 7-1). If the MCU cannot generate a clock

fast enough, it is possible to tap the OSC1/OSC2 pins

of the MCP3912 crystal oscillator directly to the crystal

of the microcontroller. When the sampling frequency is

enlarged, the phase resolution is improved, and with

the OSR increased, the phase compensation range

can be kept in the same range as the default settings.

MCU

Power Supply

Circuitry

LINE

NEUTRAL

SHUNT

Twisted

Pair

“Star” Point

Analog Ground Plane

Digital Ground Plane

MCP3912

TABLE 7-1:

SAMPLING SPEED VS.

MCLK AND OSR,

ADC PRESCALE 1:1

MCLK

(MHz)

Boost<1:0>

OSR

Sampling

Speed

(ksps)

1024

3.91

1024

3.42

1024

2.93

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