MCP3564 datasheet(80/108 Pages) MICROCHIP

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MCP3561/2/4

DS20006181C-page 80

 2019-2021 Microchip Technology Inc.

FIGURE 7-7:

Separation of Analog and

Digital Circuits on the Layout (Shown on the

UQFN Package).

When remote sensors are used to reduce the sensitivity

to external influences, such as EMI, the wires that

connect the sensor to the ADC should form a twisted

pair. Ferrite beads can be used between the digital and

analog ground planes to keep high-frequency noise

from entering the device. A low-resistance ferrite bead

is recommended.

7.4

SPI Interface Digital Crosstalk

The MCP3561/2/4 devices incorporate a high-speed

20 MHz SPI digital interface. This interface can induce

crosstalk, especially with the outer channels closer to

the SPI digital pins (for example, CH7), if it is run at full

speed without any precautions. The crosstalk is caused

by the switching noise created by the digital SPI

signals. This crosstalk would negatively impact the

SNR in this case. The noise is attenuated if proper

separation between the analog and the digital power

supplies is put in place (see Sections 7.3 “Power

Supply Design and Bypassing”).

In order to further remove the influence of the SPI

communication on measurement accuracy, it is recom-

mended to add series resistors on the SPI lines to

reduce the current spikes caused by the digital switch-

ing noise (see Figure 7-1 where these resistors have

been implemented). The resistors also help to keep the

level of electromagnetic emissions low.

The switching noise is also a linear function of the

DVDD supply voltage. In order to further reduce the

influence of the switching noise caused by SPI trans-

missions, the DVDD digital power supply voltage should

be kept as low value as possible.

The

measurement

graphs

provided

this

MCP3561/2/4 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 20 MHz interfacing.

9 10

20 19 18

CH0

CH1

SCK

SDO

REFIN-

CH2

REFIN+

SDI

IRQ/MDAT

ANALOG

DIGITAL

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