MCP3426 datasheet(29/56 Pages) MICROCHIP

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DS22226A-page 29

MCP3426/7/8

6.0

BASIC APPLICATION

CONFIGURATION

The MCP3426/7/8 devices can be used for various

precision

analog-to-digital

converter

applications.

These devices operate with very simple connections to

the application circuit. The following sections discuss

the

examples

the

device

connections

and

applications.

6.1

Connecting to the Application

Circuits

6.1.1

BYPASS CAPACITORS ON VDD PIN

For an accurate measurement, the application circuit

needs a clean supply voltage and must block any noise

signal to the MCP3426/7/8 devices. Figure 6-1 shows

an example of using two bypass capacitors (a 10 µF

tantalum capacitor and a 0.1 µF ceramic capacitor) on

the VDD line of the MCP3428. These capacitors are

helpful to filter out any high frequency noises on the

VDD line and also provide the momentary bursts of

extra currents when the device needs from the supply.

These capacitors should be placed as close to the VDD

pin as possible (within one inch). If the application

circuit has separate digital and analog power supplies,

the VDD and VSS of the MCP3426/7/8 devices should

reside on the analog plane.

6.1.2

CONNECTING TO I2C BUS USING

PULL-UP RESISTORS

The SCL and SDA pins of the MCP3426/7/8 are

open-drain configurations. These pins require a pull-up

resistor as shown in Figure 6-1. The value of these

pull-up resistors depends on the operating speed

(standard,

fast,

and

high

speed)

and

capacitance of the I2C bus line. Higher value of pull-up

resistor consumes less power, but increases the signal

transition time (higher RC time constant) on the bus.

Therefore, it can limit the bus operating speed. The

lower value of resistor, on the other hand, consumes

higher power, but allows higher operating speed. If the

bus line has higher capacitance due to long bus line or

high number of devices connected to the bus, a smaller

pull-up resistor is needed to compensate the long RC

time constant. The pull-up resistor is typically chosen

between 5 k

Ω and 10 kΩ ranges for standard and fast

modes, and less than 1 k

Ω for high speed mode

depending on the presence of bus loading capacitance.

6.1.3

I2C ADDRESS SELECTION PINS

(MCP3427 AND MCP3428)

The user can tie the Adr0 and Adr1 pins to VSS, VDD,

or left floating. See more details in Section 5.3.2

“Device Address Bits (A2, A1, A0) and Address

Selection Pins (MCP3427 and MCP3428)”.

FIGURE 6-1:

Typical Connection.

Figure 6-2 shows an example of multiple device

connections.

The

I2C

bus

capacitance

increases as the number of device connected to the I2C

bus line increases. The bus loading capacitance affects

on the bus operating speed. For example, the highest

bus operating speed for the 400 pF bus capacitance is

1.7 MHz, and 3.4 MHz for 100 pF. Therefore, the user

needs to consider the relationship between the

maximum operation speed versus. the number of I2C

devices that are connected to the I2C bus line.

FIGURE 6-2:

Example of Multiple Device

Connection on I2C Bus.

VDD

CH2-

VSS

CH3+

Adr1

Adr0

CH2+

CH3-

CH1-

CH4+

CH1+

CH4-

SDA

SCL

VDD

MCP3428

Input

Signal 2

Signal 1

Input

Signal 4

Signal 3

TO MCU

(MASTER)

I2C Address

Selection

Pins

Rp is the pull-up resistor:

Ω - 10 kΩ for f

SCL

= 100 kHz to 400 kHz

~700

Ω for f

SCL

= 3.45 MHz

C1: 0.1 µF, Ceramic capacitor

C2: 10 µF, Tantalum capacitor

SDA SCL

Microcontroller

MCP3426

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(PIC16F876)

MCP4725

MCP3427

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