MCP3221 datasheet(19/26 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS21732B-page 19

MCP3221

6.3

Device Polling

In some instances, it may be necessary to test for

MCP3221 presence on the I2C bus without performing

a conversion. This operation is described in Figure 6-4.

Here we are setting the R/W bit in the address byte to

a zero. The MCP3221 will then acknowledge by pulling

SDA low during the ACK clock and then release the

bus back to the I2C master. A stop or repeated start bit

can then be issued from the master and I2C

communication can continue.

FIGURE 6-4:

Device Polling.

6.4

Device Power and Layout

Considerations

6.4.1

POWERING THE MCP3221

VDD supplies the power to the device as well as the ref-

erence voltage. A bypass capacitor value of 0.1 µF is

recommended. Adding a 10 µF capacitor in parallel is

recommended to attenuate higher frequency noise

present in some systems.

FIGURE 6-5:

Powering the MCP3221.

6.4.2

LAYOUT CONSIDERATIONS

When laying out a printed circuit board for use with

analog components, care should be taken to reduce

noise wherever possible. A bypass capacitor from VDD

to ground should always be used with this device and

should be placed as close as possible to the device pin.

A bypass capacitor value of 0.1 µF is recommended.

Digital and analog traces should be separated as much

as possible on the board, with no traces running under-

neath the device or the bypass capacitor. Extra precau-

tions should be taken to keep traces with high-

frequency signals (such as clock lines) as far as

possible from analog traces.

Use of an analog ground plane is recommended in

order to keep the ground potential the same for all

devices on the board. Providing VDD connections to

devices in a “star” configuration can also reduce noise

by eliminating current return paths and associated

errors (Figure 6-6). For more information on layout tips

when using the MCP3221 or other ADC devices, refer

to AN688, “Layout Tips for 12-Bit A/D Converter

Applications”.

FIGURE 6-6:

VDD traces arranged in a

‘Star’ configuration in order to reduce errors

caused by current return paths.

6.4.3

USING A REFERENCE FOR

SUPPLY

The MCP3221 uses VDD as both power and a refer-

ence. In some applications, it may be necessary to use

a stable reference to achieve the required accuracy.

Figure 6-7 shows an example using the MCP1541 as a

4.096V, 2% reference.

FIGURE 6-7:

Stable Power and

Reference Configuration.

12 3 4 5 6 7 8 9

SCL

SDA

1 00 1 A2 A1A0 0

Start

Bit

Address Byte

Address bits

Device bits

R/W

Start

Bit

MCP3221 response

VDD

AIN

SCL

SDA

Microcontroller

10 µF

MCP3221

0.1 µF

VDD

RPU

VDD

Connection

Device 1

Device 2

Device 3

Device 4

VDD

4.096V

Reference

1µF

MCP1541

AIN

MCP3221

VDD

RPU

SCL

SDA

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