MCP3021 datasheet(19/26 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS21805A-page 19

MCP3021

6.3

Device Polling

In some instances, it may be necessary to test for

MCP3021 presence on the I2C bus without performing

a conversion, described in Figure 6-4. Here we are set-

ting the R/W bit in the address byte to a zero. The

MCP3021 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 MCP3021

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 MCP3021.

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

underneath the device or the bypass capacitor. Extra

precautions should be taken to keep traces with high

frequency signals (such as clock lines) as far as

possible from analog traces.

The MCP3021 should be connected entirely to the ana-

log ground place, as well as the analog power trace.

The pull-up resistors can be placed close to the

microcontroller and tied to the digital power or VCC.

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 MCP3021 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 MCP3021 uses VDD as power and also as 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.

123

456

SCL

SDA

00 1 A2A1A0 0

Start

Bit

Address Byte

Address bits

Device bits

R/W

Start

Bit

MCP3021 response

VDD

AIN

SCL

SDA

Microcontroller

10 µF

MCP3021

0.1 µF

VCC

RPU

VDD

Connection

Device 1

Device 2

Device 3

Device 4

VDD

4.096V

Reference

1µF

0.1 µF

MCP1541

AIN

MCP3021

VCC

RPU

SCL

SDA

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