MCP3302 datasheet(22/46 Pages) MICROCHIP

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MCP3302/04

DS21697E-page 22

5.5

Buffering/Filtering the Analog

Inputs

Inaccurate conversion results may occur if the signal

source for the A/D converter is not a low-impedance

source. Buffering the input will overcome the

impedance issue. It is also recommended that an

analog filter be used to eliminate any signals that may

be aliased back into the conversion results. This is

illustrated in Figure 5-9, where an op amp is used to

drive the analog input of the MCP3302/04. This

amplifier provides a low-impedance source for the

converter input and a low-pass filter, which eliminates

unwanted high-frequency noise. Values shown are for

a 10 Hz Butterworth Low-Pass filter.

Low-pass (anti-aliasing) filters can be designed using

Microchip’s interactive FilterLab® software. FilterLab

will calculate capacitor and resistor values, as well as

determine the number of poles that are required for the

application. For more information on filtering signals,

see Application Note 699 “Anti-Aliasing Analog Filters

for Data Acquisition Systems”

FIGURE 5-9:

The MCP601 Operational

Amplifier is used to implement a 2nd order anti-

aliasing filter for the signal being converted by

the MCP3302/04.

5.6

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 on the board should be

separated as much as possible, 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.

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 (see Figure 5-10). For more information on

layout tips when using the MCP3302/04,MCP3302/04,

or other ADC devices, refer to Application Note 688,

“Layout Tips for 12-Bit A/D Converter Applications”

FIGURE 5-10:

VDD traces arranged in a

‘Star’ configuration in order to reduce errors

caused by current return paths.

MCP330X

VDD

10 µF

IN-

IN+

VIN

2.2 µF

1µF

VREF

4.096V

Reference

0.1 µF

1µF

0.1 µF

MCP601

7.86 k

14.6 k

MCP1541

VDD

Connection

Device 1

Device 2

Device 3

Device 4

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