MCP3302 datasheet(20/40 Pages) MICROCHIP

20 / 40 page

MCP3302/04

DS21697B-page 20

 2002 Microchip Technology Inc.

6.5

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 imped-

ance 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 6-9, where an op amp is used to drive the ana-

log 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 6-9:

The MCP601 Operational

Amplifier is used to implement a 2nd order anti-

aliasing filter for the signal being converted by

the MCP3302/04.

6.6

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 sepa-

rated 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 possi-

ble 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 6-10). For more information on lay-

out tips when using the MCP3302/04, or other ADC

devices, refer to Application Note 688, “Layout Tips for

12-Bit A/D Converter Applications”.

FIGURE 6-10:

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

Ω

14.6 k

Ω

MCP1541

Connection

Device 1

Device 2

Device 3

Device 4

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