MCP3301-CISLASHP datasheet(19/32 Pages) MICROCHIP

19 / 32 page

DS21700C-page 19

MCP3301

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 solve 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. Using an op amp to drive

the analog input of the MCP3301 is illustrated in

Figure 6-9. This amplifier provides a low impedance

source for the converter input and 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 AN-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 MCP3301.

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 V

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 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 V

DD connections to

devices in a “star” configuration can also reduce noise

by eliminating current return paths and associated

errors (Figure 6-10). For more information on layout

tips when using the MCP3301 or other ADC devices,

refer to AN-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.

MCP3301

10 µF

IN-

IN+

2.2 µF

1µF

REF

4.096V

Reference

0.1 µF

1µF

0.1 µF

MCP601

7.86 k

14.6 k

MCP1541

Connection

Device 1

Device 2

Device 3

Device 4

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