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DS00540B-page 3

Implementing IIR Digital Filters

FIGURE 3 - SPECTRUM COMPUTED FROM

IMPULSE RESPONSE

FIGURE 4 - LOG MAGNITUDE SPECTRUM

OF IMPULSE RESPONSE

100

200

300

400

500

600

128

192

256

320

384

448

512

Frequency Response

Frequency/512 (KHz)

Frequency Response

Frequency/512 (KHz)

-90

-80

-70

-60

-50

-40

-30

-20

-10

128

192

256

320

384

448

512

Timing (Cycles)†

#of Filter Stages*775 + 16

Program Memory (locations)† #of Filter Stages*68 + 290

RAM (File Registers)

#of Filter Stages*16+16

Filter Order

Cycles

Real Time

Maximum

Program

RAM

(@ 16 MHz)

Sampling

Memory†

791

197.75 uSec

5.05 Khz

358

1566

391.5 uSec

2.55 Khz

426

2341

585.25 uSec

1.7 Khz

494

3116

779.0 uSec

1.28 Khz

562

3891

972.75 uSec

1.0 Khz

630

†: The above numbers do not include the initialization routine.

TABLE 4 - RESOURCE REQUIREMENTS

†: The above numbers do not include the initialization

routine.

PERFORMANCE

The resource requirements for filter implementations

using PIC17C42 is given below. These numbers can be

used to determine whether a higher order filter can be

executed in real time. The same information may be

used to determine the highest sampling rate possible.

FILTER APPLICATIONS

Digital filters find applications in many areas especially

involving processing of real world signals.

In some

applications like ABS systems in an automobile, digital

filtering becomes a must. In this case elimination of

noise (especially glitches and false readings of sensors)

is very critical and thus the requirement of digital signal

processing.

TABLE 3 - RESOURCE REQUIREMENTS

Digital filters are also needed in Process Control where

notch filters and low pass filters are desired because the

signals from sensors are transmitted over long lines,

especially in a very noisy environment. In this case

typically a notch filter (centering 50Hz or 60Hz) is used.

In case of eliminating background noise, a band stop

filter (e.g. 40Hz to 120Hz) is used. The sample code

given in this application note can be used to design a

feedback control system's digital compensator.

For

example, a typical DC Motor's digital compensator (like

dead beat compensator) is of second order and has the

same filter structure as is implemented in this application

note.

Author: Amar Palacherla

Logic Products Division

4-131

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