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Application Note 38

AN38-1

an38f

WHAT IS FilterCAD?

FilterCAD is designed to help users without special expertise

in filter design to design good filters with a minimum of

effort. It can also help experienced filter designers achieve

better results by providing the ability to play “what if” with

the values and configuration of various components.

With FilterCAD, you can design any of the four major filter

types (lowpass, highpass, bandpass and notch), with

Butterworth, Chebyshev, Elliptic, or custom-designed

response characteristics. (Bessel filters can be realized

by manually entering pole and Q values, but FilterCAD

cannot synthesize a Bessel response in this version.)

FilterCAD is limited to designs which can be achieved by

cascading state-variable 2nd order sections. FilterCAD

plots amplitude, phase and group-delay graphs, selects

appropriate devices and modes, and calculates resistor

values. Device selection, cascade order and modes can

be edited by the user.

LICENSE AGREEMENT/DISCLAIMER

This copy of FilterCAD is provided as a courtesy to the

customers of Linear Technology Corporation. It is licensed

for use in conjunction with Linear Technology Corporation

products only. The program is not copy protected and you

may make copies of the program as required, provided that

you do not modify the program, and that said copies are

used only with Linear Technology Corporation products.

While we have made every effort to ensure that FilterCAD

operates in the manner described in this manual, we

do not guarantee operation to be error free. Upgrades,

modifications, or repairs to this program will be strictly

at the discretion of Linear Technology Corporation. If you

encounter problems in installing or operating FilterCAD, you

may obtain technical assistance by calling our applications

department at (408) 432-1900, between 8:00 a.m. and 5:00

p.m. Pacific time, Monday through Friday. Because of the

great variety of operating-system versions, and peripherals

currently in use, we do not guarantee that you will be able

to use FilterCAD successfully on all such systems. If you

are unable to use FilterCAD, Linear Technology Corporation

does guarantee to provide design support for LTC filter

products by whatever means necessary.

Linear Technology Corporation makes no warranty,

either expressed or implied, with respect to the use of

FilterCAD or its documentation. Under no circumstances

will Linear be liable for damages, either direct or conse-

quential, arising from the use of this product or from the

inability to use this product, even if we have been informed

in advance of the possibility of such damages.

FilterCAD Download

The FilterCAD tool, although not supported, can be

downloaded at www.linear.com. Locate the downloaded

file on your computer and manually start installation in

that directory.

Your FilterCAD distribution includes the following files. If,

after installing the program, you have difficulty in running

FilterCAD, check to be sure all of the necessary files are

present.

README.DOC (Optional) if present, includes updated

information on FilterCAD not included in

this manual

INSTALL.BAT Automatic installation program—installs

FilterCAD on hard drive

FCAD.EXE Main program file for FilterCAD

FCAD.OVR Overlay file for FilterCAD—used by

FCAD.EXE

FCAD.ENC Encrypted copyright protection file—DO

NOT TOUCH!

FDPF.EXE Device-parameter file editor—used to

update FCAD.DPF (see Appendix 1)

November 1990

FilterCAD User’s Manual, Version 1.10

L, LT, LTC, LTM, Linear Technology, the Linear logo, LTspice and FilterCAD are registered

trademarks and QuikEval is a trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners.

Application Note 38

AN38-2

an38f

FCAD.DPF Device-parameter file—holds data for all

device types supported by FilterCAD

ATT.DRV AT&T graphics adapter driver

CGA.DRV IBM CGA or compatible graphics driver

EGAVGA.DRV EGA and VGA graphics drivers

HERC.DRV Hercules monochrome graphics driver

ID.DRV Identification file for all driver specifica-

tions

Note: Once you have configured FilterCAD and selected

your display type, you can delete unnecessary drivers if

you need to conserve disk space. (Be sure not to delete

any drivers.)

Before You Begin

Please check the FilterCAD program to see if it contains

the README.DOC file. This file, if present, will contain

important information about FilterCAD not included in

this manual. Please read this file before attempting to

install and use FilterCAD. To display the README file on

your screen, type:

TYPE README.DOC [Enter]

Press

[Ctrl] S

to pause scrolling. Press any key to resume scrolling. To

print a hard copy of the README file on your printer type:

TYPE README.DOC>PRN [Enter]

Procedure for FilterCAD Installation in Win7 PC

The FilterCAD installation in Win7 downloads reliably to

a target folder.

1. If an LTC program like LTspice

or QuikEval™ has been

installed then the following directory folder exists:

a. C\Program Files\LTC (in a 32-bit system)

b. C\Program Files(86)\LTC (in a 64-bit system).

If not then create a directory folder as in a. or b.

The FilterCAD download is at:

http://www.linear.com/designtools/software/#Filter

2. Start the FilterCAD download and open the FilterCAD.zip

to extract the “FilterCADv300.exe.”

“Right Click” on “FilterCAD.exe,”

and select

“Run as an administrator”

then select the following Directory:

C\Program Files\LTC (in a 32-bit system)

C\Program Files(86)\LTC (in a 64-bit system).

3. Go to

C\Program Files\LTC (in a 32-bit system)

C\Program Files(86)\LTC (in a 64-bit system)

open

“OPEN THIS FOLDER TO INSTALL FilterCAD”

and

“Run” SETUP.exe

then FilterCAD is installed in:

C\Program Files\LTC\FILTERCAD (in a 32-bit system)

C\Program Files(86)\LTC\FILTERCAD (in a 64-bit system).

END

HARDWARE REQUIREMENTS

A list of the graphics adapters and modes supported

by FilterCAD will be found in the Configuration section.

FilterCAD is a calculation-intensive program, and should

therefore, be run on the most powerful system available.

WHAT IS A FILTER?

A filter is a circuit that selectively passes a certain range

of the frequencies present at its input to its output, while

blocking (attenuating) other frequencies. Filters are nor-

mally described in terms of the frequencies that they pass.

Most filters conform to one of four common types. Lowpass

filters pass all frequencies below a specified frequency

(called the cutoff frequency) and progressively attenuate

Application Note 38

AN38-4

an38f

Figure 1.6. 6th Order Butterworth Lowpass Response

Figure 1.7. 6th Order Chebyshev Lowpass Response

Figure 1.8. 6th Order Elliptic Lowpass Response

is shown in Figure 1.5. Regrettably, real-world filters do

not meet these imaginary specifications. Different types of

filters have different characteristics, less-than-infinite rates

of attenuation versus frequency in the transition region.

In other words, the amplitude response of a given filter

has a characteristic slope. Frequencies in the passband

may also be modified, either in amplitude (“ripple”) or

in phase. Real-world filters all represent compromises:

steepness of slope, ripple, and phase shift (plus, of course,

cost and size).

FilterCAD permits the design of filters with one of three

response characteristics (plus custom responses).

These three response types, which are known as “But-

terworth,” “Chebyshev,” and “Elliptic,” represent three

different compromises among the previously described

characteristics. Butterworth filters (Figure 1.6) have the

optimum flatness in the passband, but have a slope that

rolls off more gradually after the cutoff frequency than

the other two types. Chebyshev filters (Figure 1.7) can

have a steeper initial roll off than Butterworths, but at the

expense of more than 0.4dB of ripple in the passband.

Elliptic filters (Figure 1.8) have the steepest initial roll

off of all. But exhibit ripple in both the passband and the

stopband. Elliptic filters have higher Qs, which may (if not

carefully implemented) translate to a noisier filter. These

high Qs have made elliptic filters difficult to implement

with active RC filters because of the increased stability and

center frequency accuracy requirements. Elliptic filters can

be implemented with SCFs due to their inherently better

stabilities and center frequency accuracies when compared

to active RC filters. Chebyshev and elliptic designs can

achieve greater stopband attenuation for a given number

of 2nd order sections than can Butterworths.

FREQUENCY

GAIN

– ∞

Figure 1.5. Ideal Lowpass Response

Application Note 38

AN38-5

an38f

Application Note 38

an38f

Filters are typically built up from basic building blocks

known as 1st order and 2nd order sections. Each LTC filter

contains circuitry which, together with an external clock

and a few resistors, closely approximates 2nd order filter

functions. These are tabulated in the frequency domain.

1. Bandpass function: available at the bandpass output

pin, refer to Figure 1.9.

G s

( )

= H

OBP

sω

/ Q

+ sω

/ Q

( )

+ ωo

OBP

= Gain at ω = ω

= ω

/2π; f

is the center frequency of the com-

plex pole pair. At this frequency, the phase shift

between input and output is –180°.

Q = Quality factor of the complex pole pair. It is the

ration of f

to the –3dB bandwidth of the 2nd

order bandpass function. The Q is always mea-

sured at the filter BP output.

2. Lowpass function: available at the LP output pin, refer

to Figure 1.10.

G s

( )

= H

OLP

+ s ω

/ Q

( )

+ ω

OLP

= DC gain of the LP output.

3. Highpass function: available only in mode 3 at the HP

output pin, refer to Figure 1.11.

G s

( )

= H

OHP

+ s ω

/ Q

( )

+ ω

OHP

= gain of the HP output for f →

CLK

4. Notch function: available at the N output for several

modes of operation.

G s

( )

= H

ON2

( )

+ ω

( )

+ s ω

/ Q

( )

+ ω

ON2

= gain of the notch output for f →

CLK

ON1

= gain of the notch output for f→0

= ω

/2π; f

is the frequency of the notch

occurrence.

These sections are cascaded (the output of one section

fed to the input of the next) to produce higher-order filters

which have steeper slopes. Filters are described as being

of a certain “order,” which corresponds to the number and

type of cascaded sections they comprise. For example,

an 8th order filter would require four cascaded 2nd order

sections, whereas a 5th order filter would require two

2nd order sections and one 1st order section. (The order

of a filter also corresponds its number of poles, but an

explanation of poles is outside the scope of this manual.)

Figure 1.9. 2nd Order Bandpass Section Figure 1.10. 2nd Order Lowpass Section Figure 1.11. 2nd Order Highpass Section

OBP

f(LOG SCALE)

BANDPASS OUTPUT

GAIN(V/V)

0.707 H

OBP

Q =

– f

; f

= f

–1

























= f

























OHP

0.707 H

OHP

f(LOG SCALE)

HIGHPASS OUTPUT

GAIN(V/V)

= f

× 1–













+ 1–

























–1

= f

× 1–













–1

OHP

1–

OLP

0.707 H

OLP

f(LOG SCALE)

LOWPASS OUTPUT

GAIN(V/V)

= f

× 1–













+ 1–













= f

1–

OLP

1–

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