LTC1068(RevA) データシートの表示（PDF） - Linear Technology

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LTC1068
(Rev.:RevA)

Clock-Tunable, Quad Second Order, Filter Building Blocks

Linear Technology 

W

U

MODES OF OPERATION

Linear Technology’s universal switched-capacitor filters

are designed for a fixed internal, nominal fCLK/fO ratio. The

fCLK/fO ratio is 100 for the LTC1068, 200 for the LTC1068-

200, 50 for the LTC1068-50 and 25 for the LTC1068-25.

Filter designs often require the fCLK/fO ratio of each section

to be different from the nominal ratio and in most cases

different from each other. Ratios other than the nominal

value are possible with external resistors. Operating modes

use external resistors, connected in different arrange-

ments to realize different fCLK/fO ratios. By choosing the

proper mode, the fCLK/fO ratio can be increased or de-

creased from the part’s nominal ratio.

The choice of operating mode also effects the transfer

function at the HP/N pins. The LP and BP pins always give

the lowpass and bandpass transfer functions respectively,

regardless of the mode utilized. The HP/N pins have a

different transfer function depending on the mode used.

Mode 1 yields a notch transfer function. Mode 3 yields a

highpass transfer function. Mode 2 yields a highpass

notch transfer function (i.e., a highpass with a stopband

notch). More complex transfer functions, such as lowpass

notch, allpass or complex zeros, are achieved by summing

two or more of the LP, BP or HP/N outputs. This is

illustrated in sections Mode 2n and Mode 3a.

Choosing the proper mode(s) for a particular application

is not trivial and involves much more than just adjusting

the fCLK/fO ratio. Listed here are four of the nearly twenty

modes available. To make the design process simpler and

quicker, Linear Technology has developed the FilterCAD

for Widows design software. FilterCAD is an easy-to-use,

powerful and interactive filter design program. The de-

signer can enter a few filter specifications and the program

produces a full schematic. FilterCAD allows the designer to

concentrate on the filter’s transfer function and not get

bogged down in the details of the design. Alternatively,

those who have experience with the Linear Technology

family of parts can control all of the details themselves. For

a complete listing of all the operating modes, consult the

appendices of the FilterCAD manual or the Help files in

FilterCAD. FilterCAD can be obtained free of charge on the

Linear Technology web site (www.linear-tech.com) or you

can order the FilterCAD CD-ROM by contacting Linear

Technology Marketing.

LTC1068 Series

Mode 1

In Mode 1, the ratio of the external clock frequency to the

center frequency of each 2nd order section is internally

fixed at the part’s nominal ratio. Figure 4 illustrates Mode

1 providing 2nd order notch, lowpass and bandpass

outputs. Mode 1 can be used to make high order Butter-

worth lowpass filters; it can also be used to make low Q

notches and for cascading 2nd order bandpass functions

tuned at the same center frequency. Mode 1 is faster than

Mode 3.

Please refer to the Operating Limits paragraph under Appli-

cations Information for a guide to the use of capacitor CC.

CC

R1

VIN

R3

R2

–

+

N

S

BP

LP

+

–

Σ

∫

∫

AGND

fO = RfACTLKIO; fn = fO

Q

=

R3 ;

R2

HON

=

–

RR21 ;

HOBP

=

–

R3

R1

HOLP = HON

DEVICE

RATIO

LTC1068

100

LTC1068-200 200

LTC1068-50 50

LTC1068-25 25

1068 F04

Figure 4. Mode 1, 2nd Order Filter Providing Notch,

Bandpass and Lowpass Outputs

Mode 1b

Mode 1b is derived from Mode 1. In Mode 1b (Figure 5)

two additional resistors R5 and R6 are added to lower the

amount of voltage fed back from the lowpass output into

the input of the SA (or SB) switched-capacitor summer.

This allows the filter’s clock-to-center frequency ratio to

be adjusted beyond the part’s nominal ratio. Mode 1b

maintains the speed advantages of Mode 1 and should be

considered an optimum mode for high Q designs with fCLK

to fCUTOFF (or fCENTER) ratios greater than the part’s

nominal ratio.

The parallel combination of R5 and R6 should be kept

below 5k.

Please refer to the Operating Limits paragraph under Appli-

cations Information for a guide to the use of capacitor CC.

11

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