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LT1064

Low Noise, Fast, Quad Universal Filter Building Block

Linear Technology 

LTC1064

W

U

ODES OF OPERATIO

R1

VIN

R4

R3

R2

–

+

AGND

N

S

+

–

Σ

∫

1/4 LTC1064

R2

√ √ MODE 2 (100:1):

fO

=

fCLK

100

1+

R2

R4

; fn =

fCLK

50

;

Q

=

R3

R2

1+

R2

R4

;

HOLP

=

–

1

R1

+ R2

;

BP

LP

( ) R2

HOBP

=–

R3

R1

;

HON1(f→ 0)

=

–

1

R1

+ R2

; HON2

f→

fCLK

2

=

–

R2

R1

R4

R4

∫

MODE 2 (50:1):

√ √ fO

=

fCLK

50

1+

R2

R4

;

fn

=

fCLK

50

;

Q

1.005

=

R2

R3

1 + R2

R4

–

R2

16R4

; HOLP = –

R2

R1

1

+

R2

R4

;

1064 F08

( ) R3

R2

HOBP

=

–

1

R1

– R3

16R4

;

HON1(f→

0)

=

–

1

R1

+ R2

R4

;

HON2

=

f→

fCLK

2

=

–

R2

R1

NOTE: THE 50:1 EQUATIONS FOR MODE 2 ARE DIFFERENT FROM THE EQUATIONS

FOR MODE 2 OPERATION OF THE LTC1059, LTC1060 AND LTC1061. START WITH

fO, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:

R3 =

R2

; THEN CALCULATE R1 TO SET THE DESIRED GAIN.

√ 1.005 1 + R2 + R2

Q

R4 16R4

1064 F08Eq

Figure 8. Mode 2: 2nd Order Filter Providing Notch, Bandpass and Lowpass

Mode 3a

This is an extension of Mode 3 where the highpass and

lowpass outputs are summed through two external resis-

tors RH and RL to create a notch. This is shown in Figure

9. Mode 3a is more versatile than Mode 2 because the

notch frequency can be higher or lower than the center

frequency of the 2nd order section. The external op amp of

Figure 9 is not always required. When cascading the

sections of the LTC1064, the highpass and lowpass out-

puts can be summed directly into the inverting input of the

next section. The topology of Mode 3a is useful for elliptic

highpass and notch filters with clock-to-cutoff frequency

ratios higher than 100:1. This is often required to extend

the allowed input signal frequency range and to avoid

premature aliasing.

When the internal clock-to-center frequency ratio is set at

50:1, the design equations for Q and bandpass gain are

different from the 100:1 case.

R1

VIN

R4

R3

R2

–

+

AGND

CC

HP S

BP

+–

Σ∫

∫

1/4 LTC1064

RH

MODE 3a (100:1):

MODE 3a (50:1):

LP

RG

RL

–

+

NOTCH

EXTERNAL OP AMP OR INPUT

OP AMP OF THE LTC1064,

SIDE A, B, C, D

1064 F09

√ √ fO

=

fCLK

100

R2

R4

;

fn

=

fCLK

100

RH

RL

;

HOHP

=

–

R2

R1

;

HOBP

=

–

R3

R1

;

( )( ) ( ) ( )( ) HOLP = –

R4

R1

;

HON1(f→

0)

=

RG

RL

R4

R1

;

HON2

f→

fCLK

2

=

RG

RH

R2

R1 ;

( ) √ HON(f = fO) = Q

RG

RL

HOLP

–

RG

RH

HOHP

;Q=

R3

R2

R2

R4

√ √ ( ) fO

=

fCLK

50

1+

R2

R4

; fn =

fCLK

50

RH

RL

; HOHP

f→

fCLK

2

=

–

R2

R1

;

√ R3

HOBP

=

–

1

–

R1

R3

;

HOLP(f

=

0)

=

–

R4

R1

;

Q

=

16R4

1.005 R2

R4

R2

R3

–

R2

16R4

NOTE: THE 50:1 EQUATIONS FOR MODE 3A ARE DIFFERENT FROM

THE EQUATIONS FOR MODE 3A OPERATION OF THE LTC1059,

LTC1060 AND LTC1061. START WITH fO, CALCULATE R2/R4, SET R4;

FROM THE Q VALUE, CALCULATE R3:

R3 =

R2

; THEN CALCULATE R1 TO

√ 1.005 R2 + R2 SET THE DESIRED GAIN.

Q R4 16R4

1064 F09Eq

Figure 9. Mode 3a: 2nd Order Filter Providing Highpass, Bandpass, Lowpass and Notch

10

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