LTC1064CS datasheet(10/16 Pages) LINER

10 / 16 page

LTC1064

ODES OF OPERATIO

–

∫

1064 F08

+ Σ

AGND

1/4 LTC1064

–

VIN

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.

–

∫

+ Σ

AGND

1/4 LTC1064

–

VIN

1064 F09

NOTCH

EXTERNAL OP AMP OR INPUT

OP AMP OF THE LTC1064,

SIDE A, B, C, D

–

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

1064 F08Eq

fO =

1 +

; fn =

; Q =

1 +

; HOLP = –

;

fCLK

100

MODE 2 (100:1):

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:

√

fO =

1 +

; fn =

; Q =

; HOLP = –

;

fCLK

MODE 2 (50:1):

16R4

1.005

1 +

√

–

HOBP = –

; HON1(f→ 0) = –

; HON2 = f→

16R4

1 –

R3 =

; THEN CALCULATE R1 TO SET THE DESIRED GAIN.

1 +

1.005

√

16R4

1 +

–

1 +

fCLK

()

fCLK

()

HOBP = –

; HON1(f→ 0) = –

; HON2 f→

= –

1 +

1064 F09Eq

fO =

; fn =

; HOHP =

; HOBP =

fCLK

100

MODE 3a (100:1):

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:

√

fCLK

100

R3 =

; THEN CALCULATE R1 TO

SET THE DESIRED GAIN.

1.005

√

16R4

–

16R4

1.005

√

–

HOBP = –

; HOLP(f = 0) =

Q =

16R4

1 –

–

;

–

;

fCLK

()

()()

()( )

√

fO =

1 +

; fn =

; HOHP f→

fCLK

MODE 3a (50:1):

–

;

fCLK

()

HOLP = –

; HON1(f→ 0) =

; HON2 f→

;

HON(f = fO) = Q

HOLP –

HOHP ; Q =

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