10

LTC1064

ODES OF OPERATIO

–

+

∫

∫

LP

1064 F08

AGND

NS

1/4 LTC1064

–

R1

R2

R3

R4

BP

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-

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.

–

+

∫

∫

LP

AGND

HP

S

1/4 LTC1064

–

BP

R1

R2

R3

R4

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

1 +

; Q =

1 +

;

100

R2

R4

R3

R2

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

√

R2

R4

√

R2

R4

√

1 +

; Q =

;

50

50

50

MODE 2 (50:1):

R2

R3

R2

16R4

R2

R4

1.005

1 +

√

–

R3

R1

=

R3

16R4

1 –

R2

R4

R3 =

; THEN CALCULATE R1 TO SET THE DESIRED GAIN.

1 +

+

R2

1.005

Q

√

R2

16R4

R2

R1

R2

R4

1 +

R2

R1

–

R2

R1

R2

R4

1 +

R2

R1

R2

R4

1 +

2

()

2

()

= –

R3

R1

R2

R1

R2

R1

R2

R4

1 +

1064 F09Eq

; HOBP =

100

R2

R4

MODE 3a (100:1):

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

THE EQUATIONS FOR MODE 3A OPERATION OF THE LTC1059,

FROM THE Q VALUE, CALCULATE R3:

√

R2

R4

√

√

100

R2

R4

R3 =

; THEN CALCULATE R1 TO

SET THE DESIRED GAIN.

+

R2

1.005

Q

√

R2

16R4

R2

R1

–

R2

R3

R2

16R4

R2

R4

1.005

√

–

R3

R1

Q =

R3

16R4

1 –

R4

R1

–

;

R3

R1

–

;

R3

R2

R4

R1

2

()

()()

()( )

√

R2

R4

√

1 +

=

50

50

MODE 3a (50:1):

R2

R1

–

;

2

()

()

=

;

R4

R1

R2

R1