11

LTC1067/LTC1067-50

ODES OF OPERATIO

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 (see Figure 7). Mode 3a

is more versatile than Mode 2 because the notch fre-

quency can be higher or lower than the center frequency

of the 2nd order section. The external op amp of Figure 7

is not always required. When cascading the sections of the

LTC1067, the highpass and lowpass outputs can be

summed directly into the inverting input of the next

section.

Please refer to the Operating Limits paragraph under Appli-

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

Figure 7. Mode 3a, 2nd Order Filter Providing a Highpass Notch or Lowpass Notch Output

Mode 2n

This mode extends the circuit topology of Mode 3a to

Mode 2 (Figure 8) where the highpass-notch and lowpass

outputs are summed through two external resistors, RH

and RL, to create a lowpass output with a notch higher in

frequency than the notch in Mode 2. This mode, shown in

Figure 8, is most useful in lowpass elliptic designs. When

cascading the sections of the LTC1067, the highpass-

notch and lowpass outputs can be summed directly into

the inverting input of the next section.

Please refer to the Operating Limits paragraph under Appli-

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

Figure 8. Mode 2n, 2nd Order Filter Providing a Lowpass Notch Output

–

+

Σ

∫

∫

AGND

R1

HP

BP

LP

VIN

1067 F07

+

–

S

R2

R3

R4

CC

–

+

EXTERNAL OP AMP OR INPUT OP

AMP OF THE LTC1067, SIDES A OR B

HIGHPASS

OR LOWPASS

NOTCH OUTPUT

RG

RL

RH

R2

R1

( )

RG

RH

()

fO =

; fn =

HOHPn (f = ∞) =

; HOLPn (f = 0) =

fCLK

RATIO

fCLK

RATIO

√RH

RL

√R2

R4

Q = 1.005

R3

R2

√R2

R4

( )

R4

R1

( )

RG

RL

()

NOTE: RATIO = 100 FOR LTC1067

= 50 FOR LTC1067-50

R3

(RATIO)(0.32)(R4)

(

)

1

1 –

–

+

Σ

∫

∫

AGND

R1

HP

BP

LP

VIN

1067 F08

+

–

S

R2

R3

R4

CC

–

+

EXTERNAL OP AMP OR INPUT OP AMP

OF THE LTC1067, SIDES A OR B

LOWPASS

NOTCH

OUTPUT

RG

RL

RH

fO =

fn =

HOLPn (f = 0)=

+

fCLK

RATIO

fCLK

RATIO

√ RH

RL

1 +

R2

R1

RG

RL

RG

RH

()

1

R2

R4

1 +

( )

( )

√ R2

R4

1 +

Q = 1.005

R3

R2

( )√ R2

R4

1 +

NOTE: RATIO = 100 FOR LTC1067

= 50 FOR LTC1067-50

R3

(RATIO)(0.32)(R4)

(

)

1

1 –