8

LTC1264

ODES OF OPERATIO

8. 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 8 is not always required. When cascading the

sections of the LTC1264, the highpass and lowpass

outputs can be summed directly into the inverting input of

the next section.

Please refer to the Maximum Frequency of Operation

paragraph under Applications Information for a guide to

Mode 2n

This mode extends the circuit topology of Mode 3a to

Mode 2 (Figure 9) where the highpass notch and lowpass

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 LTC1264, the highpass

notch and lowpass outputs can be summed directly into

the inverting input of the next section.

Please refer to the Maximum Frequency of Operation

paragraph under Applications Information for a guide to

Figure 7. Mode 2, 2nd Order Filter Providing

Highpass Notch, Bandpass and Lowpass Outputs

–

+

Σ

∫

∫

AGND

R1

N

BP

LP

1264 F07

+

–

S

1/4 LTC1264

R2

R3

R4

20

R4

1 +

Q = 1.005

R3

R2

R4

1 +

R3

6.42•R4

(

)

1

1 –

R2

R1

R2

R1

1

R2

R4

1 +

R3

R1

R3

6.42•R4

(

)

1

1 –

Mode 3a

This is an extension of Mode 3 where the highpass and

lowpass output are summed through two external resis-

Figure 8. Mode 3a, 2nd Order Filter Providing a Highpass Notch or Lowpass Notch Output

–

+

Σ

∫

∫

AGND

R1

HP

BP

LP

1264 G08

+

–

S

R2

R3

R4

–

+

EXTERNAL OP AMP OR

INPUT OP AMP OF THE

LTC1264, SIDES A, B, C, D

HIGHPASS

OR LOWPASS

NOTCH OUTPUT

1/4 LTC1264

R2

R1

( )

()

20

R4

Q = 1.005

R3

R2

R4

R3

6.42•R4

(

)

1

1 –

( )

R4

R1

( )

()