12

LTC1563-2/LTC1563-3

156323fa

should be much larger than RA1 to minimize the interac-

tion of this pole with the 2nd order section. This circuit is

useful in forming dual 3rd order filters and 5th order filters

with a single LTC1563 part. By cascading two parts, 7th

order and 9th order filters are achieved.

DC GAIN =

LTC1563-2: C1A = 53.9pF, C1B = 39.2pF

LTC1563-3: C1A = 35pF, C1B = 26.8pF

–R2

R1

1

2

π • R2 • C1

1563 F02

+

–

C1

INV

C2

S

1/2 LTC1563

LP

AGND

R1

(OPEN)

R2

Figure 2

RA1

≈

RB1

10

R2

RB1

RA1

R3

1563 F03

RA1 • RB1

RA1 + RB1

1

()

2

+

–

C1

INV

C2

S

1/2 LTC1563

LP

AGND

+

–

(OPEN)

C1

INV

C2

S

1/2 LTC1563

LP

1563 F04

AGND

You can also use the TEE network in both sections of the

part to make a 6th order filter. This 6th order filter does not

conform exactly to the textbook responses. Textbook

responses (Butterworth, Bessel, Chebyshev etc.) all have

three complex pole pairs. This filter has two complex pole

pairs and two real poles. The textbook response always

has one section with a low Q value between 0.5 and 0.6. By

replacing this low Q section with two real poles (two real

poles are the same mathematically as a complex pole pair

with a Q of 0.5) and tweaking the Q of the other two

complex pole pair sections you end up with a filter that is

indistinguishable from the textbook filter. The Typical

Applications section illustrates a 100kHz, 6th order pseudo-

Butterworth filter. FilterCAD is a valuable tool for custom

filter design and tweaking textbook responses.

Figure 3

What To Do with an Unused Section

If the LTC1563 is used as a 2nd or 3rd order filter, one of

the sections is not used. Do not leave this section uncon-

nected. If the section is left unconnected, the output is left

to float and oscillation may occur. The unused section

should be connected as shown in Figure 4 with the INV pin

connected to the LP pin and the S pin left open.

Figure 4