17

LTC1060

1060fb

mode 2b section (Figure 22), has a gain exceeding unity

which limits the dynamic range of the overall filter. For

very selective bandpass/bandreject filters, the mode 3a

approach, as in Figure 20, yields better dynamic range

since the external op amp helps to optimize the dynamics

of the output nodes of the LTC1060.

Figure 20. Combining Mode 3 with Mode 3a to Make The 4th Order BP Filter of Figure 21 with Improved

Dynamics. The Gain at Each Output Node is ≤ 0dB for all Input Frequencies.

Figure 21. The BP Filter of Figure 20, When Swept From a 2kHz to 20kHz Center Frequency.

1.5kHz

–30dB

–20dB

–10dB

0dB

2.5kHz

–50dB

–40dB

2kHz

1.75kHz

2.25kHz

15kHz

–30dB

–20dB

–10dB

0dB

25kHz

–50dB

–40dB

20kHz

17.5kHz

22.5kHz

TLC1060 • CMO03

LTC1060 • CM02

R42

R32

1

2

3

4

5

6

7

8

9

10

20

19

18

17

16

15

14

13

12

11

V+ = 7.5V

–7.5V

CLOCK IN

R22

LTC1060

R11

7.5V

PRECISE RESISTOR VALUES

R11 = 155.93k

R42 = 5k

NOTE: FOR CLOCK FREQUENCIES ABOVE 700kHz, A 12pF CAPACITOR ACROSS R41 AND A 20pF

CAPACITOR ACROSS R42 WERE USED TO PREVENT THE PASSBAND RIPPLE FROM ANY

ADDITIONAL PEAKING

R21 = 5k

R31 = 152k

R22 = 5.26k

R41 = 5.27k

R32 = 151.8k

–7.5V

R31

R21

R41

–

+

EXTERNAL

OP AMP

S1A

+

+

LSh

S1B

AGND

–

–

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