16

LTC1060

1060fb

Modes 2, 2a, and 2b have a notch output which frequency,

when cascading second order functions to create an

overall elliptic highpass, bandpass or notch response. The

input amplifier and its feedback resistors (R2/R4) are now

part of the resonant loop. Because of this, mode 2 and its

derivatives are slower than mode 1’s.

In mode 3 (Figure 11), a single resistor ratio (R2/R4) can

since it provides a highpass, bandpass, lowpass output

through progressive integration; notches are obtained by

summing the highpass and lowpass outputs (mode 3a,

Figure 12). The notch frequency can be tuned below or

above the center frequency through the resistor ratio

versatile and useful modes for cascading second order

sections to obtain high order elliptic filters. Figure 20

shows the two sections of an LTC1060 connected in mode

3a to obtain a clock tunable 4th order sharp elliptic

bandpass filter. The first notch is created by summing

directly the HP and LP outputs of the first section into the

inverting input of the second section op amp. The indi-

vidual Q’s are 29.6 and the filter maintains its shape and

performance up to 20kHz center frequency (Figure 21).

For this circuit an external op amp is required to obtain the

2nd notch. The dynamics of Figure 20 are excellent be-

cause the amplitude response at each output pin does not

exceed 0dB. The gain in the passband depends on the ratio

amp, meanwhile the remaining ratios are adjusted for

optimum dynamics of the LTC1060 output nodes. The

external op amp of Figure 20 is not always required. In

Figure 22, one section of the LTC1060 in mode 3a is

cascaded with the other section in mode 2b to obtain a 4th

order, 1dB ripple, elliptic bandreject filter. This configura-

tion is interesting because a 4th order function with two

different notches is realized without requiring an external

op amp. The clock-to-center frequency ratio is adjusted to

200:1; this is done in order to better approximate a linear

R,C notch filter. The amplitude response of the filter is

shown in Figure 23 with up to 1MHz clock frequency. The

0dB bandwidth to the stop bandwidth ratio is 9/1. When

the filter is centered at 1kHz, it should theoretically have a

44dB rejection with a 50Hz stop bandwidth. For a more

narrow filter than the above, the unused BP output of the

Figure 19. Cascading the Two Sections of the LTC1060 Connected in Mode 1c to Obtain a Clock Tunable 4th Order

1dB Ripple Bandpass Chebyshev Filter with (Center Frequency)/(Ripple Bw) = 20/1.

0.9kHz

–15dB

–10dB

–5dB

0dB

1.1kHz

–25dB

–20dB

1kHz

50Hz

18kHz

–15dB

–10dB

–5dB

0dB

22kHz

TLC1060 • CMO01b

–25dB

–20dB

20kHz

19kHz

21kHz

1kHz

LTC1060 • CM01

R31

R21

R52

R32

1

2

3

4

5

6

7

8

9

10

20

19

18

17

16

15

14

13

12

11

V+ = 5V

R51

R22

R62

R12

S1A

+

+

LSh

S1B

AGND

–

–

50/100

LTC1060

R61

R11

5V

PRECISE RESISTOR VALUES

R11 = 149.21k

R21 = 4.99k

R31 = 149.12k

R51 = 2.55k

R61 = 2.49k

R12 = 45.14k

R22 = 5.00k

R32 = 142.64k

R5 = 2.49k

R62 = 4.29k