11

LTC1061

Figure 17. Measured Amplitude Response of the Topology of

Figure 16, Configured to Provide a 6th Order Elliptic Highpass

Filter Operating with a Clock-to-Cutoff Frequency Ratio of 250:1.

ODES OF OPERATIO

Figure 15. Resistor Values and Amplitude Response of

the Topology of Figure 12.

passive ladder filters provide enough data to synthesize

the above filters. The measured amplitude response of

such a lowpass is shown in Figure 15 where the filter

output is taken out of side A’s Pin 1, Figure 12. The clock-

to-center frequency ratio can be either 50:1 or 100:1

because the last stage of the LTC1061 operates in Mode 1

with a center frequency very close to the overall cutoff

frequency of the lowpass filter.

In Figure 16, all three sides of the LTC1061 are connected

in Mode 3a. This topology is useful for elliptic highpass

and notch filters with clock-to-cutoff (or notch) frequency

ratio higher than 100:1. This is often required to extend the

allowed input signal frequency range and to avoid prema-

ture aliasing. Figure 16 is also a versatile, general purpose

architecture providing 3 notches and 4 pole pairs, and

there is no restriction on the location of the poles with

respect to the notch frequencies. The drawbacks, when

compared to Figure 12, are the use of an external op amp

and the increased number of the required external resis-

tors.

Figure 17 shows the measured frequency of a 6th order

highpass elliptic filter operating with 250:1 clock-to-cutoff

frequency ratio. With a 1MHz clock, for instance, the filter

yields a 4kHz cutoff frequency, thus allowing an input

frequency range beyond 100kHz. Band limiting can be

easily added by placing a capacitor across the feedback

resistor of the external op amp of Figure 16.

1

–10

9

1061 F15

–30

–60

–80

3

5

7

0

–50

–40

–20

0

–70

–90

2

4

68

10

STANDARD 1%

RESISTOR VALUES

R11 = 39.2k

R31 = 13.7k

R22 = 10k

R42 = 14k

R33 = 100k

R21 = 10k

R41 = 39.2k

R32 = 26.7k

R23 = 10k

NOTES: USE A 10pF ACROSS R42

THE ELLIPTIC LOWPASS FILTER

HAS ONLY TWO NOTCHES IN THE

STOPBAND, AND IT OPERATES

WITH A CLOCK TO CUTOFF

FREQUENCY RATIO OF 50:1.

20

19

18

17

16

15

14

13

12

11

1

2

3

4

5

6

7

8

9

10

LTC1061 F16

LTC1061

R43

R33

CLOCK INPUT

R32

R22

R42

R31

R41

R21

R11

R23

LT1056

0

–10

2.0

1061 F17

–30

–60

–80

0.5

1.0

1.5

2.5

–50

–40

–20

0

–70

–90

R11 = 105k

R31 = 47.5k

R22 = 32.4k

R42 = 52.3k

R33 = 255k

R21 = 10k

R41 = 45.3k

R32 = 28.7k

R23 = 10k

R43 = 63.4k

NOTE: FOR CLOCK FREQUENCIES

BELOW 500kHz, USE A CAPACI-

TOR IN PARALLEL WITH R21

SUCH AS (1/2

STANDARD 1%

RESISTOR VALUES

Figure 16. Using an External Op Amp to Connect

all 3 Sides of the LTC1061 in Mode 3a.