ML2111

19

and useful modes for cascading second order sections to

obtain high order elliptic filters. For very selective

bandpass/bandreject filters the mode 3a approach , as in

Figure 24, yields better dynamic range since the external

op amp helps to optimize the dynamics of the output

nodes of the ML2111.

Modes 4 and 5 are useful for constructing allpass res-

ponse filters. Mode 4, Figure 25, gives an allpass

response, but due to the sampled nature of the filter, a

slight 0.5 dB peaking can occur around the center

Figure 26. Mode 4a: 2nd Order Filter Providing Highpass, Bandpass, Lowpass, Allpass — ½ ML2111

f

f

R2

R

Q

R3

R2

R2

R

CLK

0

100 50

4

4

=™

=

™

()

;;

H

R

R

H

R2

R

OAP

OHP

== -

5

21

;;

H

R

R

4

1

;

H

R3

R

1

Figure 25. Mode 4: 2nd Order Filter Providing Allpass, Bandpass, Lowpass — ½ ML2111

+

Σ

BP

LP

V+

R1 = R2

6

15

5 (16)

AP

3 (18)

2 (19)

1 (20)

R3

R2

4 (17)

f

f

o

CLK

=

100 50

R

R

=

3

2

; H

R

R

2

1

R

R

 

2

3

2

frequency. Mode 4a (Figure 26) gives a non-inverting

output, but requires an external op amp. Mode 5 is

recommended if this response is unacceptable. Mode 5

(Figure 27) gives a flatter response than mode 4 if R1 = R2

= 0.02

´ R4.

Modes 6 and 7 are used to construct 1st order filters.

Mode 6a (Figure 28) gives a lowpass and a highpass

single pole response. Mode 6b (Figure 29) gives an

inverting and non-inverting lowpass single pole filter

response. Mode 7 (Figure 30) gives an allpass and lowpass

single pole response.

LP

V-

R1

6

15

1 (20)

R3

R2

4 (17)

R4

+

Σ

BP

5 (16)

HP

3 (18)

2 (19)

2R

R5

R

External

Op Amp