LTC1061CS datasheet(9/16 Pages) LINER

9 / 16 page

LTC1061

Another example of Mode 1b is illustrated on the front

page of the data sheet. The cascading sequence of this 6th

order bandpass filter is shown in block diagram form,

Figure 10a. the filter is geometrically centered around the

side B of the LTC1061 connected in Mode 1. This dictates

a clock-to-center frequency ratio of 50:1 or 100:1. The side

A of the IC operates in Mode 1b to provide the lower center

frequency of 0.95 and still share the same clock with the

rest of the filter. With this approach the bandpass filter can

operate with center frequencies up to 24kHz. The speed of

the filter could be further improved by using Mode 1 to lock

the higher resonant frequency of 1.05 and higher Q or 31.9

to the clock, Figure 10b, thus changing the clock to center

frequency ratio to 52.6:1.

Mode 3a – This is an extension of Mode 3 where the

highpass and lowpass outputs are summed through two

external resistors Rh and Rl to create a notch, Figure 11.

Mode 3a is very versatile because the notch frequency can

be higher or lower than the center frequency of the 2nd

order section. The external op amp of Figure 11 is not

always required. When cascading the sections of the

LTC1061, the highpass and lowpass outputs can be

summed directly into the inverting input of the next

section. Figure 12 shows an LTC1061 providing a 6th

order elliptic bandpass or notch response. Sides C and B

are connected in Mode 3a while side A is connected in

Mode 1 and uses only two resistors. The resulting filter

response is then geometrically symmetrical around either

the center frequency of side A (for bandpass responses) or

the notch frequency of side A (for notch responses).

Figure 11. Mode 3a: 2nd Order Filter Providing Highpass, Bandpass, Lowpass, Notch

ODES OF OPERATIO

Figure 10a. Cascading Sequence of the Bandpass Filter Shown

on the Front Page, with (fCLK/fO) = 50:1 or 100:1

Figure 10b. Cascading Sequence of the Same Filter for Speed

Optimization, and with (fCLK/fO) = 52.6:1

MODE 1b

MODE 1

MODE 3

SIDE A

SIDE B

SIDE C

VIN

VOUT

fO1 = 0.95

Q1 = 31.9

fO2 = 1.05

Q2 = 31.9

fO3 = 1

Q3 = 15.9

1061 F10b

MODE 1b

MODE 1

MODE 3

SIDE A

SIDE B

SIDE C

VIN

VOUT

fO1 = 0.95

Q1 = 31.9

fO2 = 1

Q2 = 15.9

fO3 = 1.05

Q3 = 31.9

1061 F10a

–

∫

AGND

VIN

1061 F11

–

NOTE: FOR Q > 5 AND fCLK > 1MHz, ADD CC SUCH AS CC ≅

–

EXTERNAL OP AMP OR

INPUT OP AMP OF THE

LTC1061, SIDES A, B, C

NOTCH

√

fO =

; fn =

; HOHP = –

; HOBP = –

; HOLP = –

HON1 (f → 0) =

; HON2

; HON (f = fO) = Q

HOLP –

HOHP

Q =

fCLK

100(50)

√R2

fCLK

→

()

fCLK

100(50)

√Rh

()

0.16

× 1.2MHz

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