LTC1060CSW datasheet(15/20 Pages) LINER

15 / 20 page

LTC1060

1060fb

COMM E TS ON THE M ODES OF OPERATIO

There are basically three modes of operation: mode 1,

mode 2, mode 3. In the mode 1 (Figure 4), the input

amplifier is outside the resonant loop. Because of this,

mode 1 and its derivatives (mode 1a, 1b, 1c) are faster

than modes 2 and 3. In mode 1, for instance, the Q errors

are becoming noticeable above 1MHz clock frequency.

Mode 1a (Figure 5), represents the most simple hook-up

of the LTC1060. Mode 1a is useful when voltage gain at the

bandpass output is required. The bandpass voltage gain,

however, is equal to the value of Q; if this is acceptable,

a second order, clock tunable, BP resonator can be achiev-

ed with only 2 resistors. The filter center frequency directly

depends on the external clock frequency. For high order

filters, mode 1a is not practical since it may require several

clock frequencies to tune the overall filter response.

Mode 1 (Figure 4), provides a clock tunable notch; the

depth is shown in Graph 14. Mode 1 is a practical

configuration for second order clock tunable bandpass/

notch filters. In mode 1, a bandpass output with a very

high Q, together with unity gain, can be obtained without

creating problems with the dynamics of the

remaining notch and lowpass outputs.

Modes 1b and 1c (Figures 6 and 7), are similar. They both

produce a notch with a frequency which is always equal to

the filter building block center frequency. The notch and

the center frequency, however, can be adjusted with an

external resistor ratio.

The practical clock-to-center frequency ratio range is:

≥≥

fCLK

500

100

; mode 1b

≥≥

fCLK

100

√2

; mode 1c

√2

The input impedance of the S1 pin is clock dependent,

and in general R5 should not be larger than 5k. Mode 1b

can be used to increase the clock-to-center frequency

ratio beyond 100:1. For this mode, a practical limit for the

(fCLK/f0) ratio is 500:1. Beyond this, the filter will exhibit

large output offsets. Mode 1c is the fastest mode of

operation: In the 50:1 mode and with (R5 = 0, R6 = ∞) the

clock-to-center frequency ratio becomes (50/√2) and cen-

ter frequencies beyond 20kHz can easily be achieved as

shown in Graph 25. Figure 19 illustrates how to cascade

the two sections of the LTC1060 connected in mode 1c to

obtain a sharp fourth order, 1dB ripple, BP Chebyshev

filter. Note that the center frequency to the BW ratio for this

fourth order bandpass filter is 20/1. By varying the clock

frequency to sweep the filter, the center frequency of the

overall filter will increase proportionally and so will the BW

to maintain the 20:1 ratio constant. All the modes of

operation yield constant Q’s; with any filter realization the

BW’s will vary when the filter is swept. This is shown in

Figure 19, where the BP filter is swept from 1kHz to 20kHz

center frequency.

Figure 17. Mode 6b: 1st Order Filter Providing Lowpass

Figure 18. Mode 7: 1st Order Filter Providing Allpass, Lowpass

1/2 LTC1060

SA/B

VIN

fC =; HOLP1 = 1 ; HOLP2 = –

fCLK R2

100(50) R3

TLC1060 • MOO14

–

LP1

(18)

(20)

LP2

(19)

S1A

(16)

V –

(17)

∫∫

1/2 LTC1060

SA/B

VIN

fP =; fz =; GAIN AT AP OUTPUT = 1 FOR 0 ≤ f ≤

HOLP = 2 x

fCLK R2

100(50) R3

TLC1060 • MOO15

–

R2=R1

R1=R2

(18)

(20)

(19)

S1A

(16)

V –

(17)

fCLK R2

100(50) R3

fCLK

∫∫

ODES OF OPERATIO

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