LTC1264CS datasheet(12/16 Pages) LINER

12 / 16 page

LTC1264

APPLICATI

I FOR ATIO

first stage and decreasing the R1 resistor of the last stage

by the same amount (multiplying the R1 resistor of the

first stage and dividing the R1 resistor of the last stage by

2 for narrowband filter, and by 5 for wideband filter is a

good rule of thumb). This adjustment may, however,

increase the filter’s passband noise.

Figure 11. Mode 1b Bandpass Filter

Figures 12 and 13 show the gain response graphs of the

40kHz Butterworth bandpass design described above. The

passband gain response graph (Figure 12) shows a 40kHz

gain of – 0.4dB and a tilted passband from 37kHz to 43kHz.

These errors are due to the 1% resistors used and the side-

to-side matching of the LTC1264 fCLK-to-fCENTER ratio

which typically is 0.4%. To adjust for 0dB gain at 40kHz,

reduce the value of R1 in the first stage by 5%. To adjust

for a flat passband, adjust by

±1% the value of R6 in stages

3 and 4. Adjusting R6 compensates for the side-to-side

matching errors. Please refer to Figure 5 equations defin-

ing fO and Q as a function of R6.

The sequence of 2nd order stages and the bandpass gain

HOBP of each stage will determine the gain peaks at the

filter’s intermediate outputs. A given internal output can

have several dB more gain than the final filter output. Gain

peaks occur around the corners of the passband. The gain

peaks can be reduced by increasing the R1 resistor of the

Figure 12. Passband Gain vs Frequency

40kHz Butterworth Bandpass

FREQUENCY (kHz)

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

–3.5

–4.0

1264 F12

MODE 1b

VS = ±7.5V

fCLK = 1MHz

fCLK/fCENTER = 25:1

Figure 13. Gain vs Frequency

40kHz Butterworth Bandpass

FREQUENCY (kHz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

1264 F13

MODE 1b

VS = ±7.5V

fCLK = 1MHz

fCLK/fCENTER = 25:1

1264 F11

OUT

INV B

HPB/NB

BPB

LPB

AGND

V+

LPA

BPA

HPA/NA

INV A

INV C

HPC/NC

BPC

LPC

V–

CLK

LPD

BPD

HPD/ND

INV D

LTC1264

fCLK

STAGE 2

STAGE 4

STAGE 3

STAGE 1

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