LTC2480CDD datasheet(34/40 Pages) LINER

34 / 40 page

LTC2480

2480f

APPLICATIO S I FOR ATIO

Figure 30. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch Mode

Figure 31. Input Normal Mode Rejection, Internal

Oscillator and 60Hz Notch Mode or External Oscillator

The SINC4 digital filter provides greater than 120dB nor-

mal mode rejection at all frequencies except DC and

integer multiples of the modulator sampling frequency

(fS). The LTC2480’s autocalibration circuits further sim-

plify the antialiasing requirements by additional normal

mode signal filtering both in the analog and digital domain.

Independent of the operating mode, fS = 256 • fN = 2048

• fOUTMAX where fN is the notch frequency and fOUTMAX is

the maximum output data rate. In the internal oscillator

mode with a 50Hz notch setting, fS = 12800Hz, with

50Hz/60Hz rejection, fS = 13960Hz and with a 60Hz notch

setting fS = 15360Hz. In the external oscillator mode, fS =

fEOSC/20. The performance of the normal mode rejection

is shown in Figures 30 and 31.

In 1x speed mode, the regions of low rejection occurring

at integer multiples of fS have a very narrow bandwidth.

Magnified details of the normal mode rejection curves are

shown in Figure 32 (rejection near DC) and Figure 33

(rejection at fS = 256fN) where fN represents the notch

frequency. These curves have been derived for the exter-

nal oscillator mode but they can be used in all operating

modes by appropriately selecting the fN value.

The user can expect to achieve this level of performance

using the internal oscillator as it is demonstrated by Fig-

ures 34, 35 and 36. Typical measured values of the normal

mode rejection of the LTC2480 operating with an internal

oscillator and a 60Hz notch setting are shown in Figure 34

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS

2480 F30

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS

2480 F31

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS

Figure 32. Input Normal Mode Rejection at DC

Figure 33. Input Normal Mode Rejection at fS = 256fN

INPUT SIGNAL FREQUENCY (Hz)

2480 F32

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fN 3fN 4fN 5fN 6fN 7fN 8fN

fN = fEOSC/5120

INPUT SIGNAL FREQUENCY (Hz)

250fN 252fN 254fN 256fN 258fN 260fN 262fN

2480 F33

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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