LTC2413IGN datasheet(33/44 Pages) LINER

33 / 44 page

LTC2413

sn2413 2413fs

APPLICATIO S I FOR ATIO

Figure 39. Input Normal Mode Rejection

Figure 38. Input Normal Mode Rejection

Figure 40. Input Normal Mode Rejection

FO = LOW OR

FO = EXTERNAL OSCILLATOR,

fEOSC = 10 • fS

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS

2413 F38

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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

INPUT SIGNAL FREQUENCY (Hz)

2413 F39

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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

INPUT SIGNAL FREQUENCY (Hz)

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

2413 F40

–10

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–120

Normal Mode Rejection and Antialiasing

One of the advantages delta-sigma ADCs offer over con-

ventional ADCs is on-chip digital filtering. Combined with

a large oversampling ratio, the LTC2413 significantly

simplifies antialiasing filter requirements.

The sinc4 digital filter provides greater than 120dB normal

mode rejection at all frequencies except DC and integer

multiples of the modulator sampling frequency (fS). The

LTC2413’s autocalibration circuits further simplify the

antialiasing requirements by additional normal mode sig-

nal filtering both in the analog and digital domain. Inde-

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

fOUTMAX where fN in the notch frequency and fOUTMAX is

the maximum output data rate. In the internal oscillator

mode, fS = 13980Hz. In the external oscillator mode, fS =

fEOSC/10.

The combined normal mode rejection performance is

shown in Figure 38. 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 39 (rejection near DC) and Figure 40

(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.

LTC2413IGN Datasheet(PDF) 33 Page - Linear Technology

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