LTC2410IGN datasheet(28/44 Pages) LINER

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LTC2410

APPLICATIO S I FOR ATIO

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 LTC2410 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

LTC2410’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 with a 50Hz notch setting, fS = 12800Hz and with a

60Hz notch setting fS = 15360Hz. In the external oscillator

mode, fS = fEOSC/10.

The combined normal mode rejection performance is

shown in Figure 36 for the internal oscillator with 50Hz

notch setting (FO = HIGH) and in Figure 37 for the internal

oscillator with 60Hz notch setting (FO = LOW) and for the

external oscillator 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 38 (rejection near DC) and

Figure 39 (rejection at fS = 256fN) where fN represents the

notch frequency. These curves have been derived for the

external oscillator mode but they can be used in all

operating modes by appropriately selecting the fN value.

The user can expect to achieve in practice this level of

performance using the internal oscillator as it is demon-

strated by Figures 40 and 41. Typical measured values of

the normal mode rejection of the LTC2410 operating with

an internal oscillator and a 60Hz notch setting are shown

in Figure 40 superimposed over the theoretical calculated

curve. Similarly, typical measured values of the normal

mode rejection of the LTC2410 operating with an internal

oscillator and a 50Hz notch setting are shown in Figure 41

superimposed over the theoretical calculated curve.

As a result of these remarkable normal mode specifica-

tions, minimal (if any) antialias filtering is required in front

of the LTC2410. If passive RC components are placed in

Figure 35. Input Signal Bandwidth Using the Internal Oscillator

Figure 36. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch

Figure 37. Input Normal Mode Rejection, Internal

Oscillator and 60Hz Notch or External Oscillator

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0.5

1.5

2.5

3.5

4.5

2410 F35

0.0

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

–3.5

–4.0

–4.5

–5.0

–5.5

–6.0

FO = HIGH

FO = LOW

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

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

2410 F36

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

FO = HIGH

FO = LOW OR

FO = EXTERNAL OSCILLATOR,

fEOSC = 10 • fS

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS

2410 F37

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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

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