LTC2430IGN datasheet(30/40 Pages) LINER

30 / 40 page

LTC2430/LTC2431

24301f

If the FO pin is driven by an external oscillator of frequency

fEOSC, Figure 29 can still be used for noise calculation if the

x-axis is scaled by fEOSC/153600. For large values of the

ratio fEOSC/153600, the Figure 29 plot accuracy begins to

decrease, but in the same time the LTC2430/LTC2431

noise floor rises and the noise contribution of the driving

amplifiers lose significance.

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 LTC2430/LTC2431 signifi-

cantly 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

LTC2430/LTC2431’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, fS = 12,800Hz with a 50Hz notch setting and fS =

15,360Hz with a 60Hz notch setting. In the external

oscillator mode, fS = fEOSC/10.

The combined normal mode rejection performance is

shown in Figure 30 for the internal oscillator with 50Hz

notch setting (FO = HIGH) and in Figure 31 for the internal

oscillator with 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 32 (rejection near DC) and Figure 33 (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 34 to 36. Typical measured values of the

normal mode rejection of the LTC2430/LTC2431 operat-

ing with an internal oscillator and a 60Hz notch setting are

shown in Figure 34 superimposed over the theoretical

calculated curve. Similarly, typical measured values of the

normal mode rejection of the LTC2430/LTC2431 operat-

ing with an internal oscillator and a 50Hz notch setting are

shown in Figure 35 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 LTC2430/LTC2431. If passive RC components are

placed in front of the LTC2430/LTC2431, the input dy-

namic current should be considered (see Input Current

section). In cases where large effective RC time constants

are used, an external buffer amplifier may be required to

minimize the effects of dynamic input current.

APPLICATIO S I FOR ATIO

Figure 30. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch

Figure 31. Input Normal Mode Rejection, Internal

Oscillator and FO = LOW or External Oscillator

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

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

2431 F30

–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

2431 F31

–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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