LTC2435-1IGN datasheet(32/40 Pages) LINER

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LTC2435/LTC2435-1

24351fa

Figure 34a. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch (LTC2435)

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 LTC2435/LTC2435-1 sig-

nificantly 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). Inde-

pendent of the operating mode, fS = 256 • fN = 1024 •

fOUTMAX where fN is the notch frequency and fOUTMAX is the

maximum output data rate. In the internal oscillator mode,

for the LTC2435, FS = 12800Hz with a 50Hz notch setting

and fS = 15360Hz with a 60Hz notch setting. For the

LTC2435-1, fS = 13980Hz (FO = LOW). In the external

oscillator mode, fS = fEOSC/10.

The normal mode rejection performance is shown in

Figure 34. 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 35 (rejection near DC) and Figure 36

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

frequency. For the LTC2435, the bandwidth is 13.6Hz

(FO = GND) and 11.4Hz (FO = VCC). The Bandwidth is

12.4Hz for the LTC2435-1 (FO = GND).

Through FO connection, the LTC2435 provides better than

110dB input differential mode rejection at 50Hz or 60Hz

±2%. While for the LTC2435-1, it has a notch frequency of

about 55Hz with better than 70db rejection over 48Hz to

62.4Hz, which covers both 50Hz ±2% and 60Hz ±2%. In

order to achieve better rejection over the range of 48Hz to

62.4Hz, a running average can be performed. By averaging

two consecutive LTC2435-1 readings, a sinc1 notch is

combined with the sinc4 digital filter, yielding the fre-

quency response shown in Figure 37. The averaging

operation still keeps the output rate with the following

algorithm:

Result 1 = average (sample 0, sample 1)

Result 2 = average (sample 1, sample 2)

Result n = average (sample n-1, sample n)

The user can expect to achieve in practice this level of

performance using the internal oscillator as it is demon-

strated by Figures 38 to 40. Typical measured values of the

normal mode rejection of the LTC2435-1 operating with an

internal oscillator and a 54.6Hz notch setting are shown in

Figure 38 and 39 superimposed over the theoretical calcu-

lated curve. The same normal mode rejection perfor-

mance is obtained for the LTC2435 with the frequency

scaled to have the notch frequency at 60Hz (FO = GND) or

50Hz (FO = VCC).

APPLICATIO S I FOR ATIO

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

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

2435 F34a

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

FO = HIGH

INPUT FREQUENCY

–60

–40

2435 F34b

–80

–100

fS/2

–120

–140

–20

Figure 34b. Input Normal Mode Rejection, Internal

Oscillator and FO = Low or External Oscillator

LTC2435-1IGN Datasheet(PDF) 32 Page - Linear Technology

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