LTC2414IGN datasheet(33/48 Pages) LINER

33 / 48 page

LTC2414/LTC2418

241418fa

APPLICATIO S I FOR ATIO

When external amplifiers are driving the LTC2414/

LTC2418, the ADC input referred system noise calculation

can be simplified by Figure 32. The noise of an amplifier

driving the LTC2414/LTC2418 input pin can be modeled

as a band limited white noise source. Its bandwidth can be

approximated by the bandwidth of a single pole lowpass

filter with a corner frequency fi. The amplifier noise spec-

tral density is ni. From Figure 32, using fi as the x-axis

selector, we can find on the y-axis the noise equivalent

bandwidth freqi of the input driving amplifier. This band-

width includes the band limiting effects of the ADC internal

calibration and filtering. The noise of the driving amplifier

referred to the converter input and including all these

effects can be calculated as N = ni • √freqi. The total system

noise (referred to the LTC2414/LTC2418 input) can now

be obtained by summing as square root of sum of squares

the three ADC input referred noise sources: the LTC2414/

LTC2418 internal noise (1

µV), the noise of the IN+ driving

amplifier and the noise of the IN– driving amplifier.

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

fEOSC, Figure 32 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 32 plot accuracy begins to

decrease, but in the same time the LTC2414/LTC2418

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 LTC2414/LTC2418 signifi-

cantly simplify 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

LTC2414/LTC2418’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 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.

Figure 33. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch

Figure 34. Input Normal Mode Rejection, Internal

Oscillator and 60Hz Notch or External Oscillator

Figure 32. Input Referred Noise Equivalent Bandwidth

of an Input Connected White Noise Source

INPUT NOISE SOURCE SINGLE POLE

EQUIVALENT BANDWIDTH (Hz)

0.1

100

10k

100k

2414/18 F32

0.1

100

FO = HIGH

FO = LOW

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

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

2414/18 F33

–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

2414/18 F34

–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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