LTC2410IGN datasheet(32/48 Pages) LINER

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LTC2410

APPLICATIO S I FOR ATIO

When external amplifiers are driving the LTC2410, the

ADC input referred system noise calculation can be simpli-

fied by Figure 36. The noise of an amplifier driving the

LTC2410 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 spectral density is ni.

From Figure 36, 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 bandwidth 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

LTC2410 input) can now be obtained by summing as

square root of sum of squares the three ADC input referred

noise sources: the LTC2410 internal noise (800nV), 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 36 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 36 plot accuracy begins to

decrease, but in the same time the LTC2410 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 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.

Figure 37. Input Normal Mode Rejection,

Internal Oscillator and 50Hz Notch

Figure 38. Input Normal Mode Rejection, Internal

Oscillator and 60Hz Notch or External Oscillator

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

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

2410 F37

–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 F38

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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

Figure 36. 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

2410 F36

0.1

100

FO = HIGH

FO = LOW

LTC2410IGN Datasheet(PDF) 32 Page - Linear Technology

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