LTC2413 datasheet(32/44 Pages) LINER

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LTC2413

sn2413 2413fs

APPLICATIO S I FOR ATIO

Input Bandwidth

The combined effect of the internal sinc4 digital filter and

of the analog and digital autocalibration circuits deter-

mines the LTC2413 input bandwidth. When the internal

oscillator is used (FO = LOW), the 3dB input bandwidth is

3.3Hz. If an external conversion clock generator of fre-

quency fEOSC is connected to the FO pin, the 3dB input

bandwidth is 0.236 • 10–6 • fEOSC.

Due to the complex filtering and calibration algorithms

utilized, the converter input bandwidth is not modeled very

accurately by a first order filter with the pole located at the

3dB frequency. When the internal oscillator is used, the

shape of the LTC2413 input bandwidth is shown in

Figure 36. When an external oscillator of frequency fEOSC

is used, the shape of the LTC2413 input bandwidth can be

derived from Figure 36, in which the horizontal axis is

scaled by fEOSC/139800.

The conversion noise (800nVRMS typical for VREF = 5V)

can be modeled as a white noise source connected to a

noise free converter. The noise spectral density is 63nV/

√Hz

for an infinite bandwidth source and 77nV/

√Hz for a single

0.5MHz pole source. From these numbers, it is clear that

particular attention must be given to the design of external

amplification circuits. Such circuits face the simultaneous

requirements of very low bandwidth (just a few Hz) in

order to reduce the output referred noise and relatively

high bandwidth (at least 500kHz) necessary to drive the

input switched-capacitor network. A possible solution is a

Figure 36. Input Signal Bandwidth Using the Internal Oscillator

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0.5

1.5

2.5

3.5

4.5

2413 F36

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

high gain, low bandwidth amplifier stage followed by a

high bandwidth unity-gain buffer.

When external amplifiers are driving the LTC2413, the

ADC input referred system noise calculation can be simpli-

fied by Figure 37. The noise of an amplifier driving the

LTC2413 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 37, 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

LTC2413 input) can now be obtained by summing as

square root of sum of squares the three ADC input referred

noise sources: the LTC2413 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 37 can still be used for noise calculation if the

x-axis is scaled by fEOSC/139800. For large values of the

ratio fEOSC/139800, the Figure 37 plot accuracy begins to

decrease, but in the same time the LTC2413 noise floor

rises and the noise contribution of the driving amplifiers

lose significance.

0.1

100

1000

INPUT NOISE SOURCE SINGLE POLE

EQUIVALENT BANDWIDTH (Hz)

0.1

100

10k

100k

2413 F37

FO = LOW

Figure 37. Input Referred Noise Equivalent Bandwidth

of an Input Connected White Noise Source

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