LTC2484CDD datasheet(33/40 Pages) LINER

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LTC2484

2484fa

APPLICATIO S I FOR ATIO

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 LTC2484 input bandwidth is shown in

Figure 28. When an external oscillator of frequency fEOSC

is used, the shape of the LTC2484 input bandwidth can be

derived from Figure 28, 60Hz mode curve in which the

horizontal axis is scaled by fEOSC/307200.

The conversion noise (600nVRMS typical for VREF = 5V)

can be modeled by a white noise source connected to a

noise free converter. The noise spectral density is 47nV

√Hz

for an infinite bandwidth source and 64nV

√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

high gain, low bandwidth amplifier stage followed by a

high bandwidth unity-gain buffer.

When external amplifiers are driving the LTC2484, the

ADC input referred system noise calculation can be sim-

plified by Figure 29. The noise of an amplifier driving the

LTC2484 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 29, 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 filter-

ing. 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 LTC2484 input) can now be obtained by

summing as square root of sum of squares the three ADC

input referred noise sources: the LTC2484 internal noise,

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 29 can still be used for noise calculation if the

x-axis is scaled by fEOSC/307200. For large values of the

ratio fEOSC/307200, the Figure 29 plot accuracy begins to

decrease, but at the same time the LTC2484 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 LTC2484 significantly

simplifies antialiasing filter requirements. Additionally,

the input current cancellation feature of the LTC2484

allows external lowpass filtering without degrading the DC

performance of the device.

Figure 29. Input Referred Noise Equivalent Bandwidth

of an Input Connected White Noise Source

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–3

–2

–1

2484 F28

–4

–5

–6

50Hz MODE

60Hz MODE

50Hz AND

60Hz MODE

Figure 28. Input Signal Bandwidth Using the Internal Oscillator

INPUT NOISE SOURCE SINGLE POLE

EQUIVALENT BANDWIDTH (Hz)

0.1

100

10k

100k

2484 F29

0.1

100

50Hz MODE

60Hz MODE

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