LTC2440_15 Datasheet, PDF(26/28 Page) Linear Technology – 24-Bit High Speed Differential ADC with Selectable Speed/Resolution

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LTC2440_15 Datasheet, PDF (26/28 Pages) Linear Technology – 24-Bit High Speed Differential ADC with Selectable Speed/Resolution

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LTC2440

APPLICATIONS INFORMATION

Example:

If an ampliï¬er (e.g. LT1219) driving the input of an LTC2440

has wideband noise of 33nV/âHz, band-limited to 1.8MHz,

the total noise entering the ADC input is:

33nV/âHz â¢ â1.8MHz = 44.3Î¼V.

When the ADC digitizes the input, its digital ï¬lter ï¬lters

out the wideband noise from the input signal. The noise

reduction depends on the oversample ratio which deï¬nes

the effective bandwidth of the digital ï¬lter.

At an oversample of 256, the noise bandwidth of the ADC

is 787Hz which reduces the total ampliï¬er noise to:

33nV/âHz â¢ â787Hz = 0.93Î¼V.

The total noise is the RMS sum of this noise with the 2Î¼V

noise of the ADC at OSR=256.

â0.93Î¼/V2 + 2Î¼V2 = 2.2Î¼V.

Increasing the oversampling ratio to 32768 reduces the

noise bandwidth of the ADC to 6.2Hz which reduces the

total ampliï¬er noise to:

33nV/âHz â¢ â6.2Hz = 82nV.

The total noise is the RMS sum of this noise with the

200nV noise of the ADC at OSR = 32768.

â82nV2 + 2Î¼V2 = 216nV.

In this way, the digital ï¬lter with its variable oversampling

ratio can greatly reduce the effects of external noise

sources.

Using Non-Autozeroed Ampliï¬ers for Lowest Noise

Applications

Ultralow noise applications may require the use of low

noise bipolar ampliï¬ers that are not autozeroed. Because

the LTC2440 has such exceptionally low offset, offset drift

and 1/f noise, the offset drift and 1/f noise in the ampli-

ï¬ers may need to be compensated for to retain the system

performance of which the ADC is capable.

The circuit of Figure 23 uses low noise bipolar ampliï¬ers

and correlated double sampling to achieve a resolution of

14nV, or 19 effective bits over a 10mV span. Each measure-

ment is the difference between two ADC readings taken

with opposite polarity bridge excitation. This cancels 1/f

noise below 3.4Hz and eliminates errors due to parasitic

thermocouples. Allow 750Î¼s settling time after switching

excitation polarity.

2440fd

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