LTC2440IGN datasheet(26/28 Pages) LINER

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LTC2440

sn2440, 2440fas

APPLICATIO S I FOR ATIO

Example:

If an amplifier (e.g. LT1219) driving the input of an

LTC2440 has wideband noise of 33nV/

√Hz, band-limited

to 1.8MHz, the total noise entering the ADC input is:

33nV/

√Hz • √1.8MHz = 44.3µV.

When the ADC digitizes the input, its digital filter filters out

the wideband noise from the input signal. The noise

reduction depends on the oversample ratio which defines

the effective bandwidth of the digital filter.

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

is 787Hz which reduces the total amplifier noise to:

33nV/

√Hz • √787Hz = 0.93µV.

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

µV

noise of the ADC at OSR=256.

√0.93µV2 + 2uV2 = 2.2µV.

Increasing the oversampling ratio to 32768 reduces the

noise bandwidth of the ADC to 6.2Hz which reduces the

total amplifier noise to:

33nV/

√Hz • √6.2Hz = 82nV.

The total noise is the RMS sum of this noise with the 200nV

noise of the ADC at OSR = 32768.

√82nV2 + 2µV2 = 216nV.

In this way, the digital filter with its variable oversampling

ratio can greatly reduce the effects of external noise

sources.

Using Non-Autozeroed Amplifiers for Lowest Noise

Applications

Ultralow noise applications may require the use of low

noise bipolar amplifiers 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 amplifiers

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 amplifiers

and correlated double sampling to achieve a resolution of

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

surement is the difference between two ADC readings

taken with opposite polarity bridge excitation. This can-

cels 1/f noise below 3.4Hz and eliminates errors due to

parasitic thermocouples. Allow 750

µs settling time after

switching excitation polarity.

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