LTC2440
26
2440fe
For more information www.linear.com/LTC2440
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 2µV
noise of the ADC at OSR=256.
√0.93µ/V
2
+ 2µV
2
= 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.
√82nV
2
+ 200nV
2
= 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 ampli
-
fiers 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 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 750µs settling time after switching
excitation polarity.
APPLICATIONS INFORMATION
