Datasheet

ADS1254
15
SBAS213B
www.ti.com
N NOISE ER ER
(Number REDUCTION IN IN
of Averages) FACTOR µVrms BITS rms
1 1 14.6µV 19.1
2 1.414 10.3µV 19.6
427.3µV 20.1
8 2.82 5.16µV 20.6
16 4 3.65µV 21.1
32 5.66 2.58µV 21.6
64 8 1.83µV 22.1
128 11.3 1.29µV 22.6
256 16 0.91µV 23.1
512 22.6 0.65µV 23.6
1024 32 0.46µV 24.1
2048 45.25 0.32µV 24.6
4096 64 0.23µV 25.1
TABLE V. Averaging.
Conversion Cycle—as used here, a conversion cycle refers
to the time period between DOUT/DRDY pulses.
Effective Resolution (ER)—of the ADS1254 in a particular
configuration can be expressed in two different units:
bits rms (referenced to output) and µVrms (referenced to
input). Computed directly from the converter's output data,
each is a statistical calculation based on a given number of
results. Noise occurs randomly; the rms value represents a
statistical measure that is one standard deviation. The ER in
bits can be computed as follows:
ER in bits rms =
20• log
2•V
Vrms noise
6.02
REF




The 2 • V
REF
figure in each calculation represents the
full-scale range of the ADS1254. This means that both units
are absolute expressions of resolution—the performance in
different configurations can be directly compared, regard-
less of the units.
f
MOD
—frequency of the modulator and the frequency the
input is sampled.
f
CLK
MOD
=
Frequency
6
f
DATA
—Data output rate.
f
f CLK
DATA
MOD
==
64 384
Frequency
Noise Reduction—for random noise, the ER can be im-
proved with averaging. The result is the reduction in noise by
the factor √N, where N is the number of averages, as shown
in Table V. This can be used to achieve true 24-bit perfor-
mance at a lower data rate. To achieve 24 bits of resolution,
more than 24 bits must be accumulated. A 36-bit accumulator
is required to achieve an ER of 24 bits. Table V uses V
REF
=
4.096V, with the ADS1254 outputting data at 20kHz, a 4096
point average will take 204.8ms. The benefits of averaging
will be degraded if the input signal drifts during that 200ms.