Conversion Cycle—as used here, a conversion cycle refers
to the time period between DOUT/DRDY pulses.
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
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:
is required to achieve an ER of 24 bits. Table V uses VREF
=
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.
2• VREF
20• log
N
NOISE
REDUCTION
FACTOR
ER
IN
Vrms
ER
IN
BITS rms
(Number
of Averages)
Vrms noise
ER in bits rms =
6.02
1
2
1
1.414
2
14.6µV
10.3µV
7.3µV
19.1
19.6
20.1
20.6
21.1
21.6
22.1
22.6
23.1
23.6
24.1
24.6
25.1
The 2 • VREF 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.
4
8
2.82
4
5.16µV
3.65µV
2.58µV
1.83µV
1.29µV
0.91µV
0.65µV
0.46µV
0.32µV
0.23µV
16
32
5.66
8
64
128
256
512
1024
2048
4096
11.3
16
fMOD—frequency of the modulator and the frequency the
input is sampled.
22.6
32
CLK Frequency
45.25
64
fMOD
=
6
TABLE V. Averaging.
fDATA—Data output rate.
fMOD CLK Frequency
fDATA
=
=
64
384
ADS1254
15
SBAS213
BB [ BURR-BROWN CORPORATION ]
