Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
12
NOISE PERFORMANCE
The ADS1255/6 offer outstanding noise performance that
can be optimized by adjusting the data rate or PGA setting.
As the averaging is increased by reducing the data rate,
the noise drops correspondingly. The PGA reduces the
input-referred noise when measuring lower level signals.
Table 1 through Table 6 summarize the typical noise
performance with the inputs shorted externally. In all six
tables, the following conditions apply: T = +25°C,
AVDD = 5V, DVDD = 1.8V, V
REF
= 2.5V, and
f
CLKIN
= 7.68MHz. Table 1 to Table 3 reflect the device
input buffer enabled. Table 1 shows the rms value of the
input-referred noise in volts. Table 2 shows the effective
number of bits of resolution (ENOB), using the noise data
from Table 1. ENOB is defined as:
ENOB +
ln
ǒ
FSRńRMS Noise
Ǔ
ln
(
2
)
where FSR is the full-scale range. Table 3 shows the
noise-free bits of resolution. It is calculated with the same
formula as ENOB except the peak-to-peak noise value is
used instead of rms noise. Table 4 through Table 6 show
the same noise data, but with the input buffer disabled.
Table 1. Input Referred Noise (μV, rms)
with Buffer On
DATA
RATE
PGA
RATE
(SPS)
1 2 4 8 16 32 64
2.5 0.247 0.156 0.080 0.056 0.043 0.037 0.033
5 0.301 0.175 0.102 0.076 0.061 0.045 0.044
10 0.339 0.214 0.138 0.106 0.082 0.061 0.061
15 0.401 0.264 0.169 0.126 0.107 0.085 0.073
25 0.494 0.305 0.224 0.149 0.134 0.102 0.093
30 0.533 0.335 0.245 0.176 0.138 0.104 0.106
50 0.629 0.393 0.292 0.216 0.168 0.136 0.122
60 0.692 0.438 0.321 0.233 0.184 0.146 0.131
100 0.875 0.589 0.409 0.305 0.229 0.170 0.169
500 1.946 1.250 0.630 0.648 0.497 0.390 0.367
1000 2.931 1.891 1.325 1.070 0.689 0.512 0.486
2000 4.173 2.589 1.827 1.492 0.943 0.692 0.654
3750 5.394 3.460 2.376 1.865 1.224 0.912 0.906
7500 7.249 4.593 3.149 2.436 1.691 1.234 1.187
15,000 9.074 5.921 3.961 2.984 2.125 1.517 1.515
30,000 10.728 6.705 4.446 3.280 2.416 1.785 1.742
Table 2. Effective Number of Bits (ENOB, rms)
with Buffer On
DATA
RATE
PGA
RATE
(SPS)
1 2 4 8 16 32 64
2.5 25.3 24.9 24.9 24.4 23.8 23.0 22.2
5 25.0 24.8 24.5 24.0 23.3 22.7 21.8
10 24.8 24.5 24.1 23.5 22.9 22.3 21.3
15 24.6 24.2 23.8 23.2 22.5 21.8 21.0
25 24.3 24.0 23.4 23.0 22.2 21.5 20.7
30 24.2 23.8 23.3 22.8 22.1 21.5 20.5
50 23.9 23.6 23.0 22.5 21.8 21.1 20.3
60 23.8 23.4 22.9 22.4 21.7 21.0 20.2
100 23.4 23.0 22.5 22.0 21.4 20.8 19.8
500 22.3 21.9 21.5 20.9 20.3 19.6 18.7
1000 21.7 21.3 20.8 20.2 19.8 19.2 18.3
2000 21.2 20.9 20.4 19.7 19.3 18.8 17.9
3750 20.8 20.5 20.0 19.4 19.0 18.4 17.4
7500 20.4 20.1 19.6 19.0 18.5 17.9 17.0
15,000 20.1 19.7 19.3 18.7 18.2 17.7 16.7
30,000 19.8 19.5 19.1 18.5 18.0 17.4 16.5
Table 3. Noise-Free Resolution (bits)
with Buffer On
DATA
RATE
PGA
RATE
(SPS)
1 2 4 8 16 32 64
2.5 23.0 22.6 22.1 21.7 21.3 20.8 19.7
5 22.3 22.4 21.9 21.3 20.7 20.3 19.3
10 22.3 22.0 21.6 21.0 20.4 19.9 18.9
15 22.0 21.7 21.3 20.7 20.1 19.3 18.7
25 21.7 21.4 21.1 20.5 19.7 19.2 18.5
30 21.8 21.3 20.8 20.4 19.8 19.0 18.1
50 21.3 21.1 20.4 19.9 19.4 18.8 17.9
60 21.3 20.9 20.5 19.8 19.3 18.8 17.8
100 20.9 20.7 20.2 19.6 19.1 18.5 17.4
500 20.1 19.6 19.1 18.6 18.0 17.3 16.3
1000 19.0 18.6 18.1 17.5 17.2 16.5 15.6
2000 18.5 18.1 17.8 17.0 16.6 16.1 15.3
3750 18.1 17.8 17.3 16.6 16.2 15.7 14.7
7500 17.7 17.3 16.9 16.2 15.8 15.3 14.4
15,000 17.3 17.0 16.5 15.9 15.5 14.9 13.9
30,000 17.1 16.7 16.4 15.9 15.4 14.6 13.8