ꢐꢕ ꢙꢚ ꢋ ꢛ ꢛ
ꢐꢕ ꢙꢚ ꢋ ꢛ ꢜ
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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
Table 2. Effective Number of Bits (ENOB, rms)
with Buffer On
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,
DATA
RATE
(SPS)
PGA
8
1
2
4
16
32
64
2.5
5
25.3
25.0
24.8
24.6
24.3
24.2
23.9
23.8
23.4
22.3
21.7
21.2
20.8
20.4
20.1
19.8
24.9
24.8
24.5
24.2
24.0
23.8
23.6
23.4
23.0
21.9
21.3
20.9
20.5
20.1
19.7
19.5
24.9
24.5
24.1
23.8
23.4
23.3
23.0
22.9
22.5
21.5
20.8
20.4
20.0
19.6
19.3
19.1
24.4
24.0
23.5
23.2
23.0
22.8
22.5
22.4
22.0
20.9
20.2
19.7
19.4
19.0
18.7
18.5
23.8
23.3
22.9
22.5
22.2
22.1
21.8
21.7
21.4
20.3
19.8
19.3
19.0
18.5
18.2
18.0
23.0
22.7
22.3
21.8
21.5
21.5
21.1
21.0
20.8
19.6
19.2
18.8
18.4
17.9
17.7
17.4
22.2
21.8
21.3
21.0
20.7
20.5
20.3
20.2
19.8
18.7
18.3
17.9
17.4
17.0
16.7
16.5
10
15
AVDD = 5V, DVDD = 1.8V, VREF = 2.5V, and fCLKIN
=
25
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:
30
50
60
100
500
1000
2000
3750
7500
15,000
30,000
ǒ
Ǔ
ln FSRńRMS Noise
ENOB +
( )
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 3. Noise-Free Resolution (bits)
with Buffer On
DATA
RATE
(SPS)
PGA
Table 1. Input Referred Noise (µV, rms)
with Buffer On
1
2
4
8
16
32
64
DATA
RATE
(SPS)
PGA
8
2.5
5
23.0
22.3
22.3
22.0
21.7
21.8
21.3
21.3
20.9
20.1
19.0
18.5
18.1
17.7
17.3
17.1
22.6
22.4
22.0
21.7
21.4
21.3
21.1
20.9
20.7
19.6
18.6
18.1
17.8
17.3
17.0
16.7
22.1
21.9
21.6
21.3
21.1
20.8
20.4
20.5
20.2
19.1
18.1
17.8
17.3
16.9
16.5
16.4
21.7
21.3
21.0
20.7
20.5
20.4
19.9
19.8
19.6
18.6
17.5
17.0
16.6
16.2
15.9
15.9
21.3
20.7
20.4
20.1
19.7
19.8
19.4
19.3
19.1
18.0
17.2
16.6
16.2
15.8
15.5
15.4
20.8
20.3
19.9
19.3
19.2
19.0
18.8
18.8
18.5
17.3
16.5
16.1
15.7
15.3
14.9
14.6
19.7
19.3
18.9
18.7
18.5
18.1
17.9
17.8
17.4
16.3
15.6
15.3
14.7
14.4
13.9
13.8
1
2
4
16
32
64
10
2.5
5
0.247 0.156 0.080 0.056 0.043 0.037 0.033
0.301 0.175 0.102 0.076 0.061 0.045 0.044
0.339 0.214 0.138 0.106 0.082 0.061 0.061
0.401 0.264 0.169 0.126 0.107 0.085 0.073
0.494 0.305 0.224 0.149 0.134 0.102 0.093
0.533 0.335 0.245 0.176 0.138 0.104 0.106
0.629 0.393 0.292 0.216 0.168 0.136 0.122
0.692 0.438 0.321 0.233 0.184 0.146 0.131
0.875 0.589 0.409 0.305 0.229 0.170 0.169
1.946 1.250 0.630 0.648 0.497 0.390 0.367
2.931 1.891 1.325 1.070 0.689 0.512 0.486
4.173 2.589 1.827 1.492 0.943 0.692 0.654
5.394 3.460 2.376 1.865 1.224 0.912 0.906
7.249 4.593 3.149 2.436 1.691 1.234 1.187
15
25
10
30
15
50
25
60
30
100
500
1000
2000
3750
7500
15,000
30,000
50
60
100
500
1000
2000
3750
7500
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
12
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