ADS1255
ADS1256
www.ti.com
SBAS288D − JUNE 2003 − REVISED AUGUST 2004
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:
Table 2. Effective Number of Bits (ENOB, rms)
with Buffer On
DATA
RATE
(SPS)
2.5
5
10
15
25
30
50
60
100
500
1000
2000
3750
7500
PGA
1
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
2
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
4
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
8
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
16
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
32
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
64
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
ln FSR RMS 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.
15,000
30,000
Table 3. Noise-Free Resolution (bits)
with Buffer On
DATA
RATE
(SPS)
2.5
5
10
15
25
30
50
60
100
500
1000
2000
3750
7500
15,000
30,000
PGA
1
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
2
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
4
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
8
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
16
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
32
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
64
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
Table 1. Input Referred Noise (µV, rms)
with Buffer On
DATA
RATE
(SPS)
2.5
5
10
15
25
30
50
60
100
500
1000
2000
3750
7500
15,000
30,000
PGA
1
0.247
0.301
0.339
0.401
0.494
0.533
0.629
0.692
0.875
1.946
2.931
4.173
5.394
7.249
9.074
10.728
2
0.156
0.175
0.214
0.264
0.305
0.335
0.393
0.438
0.589
1.250
1.891
2.589
3.460
4.593
5.921
6.705
4
0.080
0.102
0.138
0.169
0.224
0.245
0.292
0.321
0.409
0.630
1.325
1.827
2.376
3.149
3.961
4.446
8
0.056
0.076
0.106
0.126
0.149
0.176
0.216
0.233
0.305
0.648
1.070
1.492
1.865
2.436
2.984
3.280
16
0.043
0.061
0.082
0.107
0.134
0.138
0.168
0.184
0.229
0.497
0.689
0.943
1.224
1.691
2.125
2.416
32
0.037
0.045
0.061
0.085
0.102
0.104
0.136
0.146
0.170
0.390
0.512
0.692
0.912
1.234
1.517
1.785
64
0.033
0.044
0.061
0.073
0.093
0.106
0.122
0.131
0.169
0.367
0.486
0.654
0.906
1.187
1.515
1.742
12
BURR-BROWN [ BURR-BROWN CORPORATION ]
