AD7712(RevE) Ver la hoja de datos (PDF) - Analog Devices
Número de pieza
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Fabricante
AD7712 Datasheet PDF : 28 Pages
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AD7712
Tables I and II show the output rms noise for some typical notch and –3 dB frequencies. The numbers given are for the bipolar
input ranges with a VREF of +2.5 V. These numbers are typical and are generated with an analog input voltage of 0 V. The output
noise from the part comes from two sources. First, there is the electrical noise in the semiconductor devices used in the implementa-
tion of the modulator (device noise). Secondly, when the analog input signal is converted into the digital domain, quantization noise
is added. The device noise is at a low level and is largely independent of frequency. The quantization noise starts at an even lower
level but rises rapidly with increasing frequency to become the dominant noise source. Consequently, lower filter notch settings
(below 60 Hz approximately) tend to be device noise dominated while higher notch settings are dominated by quantization noise.
Changing the filter notch and cutoff frequency in the quantization noise dominated region results in a more dramatic improvement
in noise performance than it does in the device noise dominated region as shown in Table I. Furthermore, quantization noise is
2
added after the PGA, so effective resolution is independent of gain for the higher filter notch frequencies. Meanwhile, device noise is
added in the PGA and, therefore, effective resolution suffers a little at high gains for lower notch frequencies.
At the lower filter notch settings (below 60 Hz), the no missing codes performance of the device is at the 24-bit level. At the higher
settings, more codes will be missed until at 1 kHz notch setting, no missing codes performance is only guaranteed to the 12-bit level.
However, since the effective resolution of the part is 10.5 bits for this filter notch setting, this no missing codes performance should
be more than adequate for all applications.
The effective resolution of the device is defined as the ratio of the output rms noise to the input full scale. This does not remain
constant with increasing gain or with increasing bandwidth. Table II shows the same table as Table I except that the output is now
expressed in terms of effective resolution (the magnitude of the rms noise with respect to 2 × VREF/GAIN, i.e., the input full scale). It
is possible to do post filtering on the device to improve the output data rate for a given –3 dB frequency and also to further reduce
the output noise (see Digital Filtering section).
Table I. Output Noise vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3␣ dB
Gain of
Data Rate1 Frequency 1
Gain of
2
Typical Output RMS Noise (V)
Gain of
4
Gain of
8
Gain of
16
Gain of
32
Gain of Gain of
64
128
10␣ Hz2
25␣ Hz2
30␣ Hz2
50␣ Hz2
60␣ Hz2
100␣ Hz3
250␣ Hz3
500␣ Hz3
1␣ kHz3
2.62␣ Hz
6.55␣ Hz
7.86␣ Hz
13.1 Hz
15.72 Hz
26.2 Hz
65.5 Hz
131 Hz
262 Hz
1.0
1.8
2.5
4.33
5.28
13
130
0.6 × 103
3.1 × 103
0.78
1.1
1.31
2.06
2.36
6.4
75
0.26 × 103
1.6 × 103
0.48
0.63
0.84
1.2
1.33
3.7
25
140
0.7 × 103
0.33
0.25
0.5
0.44
0.57
0.46
0.64
0.54
0.87
0.63
1.8
1.1
12
7.5
70
35
0.29 × 103 180
0.25
0.25
0.25
0.41
0.38
0.38
0.43
0.4
0.4
0.46
0.46
0.46
0.62
0.6
0.56
0.9
0.65
0.65
4
2.7
1.7
25
15
8
120
70
40
NOTES
1The default condition (after the internal power-on reset) for the first notch of filter is 60 Hz.
2For these filter notch frequencies, the output rms noise is primarily dominated by device noise and as a result is independent of the value of the reference voltage.
Therefore, increasing the reference voltage will give an increase in the effective resolution of the device (i.e., the ratio of the rms noise to the input full scale is
increased since the output rms noise remains constant as the input full scale increases).
3For these filter notch frequencies, the output rms noise is dominated by quantization noise and as a result is proportional to the value of the reference voltage.
Table II. Effective Resolution vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3␣ dB
Gain of
Data Rate
Frequency 1
Gain of
2
Effective Resolution1 (Bits)
Gain of
4
Gain of
8
Gain of
16
Gain of
32
Gain of Gain of
64
128
10␣ Hz
2.62␣ Hz
22.5
21.5
21.5
21
20.5
19.5
18.5
17.5
25␣ Hz
6.55␣ Hz
21.5
21
21
20
19.5
18.5
17.5
16.5
30␣ Hz
7.86␣ Hz
21
21
20.5
20
19.5
18.5
17.5
16.5
50␣ Hz
13.1␣ Hz
20
20
20
20
19
18.5
17.5
16.5
60␣ Hz
15.72␣ Hz 20
20
20
19.5
19
18
17
16
100␣ Hz
26.2␣ Hz
18.5
18.5
18.5
18.5
18
17.5
17
16
250␣ Hz
65.5␣ Hz
15
15.5
15.5
15.5
15.5
15.5
15
14.5
500␣ Hz
131␣ Hz
13
13
13
13
13
12.5
12.5
12.5
1␣ kHz
262␣ Hz
10.5
10.5
11
11
11
10.5
10
10
NOTE
1Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 × VREF/GAIN). The above table applies for
a VREF of +2.5 V and resolution numbers are rounded to the nearest 0.5 LSB.
REV. E
–11–
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