AD7713AN Просмотр технического описания (PDF) - Analog Devices
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AD7713AN Datasheet PDF : 28 Pages
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AD7713
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 implementation of the modulator (device noise). Second,
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 12 Hz approximately) tend to
be device noise dominated while higher notch settings are domi-
nated 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 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 12 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 200 Hz notch
setting, no missing codes performance is guaranteed only 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 is the same as Table I except that the output is expressed
in terms of effective resolution (the magnitude of the rms noise
with respect to 2 ϫ 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 the Digital Filtering section).
Table I. Output Noise vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3 dB
Typical Output RMS Noise (µV)
Data Rate1 Frequency Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
2 Hz2
0.52 Hz 1.0
0.78
0.48
0.33
0.25
0.25
0.25
0.25
5 Hz2
1.31 Hz 1.8
1.1
0.63
0.5
0.44
0.41
0.38
0.38
6 Hz2
1.57 Hz 2.5
1.31
0.84
0.57
0.46
0.43
0.4
0.4
10 Hz2
2.62 Hz 4.33
2.06
1.2
0.64
0.54
0.46
0.46
0.46
12 Hz2
3.14 Hz 5.28
2.36
1.33
0.87
0.63
0.62
0.6
0.56
20 Hz3
5.24 Hz 13
6.4
3.7
1.8
1.1
0.9
0.65
0.65
50 Hz3
13.1 Hz 130
75
25
12
7.5
4
2.7
1.7
100 Hz3
26.2 Hz 0.6 ϫ 103 0.26 ϫ 103 140
70
35
25
15
8
200 Hz3
52.4 Hz 3.1 ϫ 103 1.6 ϫ 103 0.7 ϫ 103 0.29 ϫ 103 180
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
Effective Resolution* (Bits)
Data Rate
Frequency Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
2 Hz
5 Hz
6 Hz
10 Hz
12 Hz
20 Hz
50 Hz
100 Hz
200 Hz
0.52 Hz 22.5
21.5
21.5
21
20.5
19.5
18.5
17.5
1.31 Hz 21.5
21
21
20
19.5
18.5
17.5
16.5
1.57 Hz 21
21
20.5
20
19.5
18.5
17.5
16.5
2.62 Hz 20
20
20
19.5
19
18.5
17.5
16.5
3.14 Hz 20
20
20
19.5
19
18
17
16
5.24 Hz 18.5
18.5
18.5
18.5
18
17.5
17
16
13.1 Hz 15
15
15.5
15.5
15.5
15.5
15
14.5
26.2 Hz 13
13
13
13
13
12.5
12.5
12.5
52.4 Hz 10.5
10.5
11
11
11
10.5
10
10
*Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 ϫ VREF/GAIN). Table II applies for a VREF of 2.5 V
and resolution numbers are rounded to the nearest 0.5 LSB.
REV. D
–11–
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