AD7711A Analog Devices, AD7711A Datasheet - Page 11

• Manufacturer Part Number: AD7711A
• Description: CMOS, 24-Bit Sigma-Delta, Signal Conditioning ADC with RTD Current Source
• Manufacturer: Analog Devices

Specifications of AD7711A

• Resolution (bits): 24bit
• # Chan: 2
• Sample Rate: 19.5kSPS
• Interface: Ser
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip (Vref)/(PGA Gain),Uni (Vref)/(PGA Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC

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 V
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 do-
main, 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 increas-
ing frequency to become the dominant noise source. Conse-
quently, 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 added after the
PGA, so effective resolution is independent of gain for the
REV. D
First Notch of
Filter and O/P –3 dB
Data Rate
10 Hz
25 Hz
30 Hz
50 Hz
60 Hz
100 Hz
250 Hz
500 Hz
1 kHz
NOTES
1
2
3
First Notch of
Filter and O/P –3 dB
Data Rate
10 Hz
25 Hz
30 Hz
50 Hz
60 Hz
100 Hz
250 Hz
500 Hz
1 kHz
*Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 ¥ V
The default condition (after the internal power-on reset) for the first notch of filter is 60 Hz.
For 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 in-
creased since the output rms noise remains constant as the input full-scale increases).
For 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.
of 2.5 V and resolution numbers are rounded to the nearest 0.5 LSB.
3
2
2
2
2
2
3
3
3
1
Frequency
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
Frequency
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
REF
Gain of
1
1.0
1.8
2.5
4.33
5.28
13
130
0.6 ¥ 10
3.1 ¥ 10
Gain of
1
22.5
21.5
21
20
20
18.5
15
13
10.5
Table II. Effective Resolution vs. Gain and First Notch Frequency
of 2.5 V. These numbers are
Table I. Output Noise vs. Gain and First Notch Frequency
3
3
Gain of
2
0.78
1.1
1.31
2.06
2.36
6.4
75
0.26 ¥ 10
1.6 ¥ 10
Gain of
2
21.5
21
21
20
20
18.5
15
13
10.5
3
3
Typical Output RMS Noise ( V)
Gain of
4
0.48
0.63
0.84
1.2
1.33
3.7
25
140
0.7 ¥ 10
Effective Resolution* (Bits)
Gain of
4
21.5
21
20.5
20
20
18.5
15.5
13
11
–11–
3
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 the 1 kHz
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 re-
main constant with increasing gain or with increasing band-
width. 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 ¥ V
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).
Gain of
8
0.33
0.5
0.57
0.64
0.87
1.8
12
70
0.29 ¥ 10
Gain of
8
21
20
20
20
19.5
18.5
15.5
13
11
3
Gain of
16
0.25
0.44
0.45
0.54
0.63
1.1
7.5
35
180
Gain of
16
20.5
19.5
19.5
19
19
18
15.5
13
11
REF
/GAIN). The above table applies for a V
REF
Gain of
32
0.25
0.41
0.43
0.46
0.62
0.9
4.0
25
120
Gain of
32
19.5
18.5
18.5
18.5
18
17.5
15.5
12.5
10.5
/GAIN, i.e., the input full
Gain of
64
0.25
0.38
0.4
0.46
0.6
0.65
2.7
15
70
Gain of
64
18.5
17.5
17.5
17.5
17
17
15
12.5
10
AD7711A
Gain of
128
0.25
0.38
0.4
0.46
0.56
0.65
1.7
8
40
Gain of
128
17.5
16.5
16.5
16.5
16
16
14.5
12.5
10
REF
2