AD7731 Analog Devices, AD7731 Datasheet - Page 10

• Manufacturer Part Number: AD7731
• Description: Low Noise, High Throughput 24-Bit Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7731

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,(Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

AD7731
OUTPUT NOISE AND RESOLUTION SPECIFICATION
The AD7731 has a number of different modes of operation of the on-chip filter and chopping features. These options are discussed
in more detail in later sections. The part can be programmed either to optimize the throughput rate and settling time or to optimize
noise and drift performance. Noise tables for two of the primary modes of operation of the part are outlined below for a selection of
output rates and settling times. The first mode, where the AD7731 is configured with CHP = 0 and SKIP mode enabled, provides
fast settling time while still maintaining high resolution. The second mode, where CHP = 1 and the full second filter is included,
provides very low noise numbers with lower output rates. Settling time refers to the time taken to get an output that is 100% settled
to the new value after a channel change or exercising SYNC.
Output Noise (CHP = 0, SKIP = 1)
Table I shows the output rms noise for some typical output update rates and –3 dB frequencies for the AD7731 when used in
nonchop mode (CHP of Filter Register = 0) and with the second filter bypassed (SKIP of Filter Register = 1). The table is generated
with a master clock frequency of 4.9152 MHz. These numbers are typical and generated at a differential analog input voltage of 0 V.
The output update rate is selected via the SF0 to SF11 bits of the Filter Register. Table II, meanwhile, shows the output peak-to-
peak resolution in bits (rounded to the nearest 0.5 LSB) for the same output update rates. It is important to note that the numbers in
Table II represent the resolution for which there will be no code flicker within a six-sigma limit. They are not calculated based on
rms noise but on peak-to-peak noise.
The numbers are generated for the bipolar input ranges. When the part is operated in unipolar mode, the output noise will be the
same as the equivalent bipolar input range. As a result, the numbers in Table I will remain the same for unipolar ranges. To calculate
the numbers for Table II for unipolar input ranges simply subtract one from the peak-to-peak resolution number in bits.
Output
Data Rate
150 Hz
200 Hz
300 Hz
400 Hz
600 Hz
800 Hz
1200 Hz
1600 Hz
2400 Hz
3200 Hz
4800 Hz
6400 Hz
Output
Data Rate
150 Hz
200 Hz
300 Hz
400 Hz
600 Hz
800 Hz
1200 Hz
1600 Hz
2400 Hz
3200 Hz
4800 Hz
6400 Hz
–3 dB
Frequency
39.3 Hz
52.4 Hz
78.6 Hz
104.8 Hz
157 Hz
209.6 Hz
314 Hz
419.2 Hz
629 Hz
838.4 Hz
1260 Hz
1676 Hz
–3 dB
Frequency
39.3 Hz
52.4 Hz
78.6 Hz
104.8 Hz
157 Hz
209.6 Hz
314 Hz
419.2 Hz
629 Hz
838.4 Hz
1260 Hz
1676 Hz
Table II. Peak-to-Peak Resolution vs. Input Range and Update Rate (CHP = 0, SKIP = 1)
Table I. Output Noise vs. Input Range and Update Rate (CHP = 0, SKIP = 1)
SF
Word
2048
1536
1024
768
512
384
256
192
128
96
64
48
SF
Word
2048
1536
1024
768
512
384
256
192
128
96
64
48
20 ms
15 ms
7.5 ms
5 ms
3.75 ms
20 ms
15 ms
10 ms
7.5 ms
5 ms
3.75 ms
Settling
Time
10 ms
2.5 ms
1.87 ms
1.25 ms
0.94 ms
0.625 ms
0.47 ms
Settling
Time
2.5 ms
1.87 ms
1.25 ms
0.94 ms
0.625 ms
0.47 ms
Typical Output RMS Noise in V
Peak-to-Peak Resolution in Bits
2.6
3.0
3.7
4.2
5.2
6
7.8
10.9
27.1
47
99
193
17.5
17
17
16.5
16.5
16
15.5
15
14
13
12
11
1.28 V
1.28 V
1.45
1.66
2
2.3
2.9
3.3
4.3
5.4
13.9
24.4
50.3
97
17
17
16.5
16.5
16
16
15.5
15.5
14
13
12
11
–10–
640 mV
640 mV
0.87
1.02
1.26
1.46
1.78
2.1
2.6
3.5
7.3
11.4
24.5
48
17
16.5
16.5
16
16
15.5
15.5
15
14
13
12
11
320 mV
320 mV
Input Range
Input Range
0.6
0.69
0.84
1.0
1.2
1.4
1.8
2.18
3.5
5.3
12.5
24
16.5
16.5
16
15.5
15.5
15
15
14.5
14
13
12
11
160 mV
160 mV
0.43
0.48
0.58
0.69
0.85
0.98
1.27
1.51
2.22
3.1
6.5
11.8
16
16
15.5
15.5
15
14.5
14.5
14
13.5
13
12
11
80 mV
80 mV
0.28
0.32
0.41
0.46
0.58
0.66
0.82
0.94
1.24
1.9
3.3
6.6
15.5
15.5
15
15
14.5
14.5
14
14
13.5
13
11.5
11
40 mV
40 mV
0.2
0.22
0.28
0.32
0.41
0.47
0.57
0.64
0.83
1.0
1.7
3.0
15
15
14.5
14.5
14
14
13.5
13.5
13
12.5
12
11
REV. A
REV. 0
20 mV
20 mV