AD7746 Analog Devices, AD7746 Datasheet - Page 11

• Manufacturer Part Number: AD7746
• Description: 24-bit, 2 Channel Capacitance to Digital Converter
• Manufacturer: Analog Devices

Specifications of AD7746

• Resolution (bits): 24bit
• # Chan: 2
• Sample Rate: n/a
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,± 4 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
The AD7745/AD7746 resolution is limited by noise. The noise
performance varies with the selected conversion time.
Table 5 shows typical noise performance and resolution for the
capacitive channel. These numbers were generated from 1000
data samples acquired in continuous conversion mode, at an
excitation of 32 kHz, ±V
connected only to the evaluation board (no external capacitors.)
Table 5. Typical Capacitive Input Noise and Resolution vs. Conversion Time
Conversion
Time (ms)
11.0
11.9
20.0
38.0
62.0
77.0
92.0
109.6
Table 6. Typical Voltage Input Noise and Resolution vs. Conversion Time, Internal Voltage Reference
Conversion
Time (ms)
20.1
32.1
62.1
122.1
Table 7. Typical Voltage Input Noise and Resolution vs. Conversion Time, External 2.5 V Voltage Reference
Conversion
Time (ms)
20.1
32.1
62.1
122.1
Output Data
Rate (Hz)
90.9
83.8
50.0
26.3
16.1
13.0
10.9
9.1
Output Data
Rate (Hz)
49.8
31.2
16.1
8.2
Output Data
Rate (Hz)
49.8
31.2
16.1
8.2
DD
/2, and with all CIN and EXC pins
–3dB Frequency
(Hz)
87.2
79.0
43.6
21.8
13.8
10.5
8.9
8.0
–3dB Frequency
(Hz)
26.4
15.9
8.0
4.0
–3dB Frequency
(Hz)
26.4
15.9
8.0
4.0
RMS Noise
(aF/√Hz)
4.3
3.1
1.8
1.6
1.5
1.5
1.5
1.5
RMS Noise
(µV)
11.4
7.1
4.0
3.0
RMS Noise
(µV)
14.9
6.3
3.3
2.1
Rev. 0| Page 11 of 28
RMS
Noise (aF)
40.0
27.3
12.2
7.3
5.4
4.9
4.4
4.2
Table 6 and Table 7 show typical noise performance and
resolution for the voltage channel. These numbers were
generated from 1000 data samples acquired in continuous
conversion mode with VIN pins shorted to ground.
RMS noise represents the standard deviation and p-p noise
represents the difference between minimum and maximum
results in the data. Effective resolution is calculated from rms
noise, and p-p resolution is calculated from p-p noise.
P-P Noise
(µV)
62
42
28
20
P-P Noise
(µV)
95
42
22
15
P-P
Noise (aF)
212.4
137.7
82.5
50.3
33.7
28.3
27.8
27.3
Effective Resolution
(Bits)
17.6
18.2
19.4
20.1
20.5
20.7
20.8
20.9
Effective Resolution
(Bits)
17.6
18.3
19.1
19.5
Effective Resolution
(Bits)
18.3
19.6
20.5
21.1
AD7745/AD7746
P-P Resolution
(Bits)
15.2
15.9
16.6
17.3
17.9
18.1
18.2
18.2
P-P Resolution
(Bits)
15.2
15.7
16.3
16.8
P-P Resolution
(Bits)
15.6
16.8
17.7
18.3