AD7747
Rev. 0 | Page 11 of 28
OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
The AD7747 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 16 kHz, ±V
DD
× 3/8, and with all CIN and SHLD
pins connected only to the evaluation board (no external
capacitors).
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.
Table 5. Typical Capacitive Input Noise and Resolution vs. Conversion Time (Bold line represents default setting)
Conversion
Time (ms)
Output Data
Rate (Hz)
−3 dB Frequency
(Hz)
RMS Noise
(aF/√Hz)
RMS
Noise (aF)
P-P
Noise (aF)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
22.0 45.5 43.6 28.8 190 821 16.4 14.3
23.9 41.9 39.5 23.2 146 725 16.8 14.5
40.0 25.0 21.8 11.1 52 411 18.3 15.3
76.0 13.2 10.9 11.2 37 262 18.7 15.9
124.0 8.1 6.9 11.0 29 174 19.1 16.5
154.0 6.5 5.3 10.4 24 173 19.3 16.5
184.0 5.4 4.4 10.0 21 141 19.6 16.8
219.3 4.6 4.0 9.0 18 126 19.9 17.0
Table 6. Typical Voltage Input Noise and Resolution vs. Conversion Time, Internal Voltage Reference
Conversion
Time (ms)
Output Data
Rate (Hz)
−3 dB Frequency
(Hz)
RMS Noise
(μV)
P-P Noise
(μV)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
20.1 49.8 26.4 11.4 62 17.6 15.2
32.1 31.2 15.9 7.1 42 18.3 15.7
62.1 16.1 8.0 4.0 28 19.1 16.3
122.1 8.2 4.0 3.0 20 19.5 16.8
Table 7. Typical Voltage Input Noise and Resolution vs. Conversion Time, External 2.5 V Voltage Reference
Conversion
Time (ms)
Output Data
Rate (Hz)
−3 dB Frequency
(Hz)
RMS Noise
(μV)
P-P Noise
(μV)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
20.1 49.8 26.4 14.9 95 18.3 15.6
32.1 31.2 15.9 6.3 42 19.6 16.8
62.1 16.1 8.0 3.3 22 20.5 17.7
122.1 8.2 4.0 2.1 15 21.1 18.3