Datasheet

AD7745/AD7746
Rev. 0| Page 3 of 28
SPECIFICATIONS
V
DD
= 2.7 V to 3.6 V or 4.75 V to 5.25 V; GND = 0 V; EXC = 32 kHz; EXC = ±V
DD
/2; –40°C to +125°C, unless otherwise noted.
Table 1.
Parameter Min Typ Max Unit Test Conditions/Comments
CAPACITIVE INPUT
Conversion Input Range ±4.096 pF
1
Factory calibrated
Integral Nonlinearity (INL)
2
±0.01 % of FSR
No Missing Codes
2
24 Bit Conversion time ≥ 62 ms
Resolution, p-p 16.5 Bit Conversion time = 62 ms, see Table 5
Resolution Effective 19 Bit Conversion time = 62 ms, see Table 5
Output Noise, rms 2
aF/√Hz
See Table 5
Absolute Error
3
±4 fF
1
25°C, V
DD
= 5 V, after offset calibration
Offset Error
2, 4
32 aF
1
After system offset calibration,
Excluding effect of noise
4
System Offset Calibration Range
2
±1 pF
Offset Drift vs. Temperature –1 aF/°C
Gain Error
5
0.02 0.08 % of FS 25°C, V
DD
= 5 V
Gain Drift vs. Temperature
2
–28 –26 –24 ppm of FS/°C
Allowed Capacitance to GND
2
60 pF See Figure 9 and Figure 10
Power Supply Rejection 0.3 1 fF/V
Normal Mode Rejection 65 dB 50 Hz ± 1%, conversion time = 62 ms
55 dB 60 Hz ± 1%, conversion time = 62 ms
Channel-to-Channel Isolation 70 dB AD7746 only
CAPDAC
Full Range 17 21 pF
Resolution
6
164 fF 7-bit CAPDAC
Drift vs. Temperature
2
24 26 28 ppm of FS/°C
EXCITATION
Frequency 32 kHz
Voltage Across Capacitance ±V
DD
/8 V Configurable via digital interface
±V
DD
/4 V
±V
DD
× 3/8
V
±V
DD
/2 V
Average DC Voltage Across
Capacitance
<±40 mV
Allowed Capacitance to GND
2
100 pF See Figure 11
TEMPERATURE SENSOR
7
V
REF
internal
Resolution 0.1 °C
Error
2
±0.5 ±2 °C Internal temperature sensor
±2 °C External sensing diode
8
VOLTAGE INPUT
7
V
REF
internal or V
REF
= 2.5 V
Differential VIN Voltage Range ±V
REF
V
Absolute VIN Voltage
2
GND − 0.03 V
DD
+ 0.03 V
Integral Nonlinearity (INL) ±3 ±15 ppm of FS
No Missing Codes
2
24 Bit Conversion time = 122.1 ms
Resolution, p-p 16 Bits
Conversion time = 62 ms
See Table 6 and Table 7
Output Noise 3 µV rms
Conversion time = 62 ms
See Table 6 and Table 7
Offset Error ±3 µV
Offset Drift vs. Temperature 15 nV/°C
Full-Scale Error
2, 9
0.025 0.1 % of FS