AD7745ARUZ-REEL Analog Devices Inc, AD7745ARUZ-REEL Datasheet - Page 22

• Manufacturer Part Number: AD7745ARUZ-REEL
• Description: IC,Converter, Other/Special/Miscellaneous,TSSOP,16PIN
• Manufacturer: Analog Devices Inc
• Type: Capacitance-to-Digital Converterr

Specifications of AD7745ARUZ-REEL

• Design Resources: Extending the Capacitive Input Range of AD7745/AD7746 (CN0129)
• Resolution (bits): 24 b
• Data Interface: Serial
• Voltage Supply Source: Single Supply
• Voltage - Supply: 2.7 V ~ 5.25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Sampling Rate (per Second): -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7745/AD7746
PARASITIC CAPACITANCE TO GROUND
The CDC architecture used in the AD7745/AD7746 measures
the capacitance C
CIN pin. In theory, any capacitance C
affect the CDC result (see Figure 36).
The practical implementation of the circuitry in the chip
implies certain limits and the result is gradually affected by
capacitance to ground. See the allowed capacitance to GND in
the specification table for CIN and excitation. Also see the
typical performance characteristics shown in Figure 9, Figure
10, and Figure 11 .
C
13 ... 21pF
(17 ± 4pF)
C
15 ... 19pF
(17 ± 2pF)
X
X
C
C
GND1
GND2
C
17pF
C
15 ... 19pF
(17 ± 2pF)
Figure 34. Using CAPDAC in Differential Mode
CIN(+)
CIN(–)
Figure 35. Using CAPDAC in Differential Mode
Y
Y
Figure 36. Parasitic Capacitance to Ground
EXC
CIN(+)
CIN(–)
C
EXC
X
X
EXC
CIN
connected between the EXC pin and the
CAPDIFF = 1
CAPDAC(+)
17pF
CAPDAC(–)
17pF
CAPDIFF = 1
CAPDAC(+)
17pF
CAPDAC(–)
17pF
P
± 4pF
CDC
to ground should not
± 4pF
CDC
CDC
0x000000 ... 0xFFFFFF
DATA
0x000000 ... 0xFFFFFF
DATA
DATA
Rev. 0 | Page 22 of 28
PARASITIC RESISTANCE TO GROUND
The AD7745/AD7746 CDC result would be affected by a leak-
age current from the C
isolated from the ground. The influence of the leakage current
varies with the power supply voltage. The following limits can
be used as a guideline for the allowed leakage current or the
equivalent resistance between the C
A higher leakage current to ground results in a gain error, an
offset error, and a nonlinearity error. See the typical
performance characteristics shown in Figure 12 and Figure 13.
PARASITIC PARALLEL RESISTANCE
The AD7745/AD7746 CDC measures the charge transfer
between EXC pin and CIN pin. Any resistance connected in
parallel to the measured capacitance CX (see Figure 38), such as
the parasitic resistance of the sensor, also transfers charge.
Therefore, the parallel resistor is seen as an additional
capacitance in the output data. The equivalent parallel
capacitance (or error caused by the parallel resistance) can be
approximately calculated as
Where R
frequency. See the typical performance characteristics shown in
Figure 14.
R
V
V
V
R
C
GND1
GND2
DD
DD
DD
P
C
X
=
P
≈ 5 V: I
≥ 3 V: I
≥ 2.7 V: I
is the parallel resistance and C
R
P
R
P
×
EXC
Figure 37. Parasitic Resistance to Ground
C
CIN
F
Figure 38. Parasitic Parallel Resistance
GND
GND
X
1
EXC
EXC
GND
CIN
< 150 nA (that is, R
< 60 nA (that is, R
×
< 30 nA (that is, R
4
X
to ground, therefore the C
X
and ground (Figure 37).
CDC
GND
GND
GND
EXC
CDC
> 50 MΩ)
> 30 MΩ)
> 100 MΩ)
is the excitation
X
DATA
should be
DATA