Datasheet

AD7156
Rev. 0 | Page 25 of 28
PARASITIC SERIAL RESISTANCE
DATA
CDC
EXC
R
S1
CIN
R
S2
C
X
07726-055
Figure 45. Parasitic Serial Resistance
The AD7156 CDC result is affected by a resistance in series
with the measured capacitance.
The total serial resistance (R
S1
+ R
S2
in Figure 45) should be in
the order of hundreds of Ω (see Figure 14).
INPUT OVERVOLTAGE PROTECTION
EXC
GND
CDC
R
S1
CIN
R
S2
C
X
07726-056
Figure 46. AD7156 CIN Overvoltage Protection
The AD7156 capacitive input has an internal ESD protection.
However, some applications may require an additional over-
voltage protection, depending on the application-specific
requirements. Any additional circuit in the capacitive front
end must be carefully designed, especially with respect to the
limits recommended for maximum capacitance to ground,
maximum serial resistance, maximum leakage, and so on.
INPUT EMC PROTECTION
EXC
CDC
CIN
C
X
10k
82k39k
47pF
22pF68pF
GND
07726-057
Figure 47. AD7156 CIN EMC Protection
Some applications may require an additional input filter for
improving EMC. Any input filter must be carefully designed,
considering the balance between the system capacitance
performance and system electromagnetic immunity.
Figure 47 shows one of the possible input circuit configurations
for significantly improving the system immunity against high
frequency noise while only slightly affecting the AD7156
performance in terms of additional gain and offset error.
POWER SUPPLY DECOUPLING AND FILTERING
CDC
GND
SDA
SCL
0.1µF 10µF
1k
V
DD
1k 1k
0
7726-058
Figure 48. AD7156 V
DD
Decoupling and Filtering
The AD7156 has good dc and low frequency power supply
rejection but may be sensitive to higher frequency ripple and
noise, specifically around the excitation frequency and its
harmonics. Figure 48 shows a possible circuit configuration
for improving the system immunity against ripple and noise
coupled to the AD7156 via the power supply.
If the serial interface is connected to the other circuits in the
system, it is better to connect the pull-up resistors on the other
side of the V
DD
filter than to connect to the AD7156. If the
AD7156 is used in standalone mode and the serial interface
is not used, it is better to connect the pull-up resistors directly
to the AD7156 V
DD
.