AD7151
Rev. 0 | Page 24 of 28
PARASITIC SERIAL RESISTANCE
DATA
CDC
EXC
R
S1
CIN
R
S2
C
X
07086-034
Figure 45. Parasitic Serial Resistance
The AD7151 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 on the order of hundreds of Ω.
See
Figure 12.
INPUT OVERVOLTAGE PROTECTION
EXC
GND
CDC
R
S1
CIN
R
S2
C
X
07086-035
Figure 46. AD7151 CIN Overvoltage Protection
The AD7151 capacitive input has an internal ESD protection.
However, some applications may require an additional
overvoltage 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
10kΩ
82kΩ39kΩ
47pF
22pF68pF
GND
07086-036
Figure 47. AD7151 CIN EMC Protection
Some applications may require an additional input filter for
improving electromagnetic compatibility (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
significantly improving the system immunity against high
frequency noise and slightly affecting the AD7151 performance
in terms of additional gain and offset error.
POWER SUPPLY DECOUPLING AND FILTERING
CDC
GND
SDA
SCL
0.1µF 10µF
1kΩ
DD
VDD
1kΩ
1kΩ
07086-058
Figure 48. AD7151 V
DD
Decoupling and Filtering
The AD7151 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 AD7151 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 connect to the AD7151. If the
AD7151 is used in standalone mode and the serial interface is
not used, it is better to connect the pull-up resistors directly to
the AD7151 V
DD
.
