AD7150
Rev. 0 | Page 24 of 28
PARASITIC SERIAL RESISTANCE
DATA
CDC
EXC
R
S1
CIN
R
S2
C
X
06517-055
Figure 47. Parasitic Serial Resistance
The AD7150 CDC result is affected by a resistance in series
with the measured capacitance. The total serial resistance (R
S1
+
R
S2
in Figure 47) should be on the order of hundreds of Ω.
See
Figure 14.
INPUT OVERVOLTAGE PROTECTION
EXC
GND
CDC
R
S1
CIN
R
S2
C
X
06517-056
Figure 48. AD7150 CIN Overvoltage Protection
The AD7150 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
06517-057
Figure 49. AD7150 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 49 shows one of the possible input circuit configurations
significantly improving the system immunity against high
frequency noise and slightly affecting the AD7150 performance
in terms of additional gain and offset error.
POWER SUPPLY DECOUPLING AND FILTERING
CDC
GND
SDA
SCL
0.1µF 10µF
1kΩ
DD
1kΩ 1kΩ
6517-058
Figure 50. AD7150 V
DD
Decoupling and Filtering
The AD7150 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 50 shows a possible circuit configuration for
improving the system immunity against ripple and noise
coupled to the AD7150 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 AD7150. If the
AD7150 is used in standalone mode and the serial interface is
not used, it is better to connect the pull-up resistors directly to
the AD7150 V
DD
.
