AD7156 datasheet(25/28 Pages) AD

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AD7156

Rev. 0 | Page 25 of 28

PARASITIC SERIAL RESISTANCE

DATA

CDC

EXC

RS1

CIN

RS2

Figure 45. Parasitic Serial Resistance

The AD7156 CDC result is affected by a resistance in series

with the measured capacitance.

The total serial resistance (RS1 + RS2 in Figure 45) should be in

the order of hundreds of Ω (see Figure 14).

INPUT OVERVOLTAGE PROTECTION

EXC

GND

CDC

RS1

CIN

RS2

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

10kΩ

82kΩ

39kΩ

47pF

22pF

68pF

GND

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

1kΩ

VDD

1kΩ

Figure 48. AD7156 VDD 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 VDD 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 VDD.

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