AD7151 datasheet(24/28 Pages) AD

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AD7151

Rev. 0 | Page 24 of 28

PARASITIC SERIAL RESISTANCE

DATA

CDC

EXC

RS1

CIN

RS2

Figure 45. Parasitic Serial Resistance

The AD7151 CDC result is affected by a resistance in series

with the measured capacitance. The total serial resistance (RS1 +

RS2 in Figure 45) should be on the order of hundreds of Ω.

See Figure 12.

INPUT OVERVOLTAGE PROTECTION

EXC

GND

CDC

RS1

CIN

RS2

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

10kΩ

82kΩ

39kΩ

47pF

22pF

68pF

GND

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

VDD

1kΩ

Figure 48. AD7151 VDD 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 VDD 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 VDD.

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