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AD7156

Rev. 0 | Page 24 of 28

HARDWARE DESIGN CONSIDERATIONS

OVERVIEW

The AD7156 is an interface to capacitive sensors.

On the input side, Sensor CX can be connected directly between

the AD7156 EXC and CIN pins. The way it is connected and the

electrical parameters of the sensor connection, such as parasitic

resistance or capacitance, can affect the system performance.

Therefore, any circuit with additional components in the capacitive

front end, such as overvoltage protection, has to be carefully

designed, considering the AD7156 specified limits and informa-

tion provided in this section.

On the output side, the AD7156 can work as a standalone device,

using the power-up default register settings and flagging the

result on the digital outputs. Alternatively, the AD7156 can be

interfaced to a microcontroller via the 2-wire serial interface,

offering flexibility by overwriting the AD7156 register values

from the host with a user-specific setup.

PARASITIC CAPACITANCE TO GROUND

DATA

CDC

EXC

CGND1

CIN

CGND2

Figure 42. Parasitic Capacitance to Ground

The CDC architecture used in the AD7156 measures the

capacitance, CX, connected between the EXC pins and the

CIN pins. In theory, any capacitance, CGND, to ground should

not affect the CDC result (see Figure 42).

The practical implementation of the circuitry in the chip implies

certain limits, and the result is gradually affected by capacitance

to ground (for information about the allowed capacitance to

GND for CIN and information about excitation see Table 1

and Figure 4 to Figure 9).

PARASITIC RESISTANCE TO GROUND

DATA

CDC

EXC

RGND1

CIN

RGND2

Figure 43. Parasitic Resistance to Ground

The AD7156 CDC result is affected by a leakage current from

CX to ground; therefore, CX should be isolated from the ground.

The equivalent resistance between CX and ground should be

maximized (see Figure 43). For more information, see Figure 10

to Figure 13.

PARASITIC PARALLEL RESISTANCE

DATA

CDC

EXC

CIN

Figure 44. Parasitic Parallel Resistance

The AD7156 CDC measures the charge transfer between the

EXC and CIN pins. Any resistance connected in parallel to the

measured capacitance, CX (see Figure 44), such as the parasitic

resistance of the sensor, also transfers charge. Therefore, the

parallel resistor is seen as an additional capacitance in the

output data. The equivalent parallel capacitance (or error

caused by the parallel resistance) can be approximately

calculated as

EXC

where:

RP is the parallel resistance.

fEXC is the excitation frequency.

For additional information, see Figure 15.

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