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AD7151

Rev. 0 | Page 23 of 28

HARDWARE DESIGN CONSIDERATIONS

OVERVIEW

The AD7151 is an interface to capacitive sensors.

On the input side, the sensor (CX) can be connected directly

between the AD7151 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 AD7151 specified limits and information provided in this

section.

On the output side, the AD7151 can work as a standalone

device, using the power-up default register settings and flagging

the result on digital outputs. Alternatively, the AD7151 can be

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

offering flexibility by overwriting the AD7151 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 AD7151 measures the

capacitance CX connected between the EXC pin and the CIN

pin. 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 (see Table 1 for information about the

allowed capacitance to GND for CIN and information about

excitation).

See Figure 4, Figure 5, Figure 6, and Figure 7.

PARASITIC RESISTANCE TO GROUND

DATA

CDC

EXC

RGND1

CIN

RGND2

Figure 43. Parasitic Resistance to Ground

The AD7151 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).

See Figure 8, Figure 9, Figure 10, and Figure 11.

PARASITIC PARALLEL RESISTANCE

DATA

CDC

EXC

CIN

Figure 44. Parasitic Parallel Resistance

The AD7151 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 and fEXC is the excitation

frequency.

See Figure 13.

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