AD7747_17 datasheet(22/29 Pages) AD

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AD7747

Rev. 0 | Page 21 of 28

CIRCUIT DESCRIPTION

VIN(+)

VDD

CIN1(+)

VIN(–)

SHLD

GND

SDA

SCL

RDY

CIN1(–)

REFIN(+)

REFIN(–)

TEMP

SENSOR

24-BIT Σ-Δ

GENERATOR

DIGITAL

FILTER

I2C

SERIAL

INTERFACE

EXCITATION

CONTROL LOGIC

CALIBRATION

VOLTAGE

REFERENCE

CAP DAC 1

CAP DAC 2

CLOCK

GENERATOR

MUX

AD7747

Figure 24. AD7747 Block Diagram

OVERVIEW

The AD7747 core is a high precision converter consisting of a

second-order (Σ-Δ or charge balancing) modulator and a third-

order digital filter. It works as a CDC for the capacitive inputs

and as a classic ADC for the voltage input or for the voltage

from a temperature sensor.

In addition to the converter, the AD7747 integrates a multi-

plexer, an excitation source and CAPDACs for the capacitive

inputs, a temperature sensor and a voltage reference for the

voltage and temperature inputs, a complete clock generator,

a control and calibration logic, and an I2C-compatible serial

interface.

CAPACITANCE-TO-DIGITAL CONVERTER

Figure 25 shows the CDC simplified functional diagram. The

measured capacitance CX is connected between the Σ-Δ modu-

lator input and ground. A square-wave excitation signal is

applied on the CX during the conversion and the modulator

continuously samples the charge going through the CX. The

digital filter processes the modulator output, which is a stream

of 0s and 1s containing the information in 0 and 1 density. The

data from the digital filter is scaled, applying the calibration

coefficients, and the final result can be read through the serial

interface.

DIGITAL

FILTER

24-BIT Σ-Δ

MODULATOR

CLOCK

GENERATOR

CAPACITANCE TO DIGITAL CONVERTER

(CDC)

EXCITATION

DATA

SHLD

CIN

Figure 25. CDC Simplified Block Diagram

ACTIVE AC SHIELD CONCEPT

The AD7747 measures capacitance between CIN and ground.

That means any capacitance to ground on signal path between

the AD7747 CIN pin(s) and sensor is included in the AD7747

conversion result.

The parasitic capacitance of the sensor connections can easily

be in the same, if not even higher, order as the capacitance of

the sensor itself. If that parasitic capacitance is stable, it can be

treated as a nonchanging capacitive offset. However, the para-

sitic capacitance of sensor connections is often changing as a

result of mechanical movement, changing ambient temperature,

ambient humidity, etc. These changes are seen as drift in the

conversion result and may significantly compromise the system

accuracy.

To eliminate the CIN parasitic capacitance to ground, the

AD7747 SHLD signal can be used for shielding the connection

between the sensor and CIN, as shown in Figure 25. The SHLD

output is basically the same signal waveform as the excitation of

the CIN pin; the SHLD is driven to the same voltage potential

as the CIN pin. Therefore, there is no ac current between CIN

and SHLD pins, and any capacitance between these pins does

not affect the CIN charge transfer. Ideally, the CIN to SHLD

capacitance does not have any contribution to the AD7747 result.

To get the best result, locate the AD7747 as close as possible to

the capacitive sensor. Keep the connection between the sensor

and AD7747 CIN pin, and also the return path between sensor

ground and the AD7747 GND pin, short. Shield the PCB track

to the CIN pin and connect the shielding to the AD7747 SHLD

pin. In addition, if a shielded cable is used for sensor connection,

the shield should be connected to the AD7747 SHLD pin.

CAPDAC

The AD7747 CDC full-scale input range is ±8.192 pF. For sim-

plicity of calculation, however, the following text and figures use

±8 pF. The part can accept a higher capacitance on the input

and the common-mode or offset (nonchanging component)

capacitance can be balanced by programmable on-chip CAPDACs.

DATA

CDC

SHLD

CIN(+)

CIN(–)

CAPDAC(+)

CAPDAC(–)

Figure 26. Using a CAPDAC

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