AD7746 datasheet(20/28 Pages) AD

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AD7745/AD7746

Rev. 0 | Page 20 of 28

CIRCUIT DESCRIPTION

DIGITAL

FILTER

24-BIT

Σ-∆

MODULATOR

CLOCK

GENERATOR

MUX

TEMP

SENSOR

CAP DAC

VOLTAGE

REFERENCE

CONTROL LOGIC

CALIBRATION

AD7745

VDD

REFIN(+) REFIN(–)

GND

SDA

SCL

RDY

VIN(+)

VIN(–)

CIN1(+)

CIN1(–)

EXCA

EXCB

I2C

SERIAL

INTERFACE

EXCITATION

Figure 26. AD7745 Block Diagram

DIGITAL

FILTER

24-BIT

Σ-∆

MODULATOR

MUX

TEMP

SENSOR

CAP DAC

VOLTAGE

REFERENCE

CONTROL LOGIC

CALIBRATION

AD7746

VDD

REFIN(+) REFIN(–)

GND

SDA

SCL

RDY

VIN(+)

VIN(–)

CIN1(+)

CIN1(–)

CIN2(+)

CIN2(–)

EXC1

EXC2

CLOCK

GENERATOR

I2C

SERIAL

INTERFACE

EXCITATION

Figure 27. AD7746 Block Diagram

OVERVIEW

The AD7745/AD7746 core is a high precision converter con-

sisting of a second order (Σ-Δ or charge balancing) modulator

and a third order digital filter. It works as a CDC for the capa-

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

the voltage from a temperature sensor.

In addition to the converter, the AD7745/AD7746 integrates a

multiplexer, an excitation source and CAPDACs for the capa-

citive inputs, a temperature sensor, a voltage reference for the

voltage and temperature inputs, a complete clock generator, a

control and calibration logic, and an I2C-compatible serial

interface.

The AD7745 has one capacitive input, while the AD7746 has

two capacitive inputs. All other features and specifications are

identical for both parts.

CAPACITANCE-TO-DIGITAL CONVERTER

Figure 28 shows the CDC simplified functional diagram. The

measured capacitance CX is connected between the excitation

source and the Σ-Δ modulator input. 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.

The AD7745/AD7746 is designed for floating capacitive

sensors. Therefore, both CX plates have to be isolated from

ground.

DIGITAL

FILTER

24-BIT

Σ-∆

MODULATOR

CLOCK

GENERATOR

CAPACITANCE TO DIGITAL CONVERTER

(CDC)

EXCITATION

DATA

EXC

CIN

Figure 28. CDC Simplified Block Diagram

EXCITATION SOURCE

The two excitation pins EXCA and EXCB are independently

programmable. They are identically functional and therefore

either of them can be used for the capacitive sensor excitation.

On the 2-channel AD7746 using a separate excitation pin for

each capacitive channel is recommended.

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