AD7746 datasheet(24/28 Pages) AD

24 / 28 page

AD7745/AD7746

Rev. 0 | Page 24 of 28

If the capacitive sensor can be considered to be at the same

temperature as the AD7745/AD7746 chip, the internal

temperature sensor can be used as a system temperature sensor.

That means the complete system temperature drift

compensation can be based on the AD7745/AD7746 internal

temperature sensor without need for any additional external

components. See the typical performance characteristics in

Figure 18.

EXTERNAL TEMPERATURE SENSOR

DIGITAL

FILTER

AND

SCALING

24-BIT

Σ-∆

MODULATOR

CLOCK

GENERATOR

EXTERNAL

TEMPERATURE

SENSOR

VOLTAGE

REFERENCE

DATA

I ... N

× I

∆V

VDD

VIN (–)

VIN (+)

RS2

RS1

2N3906

Figure 41. Transistor as an External Temperature Sensor

The AD7745/AD7746 provide the option of using an external

transistor as a temperature sensor in the system. The ∆VBE

method, which is similar to the internal temperature sensor

method, is used. However, it is modified to compensate for the

serial resistance of connections to the sensor. Total serial

resistance (RS1 + RS2 in Figure 41) up to 100 Ω is compensated.

The VIN(–) pin must be grounded for proper external

temperature sensor operation.

The AD7745/AD7746 are factory calibrated for

Transistor 2N3906 with the ideality factor nf = 1.008.

See the typical performance characteristics shown

in Figure 19.

VOLTAGE INPUT

DIGITAL

FILTER

24-BIT

Σ-∆

MODULATOR

CLOCK

GENERATOR

ANALOG TO DIGITAL CONVERTER

(ADC)

VOLTAGE

REFERENCE

DATA

VIN(–)

VIN(+)

REFIN(–)

REFIN(+)

RREF

RTD

VDD

GND

Figure 42. Resistive Temperature Sensor Connected to the Voltage Input

The AD7745/AD7746 Σ-Δ core can work as a high resolution

(up to 21 ENOB) classic ADC with a fully differential voltage

input. The ADC can be used either with the on-chip high

precision, low drift, 1.17 V voltage reference, or an external

reference connected to the fully differential reference input pins.

The voltage and reference inputs are continuously sampled by a

Σ-Δ modulator during the conversion. Therefore, the input

source impedance should be kept low. See the application

example in Figure 42.

VDD MONITOR

Along with converting external voltages, the AD7745/AD7746

Σ-Δ ADC can be used for monitoring the VDD voltage. The

voltage from the VDD pin is internally attenuated by 6.

TYPICAL APPLICATION DIAGRAM

DIGITAL

FILTER

24-BIT

Σ-∆

MODULATOR

CLOCK

GENERATOR

MUX

TEMP

SENSOR

CAP DAC

EXCITATION

VOLTAGE

REFERENCE

CONTROL LOGIC

CALIBRATION

AD7745

VDD

REFIN(+) REFIN(–)

GND

SDA

SCL

RDY

VIN(+)

VIN(–)

CIN1(+)

CIN1(–)

EXCA

EXCB

HOST

SYSTEM

0.1

µF

3V/5V

POWER SUPPLY

I2C

SERIAL

INTERFACE

10k

Ω

10k

Ω

Figure 43. Basic Application Diagram for a Differential Capacitive Sensor

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