AD7785 datasheet(29/32 Pages) AD

29 / 32 page

AD7785

Rev. 0 | Page 29 of 32

APPLICATIONS INFORMATION

The AD7785 provides a low cost, high resolution analog-to-

digital function. Because the analog-to-digital function is

provided by a ∑-Δ architecture, the part is more immune to

noisy environments, making it ideal for use in sensor

measurement and industrial and process control applications.

TEMPERATURE MEASUREMENT USING A

THERMOCOUPLE

Figure 20 outlines a connection from a thermocouple to the

AD7785. In a thermocouple application, the voltage generated

by the thermocouple is measured with respect to an absolute

reference, so the internal reference is used for this conversion.

The cold junction measurement uses a ratiometric configuration,

so the reference is provided externally.

Because the signal from the thermocouple is small, the AD7785

is operated with the in-amp enabled to amplify the signal from

the thermocouple. As the input channel is buffered, large

decoupling capacitors can be placed on the front end to

eliminate any noise pickup that may be present in the

thermocouple leads. The AD7785 has a reduced common-

mode range with the in-amp enabled, so the bias voltage

generator provides a common-mode voltage so that the voltage

generated by the thermocouple is biased up to AVDD/2.

The cold junction compensation is performed using a thermis-

tor. The on-chip excitation current supplies the thermistor. In

addition, the reference voltage for the cold junction

measurement is derived from a precision resistor in series with

the thermistor. This allows a ratiometric measure-ment so that

variation of the excitation current has no effect on the

measurement (it is the ratio of the precision reference resistance

to the thermistor resistance that is measured).

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

Σ-Δ

ADC

AD7785

AIN2(+)

REFIN(+)

REFIN(–)

AIN2(–)

AVDD

GND

MUX

BAND GAP

REFERENCE

INTERNAL

CLOCK

CLK

GND

AVDD

IN-AMP

BUF

REFIN(+) REFIN(–)

VBIAS

AIN1(+)

AIN1(–)

THERMOCOUPLE

JUNCTION

RREF

IOUT2

Figure 20. Thermocouple Measurement Using the AD7785

AD7785 Datasheet(PDF) 29 Page - Analog Devices

AD7785 Datasheet(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL