AD7785 datasheet(31/33 Pages) AD

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AD7785

Rev. 0 | Page 30 of 32

TEMPERATURE MEASUREMENT USING AN RTD

To optimize a 3-wire RTD configuration, two identically

matched current sources are required. The AD7785, which

contains two well-matched current sources, is ideally suited to

these applications. One possible 3-wire configuration is shown

in Figure 21. In this 3-wire configuration, the lead resistances

result in errors if only one current is used, as the excitation

current flows through RL1, developing a voltage error between

AIN1(+) and AIN1(–). In the scheme outlined, the second RTD

current source is used to compensate for the error introduced

by the excitation current flowing through RL1. The second RTD

current flows through RL2. Assuming RL1 and RL2 are equal

(the leads would normally be of the same material and of equal

length), and IOUT1 and IOUT2 match, the error voltage across

RL2 equals the error voltage across RL1, and no error voltage

is developed between AIN1(+) and AIN1(–). The voltage is

developed twice across RL3. However, because this is a

common- mode voltage, it does not introduce errors. The

reference voltage for the AD7785 is also generated using one of

these matched current sources. It is developed using a precision

resistor and applied to the differential reference pins of the

ADC. This scheme ensures that the analog input voltage span

remains ratiometric to the reference voltage. Any errors in the

analog input voltage due to the temperature drift of the excita-

tion current are compensated by the variation of the reference

voltage.

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

AD7785

IOUT1

REFIN(+)

REFIN(–)

AVDD

GND

BAND GAP

REFERENCE

INTERNAL

CLOCK

CLK

GND

AVDD

IN-AMP

BUF

REFIN(+) REFIN(–)

AIN1(+)

AIN1(–)

RREF

IOUT2

RL2

RL1

RTD

RL3

Σ-Δ

ADC

Figure 21. RTD Application Using the AD7785

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