Datasheet

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.
06721-021
DOUT/RDY
DIN
SCLK
CS
DV
DD
SERIAL
INTERFACE
AND
CONTROL
LOGIC
AD7785
IOUT1
REFIN(+)
REFIN(–)
AV
DD
GND
BAND GAP
REFERENCE
INTERNAL
CLOCK
CLK
GND
GND
A
V
DD
IN-AMPBUF
REFIN(+) REFIN(–)
AIN1(+)
AIN1(–)
R
REF
IOUT2
RL2
RL1
RTD
RL3
Σ-Δ
ADC
Figure 21. RTD Application Using the AD7785