CN0287 Datasheet, PDF (4/9 Pages) Analog Devices – Devices Connected

CN-0287
Circuit Note
Temperature measurement is a high precision and low speed
application, therefore there is adequate settling time available to
switch the single current source between all 4 channels,
providing excellent channel-to-channel matching, low cost, and
small PCB footprint.
The ADG738 is an 8-to-1 multiplexer that switches the current
source between channels. In order to support the 2-, 3-, and 4-
wire RTD configurations, each of the four channels need two
switches.
In many applications, the RTD may be located remotely from
the measurement circuit. The resistance from the long lead
wires can generate large errors, especially for low resistance
RTDs. In order to minimize the effect of the lead resistance, a 3-
wire RTD configuration is supported as shown in Figure 3.
R_W1
R_W2
RTD
R_RTD
R_W3
+5V
ADG738
S1 D
CURRENT SOURCE
S2
I_EXC
AD7193
JP[x]
+5V
I_EXC
R_REF
AIN1
AIN2
REFIN2(+)
REFIN2(-)
Figure 3. Connector and Jumper Configuration for3-Wire RTD Sensor
With S1 of the ADG738 closed and S2 opened, the voltage at
the input of AD7193 is V1. With S1 opened and S2 closed, the
voltage on the input of AD7193 is V2, The voltage across the
RTD sensor is VRTD, the exciting current from the current source is
IEXC. V1 and V2 contain the error generated by the lead resistance
as shown below:
V₁ = ( R _RTD + R_W3 ) × I _EXC
(1)
V ₂ = ( R_W2 + R _RTD + R_W3 ) × I _EXC
(2)
V _RTD = R _RTD × I _EXC
(3)
Assuming RW1 = RW2 = RW3 and combining Equations 1, 2, and 3
yields:
VRTD = 2V1 – V2
(4)
RRTD = VRTD/IEXC = (2V1 – V2)/IEXC
(5)
Equation 5 shows that the 3-wire configuration requires two
separate measurements (V1 and V2) in order to calculate RRTD,
thereby decreasing the output data rate. In most applications
this is not a problem.
The 4-wire RTD connection requires two extra sense lines, but
is insensitive to wiring resistances and only requires one
measurement.
Figure 4 summarizes the connector configuration and jumper
placements for RTD 2-wire, RTD 3-wire, RTD 4-wire, and
thermocouple applications.
RTD 2-WIRE
RTD 3-WIRE
RTD 4-WIRE
THERMOCOUPLE
RTD
CNx
1 ^JPx
2
1
3
2
4
3
RTD
CNx
1 ^JPx
2
1
3
2
4
3
RTD
CNx
1 ^JPx
2
1
3
2
4
3
+
TC
–
CNx
1 ^JPx
2
1
3
2
4
3
Figure 4. Connector Configuration and Jumper Placements for EVAL-CN0287-SDPZ Board
Rev. C | Page 4 of 9