CN-0287

Circuit Note

Rev. C | Page 4 of 9

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.

Figure 3. Connector and Jumper Configuration for3-Wire RTD Sensor

With S1 of the ADG738 closed and S2 opened, the voltage at

as shown below:

EXC

W3

RTD

1

I

R

R

V

×

+

=

)

(

(1)

EXC

W3

RTD

W2

2

I

R

R

R

V

×

+

+

=

)

(

(2)

EXC

RTD

RTD

I

R

V

×

=

(3)

yields:

(4)

(5)

Equation 5 shows that the 3-wire configuration requires two

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.

Figure 4. Connector Configuration and Jumper Placements for EVAL-CN0287-SDPZ Board

AD7193

REFIN2(-)

REFIN2(+)

AIN2

AIN1

S1

D

ADG738

RTD

S2

JP[x]

+5V

CURRENT SOURCE

+5V

RTD 2-WIRE

RTD 3-WIRE

RTD 4-WIRE

THERMOCOUPLE

JPx

2

1

3

2

1

3

4

CNx

RTD

RTD

JPx

2

1

3

2

1

3

4

CNx

JPx

2

1

3

2

1

3

4

CNx

JPx

2

1

3

2

1

3

4

CNx

TC

+

–

RTD