100µA

REF200

+5V

100µA

+5V

R3 78.7

INA326

R1

R1

148

Ÿ@

150C

14.8mV

+

4.84V

+2mV-

+

-

PCB

3 Wire

RTD

Large Lead R

300m

10

Ÿ

-1mV+

-

14.8mV

+

-

10

Ÿ

- 1mV+

10

Ÿ

100µA

REF200

+5V

100µA

3 Wire

RTD

+5V

R3 78.7

INA326

R1

R1

148

Ÿ@

150C

14.8mV

+

+

6.93mV

4.84V

G = 2(R2/R1)

- 7.87mV +

200µA

+

-

-

-

REF200

www.ti.com

SBVS020B – SEPTEMBER 2000 – REVISED JULY 2015

Figure 13. RTD Amplifier with Maximum Output Condition

8.2.2.1 Lead Resistance Cancelation (3-Wire RTD)

Figure 14 shows the 3-wire RTD configuration can be used to cancel lead resistance. The resistance in each

lead must be equal to cancel the error. Also, the two current sources in the REF200 must be equal. Notice that

the voltage developed on the two top leads of the RTD are equal and opposite polarity so that the amplifiers

input is only from the RTD voltage. In this example, the RTD drop is 14.8 mV and the leads each have 1 mV.

Notice that the 1 mV drops cancel. Finally, notice that the voltage on the 3rd lead (2 mV) creates a small shift in

the common mode voltage. In some applications, a larger resistor is intentionally added to shift the common-

mode voltage. However, the INA326 has a rail-to-rail common mode range, so it can accept common-mode

voltages near ground.

Figure 14. 3-Wire RTD Configuration Cancels Lead Resistance

Copyright © 2000–2015, Texas Instruments Incorporated

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