100µA

REF200

+5V

100µA

3 Wire

RTD

+5V

R3 78.7

INA326

R1

R1

80.3

Ÿ@

-50C

8.03mV

+

+

160µ V

0.111V

G = 2(R2/R1)

- 7.87mV +

+

200µA

-

-

-

REF200

SBVS020B – SEPTEMBER 2000 – REVISED JULY 2015

www.ti.com

Typical Application (continued)

8.2.1 Design Requirements

The design requirements are as follows:

•

Supply Voltage: 5 V

•

RTD temperature range: –50°C to +125°C

•

RTD resistance range 80.3

Ω to 147.9 Ω

•

Output: 0.1 V to 4.9 V

The design goals and performance are summarized in Table 1. Figure 15 depicts the measured transfer function

of the design.

Table 1. Comparison of Design Goals, Calculations, Simulation, and Measured Performance

RTD

GOAL

CALCULATED

SIMULATED

MEASURED

80.3

Ω

0.1 V

0.112 V

0.117 V

0.11 3 V

142.9

Ω

4.9 V

4.83 V

4.82 V

4.862 V

8.2.2 Detailed Design Procedure

Figure 12 and Figure 13 shows the schematic of the RTD amplifier for minimum and maximum output conditions.

This circuit was designed for a –50°C to 150°C RTD temperature range. At –50°C the RTD resistance is 80.3

Ω

drop that opposes the RTD drop. The drop across R3 is used to shift amplifiers input differential voltage to a

minimum level. The output is the differential input multiplied by the gain (Vout = 698

∙ 160 μV = 0.111 V). At

150°C, the RTD resistance is 148

Figure 13). For more detailed design procedures and results, refer to the reference guide, RTD to Voltage

Reference Design Using Instrumentation Amplifier and Current Reference (TIDU969).

Figure 12. RTD Amplifier with Minimum Output Condition

10

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