Circuit Note

CN-0337

Circuits from the Lab® reference designs are engineered and

tested for quick and easysystemintegrationto helpsolve today’s

analog, mixed-signal, and RF design challenges. For more

informationand/orsupport,visit www.analog.com/CN0337.

Devices Connected/Referenced

AD8608

Precision, Low Noise, CMOS, Rail to Rail

Input/Output Quad Op Amp

AD7091R

1 MSPS, Ultralow Power, 12-Bit ADC

ADuM5401

4-Channel, 2.5 kV Isolators with

Integrated DC-to-DC Converter

12-Bit, 300 kSPS, Single-Supply, Fully Isolated RTD Temperature Measurement

System with 3-Wire Compensation

Rev. 0

CircuitsfromtheLab®referencedesignsfromAnalogDeviceshavebeendesignedandbuiltbyAnalog

Devices engineers. Standard engineering practices have been employed in the design and

constructionofeachcircuit,andtheirfunctionandperformancehavebeentestedandverifiedinalab

environment at room temperature. However, you are solely responsible for testing the circuit and

determiningitssuitabilityandapplicabilityforyouruseandapplication.Accordingly,innoeventshall

AnalogDevicesbeliablefordirect,indirect,special,incidental,consequentialorpunitivedamagesdue

toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

www.analog.com

Fax: 781.461.3113

©2014 Analog Devices, Inc. All rights reserved.

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN0337 Circuit Evaluation Board (EVAL-CN0337-PMDZ)

SDP/PMD Interposer Board (SDP-PMD-IB1Z)

System Demonstration Platform (EVAL-SDP-CB1Z)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 is a completely isolated 12-bit,

300 kSPS RTD temperature measuring system that uses only

three active devices. The system processes the output of a Pt100

RTD and includes an innovative circuit for lead-wire

compensation using a standard 3-wire connection. The circuit

operates on a single 3.3 V supply. The total error after room

temperature calibration is less than ±0.24% FSR for a ±10°C

change in temperature, making it ideal for a wide variety of

industrial temperature measurements.

The small footprint of the circuit makes this combination an

industry-leading solution for temperature measurements where

accuracy, cost, and size play a critical role. Both data and power

are isolated, thereby making the circuit robust to high voltages

and also ground-loop interference often encountered in harsh

industrial environments.

The novel circuit for 3-wire RTD lead wire compensation was

developed by Hristo Ivanov Gigov, Associate Professor and PhD,

and Stanimir Krasimirov Stankov, Engineer and PhD Student,

Department of Electronic Engineering and Microelectronics,

Technical University of Varna, Varna, Bulgaria.

U1C

1/4

AD8608

U1D

1/4

AD8608

U1A

1/4

AD8608

U1B

1/4

AD8608

R5

VREF

GND_ISO

TP1

0.1V TO 2.4V

+3.3V

+3.3V

ISOLATION

ADuM5401

(C-GRADE)

U3

AD7091R

J1

PMOD CON

12-PIN

J2

A

B

+3.3V

GND_ISO

GND_ISO

+3.3V

VIN

CS

SCLK

CONVST

SDO

VREF

2kΩ

R10

1kΩ

R8

26.7kΩ

R4

39.2kΩ

R11

51Ω

R3

1kΩ

R1

100Ω

R1′

R6′

R1′ = R1ǁR2

R6′ = R6ǁR12

R9

1.1kΩ

R12

39.2kΩ

C10

0.1µF

C11

0.1µF

C9

4.7nF

C12

1µF

GND_ISO

GND_ISO

R2

1.91kΩ

RTD

(Pt100)

LINE

0ºC TO 300°C

100Ω TO 212.05Ω

INPUT

1

2

3

1

2

3

r

1

r

2

r

3

+3.3V_IN

SS

SCK

CONVST

MISO

GND

GND

R6

2k

Ω

Figure 1. Resistance Deviation to Digital Conversion with Isolation Using Pt100 RTD Sensor

(All Connections and Decoupling Not Shown)