AN-1559| Application Note
Practical RTD Interface Solutions
Practical RTD Interface Solutions
National Semiconductor Application Note 1559 Chris Eckert, Ron Bax June 2007
1.0 Purpose
This application note is intended to review Resistance Temperature Devices and commonly used interfaces for them. In an industrial environment, longitudinal noise in a wide spectrum may exist requiring good common mode rejection at the circuit interface. A brief discussion of longitudinal balance and how to improve the interface response to longitudinal noise is presented. Circuit configurations that provide better longitudinal balance along with design recommendations for optimal performance are presented.
0 C. From -200 C to 0 C their Resistance vs Temperature characteristic is described by a 3rd order polynomial R=R0(1+At+Bt2+C(t-100)t3) From 0 C to 850 C the device is described by a 2nd order polynomial: R=R0(1+At_Bt2) The coefficients are : A = 3.9083 10-3 C-1 B = -5.775 10 -7 C-1 C = -4.183 10-12 C-1 2.4 RECOMMENDATIONS For precision applications, remember that these sensors are not exactly linear. The above polynomials can be used in a microcomputer after the information has been digitized to get accurate temperature readings. For precision applications, use the higher valued resistance sensors for increased sensitivity and resolution (PT1000). Higher resistance sensors may require higher single supply voltage, or even 2 supplies. Allow sufficient headroom for linear performance. Choose current drive levels that minimize sensor self heating. Acceptable current levels for each type of sensor and allowable measurement error can be easily calculated. When choosing a sampling interval for the data
acquisition system, remember that these sensors have response times.
2.0 RTD Review
2.1 RTD DESCRIPTION An RTD (Resistance Temperature Device) can be fabricated in several ways, but the most common are based on the temperature characteristics of Platinum. It has a predictable temperature characteristic that can be controlled and configured in a number of different ways. Platinum wire wound around glass or ceramic and platinum thin film chip designs are common. Many kinds of RTD designs are available to designers but this discussion will be limited to some of the most common. 2.2 RTD STANDARDS An international standard, EN 60751, defines the detailed electrical characteristics of platinum temperature sensors. The standard contains tables of resistance vs temperature, tolerances, curves and temperature ranges. 2.3 RTD SPECIFICATIONS Three common RTDs are the PT100, PT500 and the PT1000. The 100, 500 and 1000 represent the resistances of each at
3.0 2, 3 and 4 Wire Configurations
Temperature sensors can be configured in several ways:
30002301
FIGURE 1. Standard Schematic
AN-1559
2 wire This configuration is used when the parasitic resistance of the connection wires is known and doesn't change. This can be compensated for by a computation later in the signal path.
2007 National Semiconductor Corporation 300023
3 wire This configuration allows the designer to monitor one side of the current loop with a Kelvin connection. The voltage drop in the resistance of the loop is measured and compensated for. For users with only three wires available.
www.national.com
AN-1559 Application Note national Download PDF
Add this permalink to your bookmarks for future download of AN-1559 ApplicationNote
Permalink: http://application.emcelettronica.com/national/AN-1559
