Design Article
Comment
DadOf3TeenieBoppers
Accuracy is not the greatest in the world, but at least one would have a ...
SPLatMan
Switching between two currents, rather than using 2 (hopefully matched) ...
Make accurate temperature measurements using semiconductor junctions
Sachin Gupta and Umanath Kamath, Cypress Semiconductor
6/20/2012 10:02 AM EDT
High frequency noise is another source of error in measured value. When diodes are sitting at remote location connected using long wires, it is very much susceptible to the noise. Long wires act like antenna. To avoid noise pick up, twisted pair wires should be used to reduce the effective length of antenna. Wires used for excitation should be paired together and wires used for voltage measurement should be routed together. If noise is very high at the place where system is deployed, it is good to have a shield around the twisted pair to ground the noise before they get coupled to the wires.
In summary following points should be taken care while using diodes for temperature measurement:
- Four wires measurement must be done with measurement wires connected to diode as close as possible.
- ADC must be calibrated for offset and gain error.
- DAC must be calibrated for gain error.
- Instead of standard two lead diode, diode connected transistor must be used.
- A transistor, which has fairly constant hFE over collector current, must be used.
- To deal with high frequencies, twisted pair wires must be used. Shielding must be used if noise is very high.
Conclusion
Diodes being the most inexpensive are the potential candidates for low cost temperature measurement applications. This article talks about the points to be taken care, to make accurate measurement using them. SoCs offer higher integrations while implementing remote sensing diode applications as they have internal ADCs, DACs, multiplexer.
About the Authors
Sachin Gupta is a Senior Applications Engineer with Cypress Semiconductor. He loves working on different types of analog and digital circuits, as well as synthesizable codes. He can be reached at sgup@cypress.com.
Umanath Kamath works as an Applications Engineer with Cypress Semiconductors. He enjoys building analog and digital circuits and systems using different types of sensors.
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SPLatMan
6/28/2012 3:14 AM EDT
Switching between two currents, rather than using 2 (hopefully matched) transistors at different currents, is a neat trick in these days of microprocessors. I will keep it in mind.
That said, this method would really only be economical if the design already contains a high resolution ADC. If you only have a 10 or 12-bit ADC as is usually included in a microcontroller chip, then a $0.25 thermistor plus a fixed resistor is a better bet. Even cheap (1%) thermistors have an accuracy of about 1'F (rule of thumb: 1'F/% @25'C) and a 10 bit ADC will give a resolution of way less than a degree. The downside is that the processor will have to do some linearisation.
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DadOf3TeenieBoppers
7/12/2012 2:07 PM EDT
Accuracy is not the greatest in the world, but at least one would have a ballpark figure +/- 2% to 3%) of the core temperature of a processor or FPGA with a temperature sensing diode built in. This way an alarm can be raised as the temperature begins to rise into the area where the lifespan of an IC begins to drop off dramatically.
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