Summary and References

SUMMARY

This training covers thermal effects on Printed Circuit Boards (PCB) encountered in high (DC) precision op amp circuits: causes, effects, and fixes have been covered.

Thermocouple junctions are everywhere on a PCB. The Seebeck effect tells us that these junctions create a thermoelectric voltage. This was shown to produce a voltage across resistors (and other components) in the presence of a temperature gradient.

Preventing large thermoelectric voltages from occurring is usually the most efficient way to deal with thermocouple junctions. The amount of heat generated on the PCB can be reduced and the heat flow can be redirected away from critical circuit areas. It is also beneficial to keep any temperature changes from occurring too quickly. Any remaining thermoelectric voltage effects need to be reduced. Choosing metals, in critical areas, to have approximately the same work function, will minimize the thermoelectric coefficients of the metal junctions. Critical components can be oriented so that they follow constant temperature contour lines. It is possible to cancel most of the thermoelectric voltage effects at the input of op amps by correctly orienting them. It also helps to space the smaller components closely together.

Once a design has been implemented on a PCB, it is important to measure its thermal response. Information on where to focus the design effort can greatly speed up the design process. Information has been provided on measuring temperature, thermal gradients, package 𝜃JA‘s, and troubleshooting tips and tricks.

Leakage current effects also need to be minimized. Methods to accomplish this for surface and bulk leakage currents have been shown. Other effects are also discussed.

A design example, using the MCP6V01 (Thermocouple Auto-zeroed Reference Design PCB), illustrates the theory and recommendations. The circuit operation is described and then the PCB layout choices are covered in detail.

References to the literature and the design example’s schematic are also provided.

REFERENCES

Related Application Notes

AN990 Analog Sensor Conditioning Circuits–An Overview
AN1177 Op Amp Precision Design: DC Errors
AN1228 Op Amp Precision Design: Random Noise

Other Application Notes

AN684 Single Supply Temperature Sensing with Thermocouples
AN679 Temperature Sensing Technologies

Other Reference

User’s Guide, MCP6V01 Thermocouple Auto-Zeroed Reference Design

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