Whenever two dissimilar metals are connected together a thermocouple is formed. A thermocouple generates a small DC voltage that is proportionate to temperature. A parasitic thermocouple is an unintended junction of two different metals that may introduce errors. Printed circuit boards contain hundreds of parasitic thermocouples. For example, surface mount resistors normally have a tin-plated nickel end cap. This end cap is soldered to a copper trace. The copper junction with the nickel-tin end cap creates a thermocouple. There are potentially other thermocouples in this simple component where the end-cap contacts the film resistor (see Figure 3-11). Most PCB designs have hundreds if not thousands of similar components. The presence of hundreds of parasitic thermocouples may seem like a serious accuracy concern for precision DC circuits, but it generally is not a significant issue because the thermocouple voltages cancel each other when the PCB temperature is uniform. Thus, the parasitic thermocouple effect is a concern only for precision DC systems that have a temperature gradient on the PCB. This gradient may be due to high power dissipation on a localized portion of the PCB or an adjacent heat source that is not uniformly applied to the PCB.

Figure 3-12 and Figure 3-13 show a single horizontally modeled resistor with a vertical and horizontal temperature gradient. For the vertical temperature gradient, the parasitic thermocouples are at the same temperature, so the junction voltages cancel each other (Verror = 0 V). Conversely, with the horizontal temperature gradient the two junctions are no longer at the same temperature, so the errors do not fully cancel out (Verror = 20 µV).

Figure 3-14 and Figure 3-15 show a low thermal EMF resistor layout. This layout can be used in precision DC applications that have large temperature gradients to minimize the parasitic thermocouple effect. This two-resistor series layout can replace the single resistor shown in Figure 3-12 and Figure 3-15. Figure 3-14 shows that with a vertical temperature gradient the two resistors each develop an equal error voltage, but the two error voltages cancel out each other (Verror = +10 µV - 10 µV = 0 V). Figure 3-15 shows that with a horizontal temperature gradient the two resistors are each at a different temperature, but each resistor is at a constant temperature. That is, the resistor on the left is at approximately 65°C and the resistor on the right is at approximately 75°C. The important point here is that each individual resistor is at a uniform temperature so the thermocouples on that resistor cancel each other for a net error of 0 V.