Circuit improves on temperature measurement

摘要

When current pulses with a stable I_(HIGH)/I_(LOW) ratio modulate a semiconductor junction, the ensuing voltage difference (for example, ΔV_(BE) for a bipolar transistor) is a linear function of the absolute (Kelvin) temperature, T. You can use this truism to make accurate temperature measurements. Technical literature has thor-oughly covered the relationship (refer-ences 1 to 4) and has numerous imple-mentations. This Design Idea suggests some areas for improvement and design variations on the basic idea. The principal equation describing the phenomenon is as follows: ΔV=86.4 x T x ln(I_(HIGH)/ I_(LOW)), expressed in microvolts. Setting the current ratio I_(HIGH)/I_(LOW) =10 results in ΔV of approximately 200 μV per degree. The key issue in the practical implementation of the idea is to switch current with a highly stable I_(HIGH)/I_(LOW) ratio, which you can do by using a number of discrete components. This Design Idea suggests a digitally controlled and integrated approach (Figure 1).