Posted on Feb 13, 2013

The DVM-to-temperature adapter is built around a single IC, National`s LMlO. That micropower IC contains a stable 0.2 V reference, a reference amplifier and a general-purpose op amp. The circuit is designed for a linear temperature range of 0 to 100°C (32 to 212°F). The 0.2-V reference and reference amplifier provide a stable, fixed-excitation voltage to the Wheatstone bridge. The voltage is determined by a feedback network consisting of Rl through R6. Switch S2a configures the feedback to increase the voltage from 0.6 Von the Celsius range to 1.08 Von the Fahrenheit range.

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These differences compensate for the fact that one degree Fahrenheit produces a smaller resistance change than does one degree Celsius. Resistors Rl through R16 also form the fixed leg of the Wheatstone bridge, nulling the bridge output at zero degrees. Since 0°C is different from 0°F, S2b is used to select the appropriate offset. The LMlO"s op amp, along with R9 through Rl2, form a differential amplifier that boosts the bridge output to 10 m V per degree. Since a single supply is used, and since the output must be able to swing both positive and negative, the output is referenced to the bridge supply voltage, rather than to the common supply.

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