Design Ideas:October 27, 1994

Temperature measurements using RTDs (resistance-temperature detectors) generally employ bridge circuits with stable power supplies for signal conditioning. Unfortunately, this scheme generally produces nonlinear outputs. A linear bridge configuration that uses two identical current sources depends on the accuracy of matching the sources.

Fig 1a shows a linear bridge configuration with a single current source. The circuit eliminates the nonlinearity problem of constant voltage excitation as well as errors due to nonmatching current sources. The accuracy depends on the stability of the current source. To overcome this dependency, Fig 1a derives the dual-slope ADC's reference voltage from the IR drop. Any change in the constant current changes the reference voltage of the ADC, thereby compensating the current-source variation. Referring to Fig 1a,

In the dual-slope ADC circuit of Fig 1a,

where K is a constant. Hence, the digitized value is directly proportional to δR, which is proportional to temperature. Deviations in the excitation current have little effect on the output. You can extend the same technique to make precise resistance measurements. The circuit in Fig 1b can measure up to 2 kÍ with a conversion factor of 1 mV/Í. (DI #1609)