Fig 1's simple arrangement of active-R oscillators accurately converts temperature into time. Most designers use these types of oscillators for low-grade oscillators that don't have stringent waveshape and frequency-stability requirements. However, the three-phase symmetrical active-R oscillator actually can generate an excellent triangular waveform. In this case, the oscillators become relaxation generators in which the input stage's constant bias current periodically charges and discharges the op amp's compensation capacitor.

Furthermore, the period of oscillation, T, is very predictable. You can express it as T=AØ, where Ø is the absolute temperature and A is a constant depending on the op amp's slew rate, the threshold voltage necessary to commute the input stage, and by the external fixed gain.

The LM246 programmable quad op amps provide for good thermal tracking. The supply voltage was ñ11V. The resistor values inFig 1 produced the following results:

• oscillation period of 7.06 µsec at 25°C
• sensitivity of 26 nsec/°C over an 85°C range
• uncalibrated temperature error of 1.8°C at 25°C
• uncalibrated temperature error of 3.2°C over the 0 to 85°C range
• temperature error of 0.4°C with 25°C calibration (by adjusting R_SET) over 0 to 85°C range
• nonlinearity of 0.3°C. (DI#1679)