Temperature to frequency converter - °C into Hz

Although a temperature to voltage converter may be more common, a temperature to frequency converter is much more useful when digital circuits are used for temperature measure- ment. This type of converter can be connected to either a frequency counter or even a microprocessor, without the need for an additional A/D converter. The circuit described here is remarkably accurate. A 10 Hz/°C conversion factor is maintained within 3 Hz, throughout the 5° to 100°C range. A "pseudo" zener diode, the tempera- ture dependent LM 335, is used as the temperature sensor. The IC comes in a plastic transistor package. The ADJ pin is not used in this application. The °C into Hz voltage across this "zener diode" is directly related to the absolute temperature in degrees Kelvin: ULNA 335 = 10 • T (mV) Therefore, at 0°C the voltage will be exactly 2.73 V. In order that the voltage to frequency converter can be calibrated in degrees centigrade, this temperature to frequency converter 7-20 - ...