In the figurewe present an interesting application suggested by Microchip (www.microchip.com) in which the voltage reference for an A / D converter is determined by the temperature of a sensor. The circuit uses a thermistor in parallel with a resistor as a sensor in order to adapt the system's response. The circuit gain, under these conditions, is given by the formula:
Where:
Vout: amp is the output voltage on the operational amplifier
Vin: amp is the voltage applied to the input
Rntc is the resistance displayed by the thermistor at the temperature taken as a reference.
In this circuit, a precision 2.5 V voltage reference is used to generate a voltage of 0.276 V at the input of the operational amplifier. When the NTC temperature is 0oC the resistance of the thermistor is approximately 32,650 ohms. The value of the parallel resistor and the 10k ohm metal film resistor is 7655.38 ohms. This gives a gain for the 14.94 V / V circuit. When the temperature of the NTC is 50º C the resistance of the thermistor will be approximately 3601 ohms and with that the gain of the amplifier will be 5.8226 V / V. With these characteristics we have the following formula for the transfer function of the converter that digitizes the input signal:
