Thermistors and silicon temperature sensors are widely used forms of analog-output temperature sensors. A silicon temperature sensor is a far better choice than a thermistor when a linear relationship between voltage and temperature is needed. Over a narrow temperature range, however, thermistors can provide reasonable linearity and good sensitivity.
Many circuits originally constructed with thermistors have over time been updated using silicon temperature sensors.
Silicon temperature sensors come with different output scales and offsets. Some, for example, are available with output transfer functions that are proportional to K, others to C or F. Some of the C parts provide an offset so that negative temperatures can be monitored using a single-ended supply. Thermistors and silicon temperature sensors continue to enjoy popularity due to low cost and convenience of use in many situations. The problem for many sensor circuits, however, is putting their output in a form that lends itself to data conversion -or even basic signal conditioning.
To invert the output of an analog temperature sensor (in producing a signal for a temperature- compensated power supply, for example) you might choose an inverting op amp, but that approach may require two carefully picked feedback resistors to meet the specified accuracy.
Figure 1: The need for a pair of matched resistors handicaps the classic inverting amplifier circuit
An inverting op amp operates with a noise gain of two, which produces twice as much output offset voltage as does a unity-gain buffer. Figure 1 depicts the traditional method for inverting an analog output such as that of a temperature sensor. It also depicts a single-supply system, which has become the norm. As reference voltage for the op amp's non-inverting input, the circuit also requires a voltage source, a voltage reference, or a resistive voltage divider.
As an alternative, the circuit of Figure 2 inverts the output of an analog temperature sensor precisely, without need for precision components.
Figure 2: This connection provides a precise inversion of the temperature sensor output without the need for precision components
Connecting the sensor output to the amplifier's inverting input forces the sensor's ground pin to swing negative with respect to the reference voltage. Thus, at the temperature corresponding to a sensor output of 0V, the circuit output equals VREF. The circuit output implements the exact inverse of the scale factor of the temperature sensor, swinging negative with respect to the reference. When the sensor output is 750mV at 25C (typical for a 10mV/ C sensor with 500mV offset at 0C), the circuit output is VREF-750 mV. Components R and C ensure stability in the amplifier.
Voltage across the temperature sensor must never drop below the sensor's minimum rated supply voltage. That minimum occurs at the coldest temperature expected, for which the sensor's negative pin is driven most positive. However, that restriction is eased by the low minimum operating voltage of the MAX6607/ MAX6608 devices shown (1.8V).
Thus, VS must be at least 1.8 volts greater than VREF at all times. See Table 1. Because this circuit varies the sensor's supply voltage, the sensor introduces a slight additional error in proportion to its power supply rejection ratio.
Table 1: Maximum VREF for various supply voltages