Solving Sensor Offset Problems

Design engineers sometimes have to interface the TC7106 and
similar ADCs to “non-ideal” sensors. A very common problem is
that the sensor often does not give a “zero” output where the design
wants a zero reading.
One example of a “non-ideal” sensor is a diode used as a temperature
sensor. The diode typically changes –2mV per degree Celsius,
but the change is from the diode’s forward voltage of 600mV or so.
In order for the display to read “000” at 0.0 degrees, an offset must
be provided.
The differential inputs of the TC7106 yield an easy solution to the
offset problem. Figure 1 shows a simple thermometer with a diode
sensor. Because the diode voltage decreases as temperature
increases, IN LO is connected to the diode temperature sensor.
The IN HI input is connected to a trimpot which is used to cancel the
diode’s forward voltage.