For a project I require simple temperature sensing. The specifications are as follows:
- temperature range: 50-250 Celsius
- costs resonably low (< 10 US)
- accuracy +/- 2 degrees
for cost and availability reasons I selected the following RTD: (farnell)
The trick now is to get the temperature information into a micro controller. This will be accomplished using an Wheatstone bridge and an MCP6N11 (farnell). The gain of the amplifier has to be around 6. A proposed schematic can be found below (note that the RTD is placed between "R_uit" and "R_in").
There rests only one question. How do I properly set Vref? Considering that the micro controller can measure between 0 and 5V, which I prefer to utilize fully. In the datasheet is stated that Vref "lifts" the output voltage. With that in mind, isn't it best to just ground Vref? It is provided that the MCP6N11 is a rail-to-rail device.
In the User guide of the demonstration board they talk about setting Vref around 1.8V when Vdd is 5V.
This should get you close. By my calcs, in your range the thermistor will be going from about 1 ohm to about 6 ohms. With a 1K resistor to hold currents down to milliamps, you need a gain of about 35 to keep your max voltage at 1V. This circuit has a gain of 34 using standard 5% resistor values. You might need to change to a rail-to-rail op amp if you're going to power it with a one-sided supply.
Best Answer
Vref of an amplifier determines its zero point; when the input is 0, the output will be equal to Vref. If the polarity to be measured is equal in both directions then Vref should be 2.5V for maximum range with a 5V ADC, otherwise it should be biased appropriately given the input.