I am finally getting to the topic of integrated temperature sensors. Before we get to actual measurements, for completeness I thought I should mention some devices that monitor temperature but act as limit detectors with digital outputs to indicate when the temperature has transgressed. Most have programmable set points that can be fixed by resistors or through communication with a host micro.
An example of the former would be the TMP01 from Analog Devices, or even the system management device from Linear Technology, the LT2995, which includes voltage monitors. System management devices like LTC2974 interface through an I2C bus and will monitor four external temperatures.
There is a unique device from Analog Devices, the TMP05, which has a ratiometric PWM output proportional to the temperature. The unique feature is that they can be chained together requiring only two microcomputer I/O pins for a large number of temperature sensors. As he points out, the distance between the sensors can be increased with the use of line driver/receivers and I would add that you could isolate as well.
You would think that when it came to semiconductor temperature sensors you only had five parameters to play with: package, accuracy, temperature range, internal peripherals, and serial interface. There are a host of products like the TC77 from Microchip with a serial number and SPI interface, the TMP175 from TI with I2C interface or DS18S20 from Maxim with a 1-wire interface and programmable trip points. But there are another two factors to consider.
The first is how you are going to thermally connect the sensor to the object whose temperature you are trying to read. True, this is something to consider with all sensors, but the IC can need additional connections and normally needs a PCB. Sure, flying leads are possible, but that has to be part of your manufacturing process and subject to frequent failure. The second is a sort of related issue. Where is the temperature actually being measured? The DS18S20 actually measures the temperature of the ground lead so that any conducted heat on the PCB will affect the temperature readings.
Some functions are built around their need to know the temperature in order to derive another measurement. Besides thermocouples, another such function is relative humidity (RH). When I had a project that needed to measure RH, specialized ICs were not available, but now there seem to be a few, all of which include a temperature sensor. Jon Titus did a review of the Honeywell offering. Measurement Specialities produces a similar device with a digital interface, the HTU21D, and a purely analog version, the HTG3500.
There are also sensor digitizers that take a sensor input and convert it to a digital (serial data) format. The MAX31865 is one such device for RTDs and the MAX31855 is the matching part for thermocouples, including cold junction compensation.
Temperature Measurement 5 will have some conclusions and “gotchas.”