In my first blog in this series, I discussed the generalities of the different temperature measurement techniques. In this blog I will deal with converting the temperature into electronic signals and start with purely analog conversion. All the major analog semiconductor manufacturers produce standalone devices — Analog Devices, Linear Technology, Maxim Integrated, and Texas Instruments amongst them. Just search for both the data sheets and application notes on their websites.
Some of these designs even can detect open-circuit on the sensors and linearize the output. Three design ideas demonstrates how linearization can be achieved for an RTD (IC generates second order polynomial), with an improvement using a spreadsheet (Design an RTD interface with a spreadsheet), and an improvement (A better approach to designing an RTD interface with a spreadsheet) proposed by yours truly.
Using current sources allows a modification of the Wheatstone bridge approach and is useful with 3-wire RTDs. The XTR103 in Figure 1 has two current sources (0.8mA) which are at a low value to prevent self-heating of the RTD. I have also seen the current pulsed to reduce the self-heating. Rz is mounted on the PCB and is equal to the value of the RTD at its minimum temperature of the desired range. One current source drives the RTD and the second drives Rz.
The voltage across the RTD increases with temperature, while the voltage across Rz remains constant, so with the use of a differential amplifier across both resistors you can measure only the change in voltage from its minimum range. The currents join on the return path of the RTD and pass through Rcm. This shifts the common mode of the voltage difference up into the operational range of the difference amplifier.
You can see from the notes on the figure that the line resistance is cancelled out. It is simple enough to replicate parts of this for your design by creating your own pair of current sources, as you can see in Figure 2. Also of interest is the fact that the circuit in Figure 2 has the ability to detect if any one of the RTD wires is broken. Although you may not use this exact approach in your design, the possibility of cable failure is something you need to consider.
Let us move our focus to the conversion of the measured RTD signal to digital format.
Modern A/D devices cater for devices like RTDs and include current sources. Since temperature changes very slowly, the Delta-Sigma ADC is ideal for the application, and today there are some with very high resolution. Some also allow for differential inputs, but because of the high resolution it is possible to use a different technique if there is no differential input option. Simply use an ADC system with two input channels, and the micro can numerically subtract one channel from another to give you the difference.
With the high resolution it is still possible to have meaningful results. Let’s assume you have a 20bit ADC with a span of 2.5V — the LSB represents 2.4µV which is 2.4mΩ at 1mA current through the RTD. With a DIN 43760 RTD the sensitivity is about 0.385Ω/°C, so theoretically you could resolve to milli-degrees. There is yet another approach if you have sufficient resolution and inputs. You could configure a precision resistor in series with the RTD and measure the voltage across it to give the actual current flowing. We will see more on these ADC systems in a while.
A thermocouple produces a very small signal and will benefit from a noise-immune amplifier as well as quite a bit of gain, since the signal is so small. As always, protection against surges is good practice. It will also need (as previously mentioned) cold junction compensation.
Here is a suggestion from Linear: Ultraprecise Instrumentation Amplifier Makes Robust Thermocouple Interface. And Analog Devices has a ton of information here: Thermocouple Interface.
In years gone by you would condition the signal based on thermocouple type and temperature range so that the signal would be within the common mode of the input and with the maximum possible output and then feed the signal to an ADC.
In many applications the temperature range and thermocouple type can be pre-determined. My employer sells signal conditioning modules, but the Holy Grail is to have a universal input that can cater to all types and ranges so that we only need to make one product and differentiate by software. So the philosophy has changed to use the high-resolution Delta Sigma ADCs as well. Let’s assume the range of the ADC is 1.2 volts and a resolution of 24 bits, so the least significant bit is equal to 72 nV. Further let’s assume that you only want a 250°C range which is about 10 mV for a J-type thermocouple.
That range would be represented by a count of 10mV/72nV=139000 which is about a 17 bit resolution. Now, as promised, we will still get to these ADC systems, but I think it would be better to point out here that the current source on the ADC system (as mentioned in the RTD discussion) can be used to periodically test the integrity of the thermocouple connection, although obviously not at the same time as taking a temperature measurement!
On a practical note, it is possible to use lookup tables or formulae to figure out the temperature, but most published tables take the temperature and give you the voltage when, in fact, you are measuring the voltage and want the temperature. A great resource is the National Institute of Standards and Technology (NIST) Thermocouple Database: NIST ITS-90 Thermocouple Database. I still have its program to produce the inverse tables, but that doesn’t seem to be available any more. However it should be no problem to generate any table using a spreadsheet and the different coefficients, either normal or inverted.
Shifting our focus once again, this time to thermistors. It seems to me that because the thermistor is so simple and cheap, most designers opt for a simple resistor divider technique, but I could be wrong. A single current source driving the thermistor also seems to have some traction in the industry. Again, the current through it can cause self-heating, and, given the sensitivity of the thermistor, this heating could be more problematic than in applications using the RTD.
As you can tell, although they are far from rare I have never used one. I have heard reports, though, that there are tremendous variations among manufacturers for similar parts, so your design had better cater to the possibility of variations during the product’s life. Any technique used to energize an RTD can be used on a thermistor as well, and I won’t spend any more time discussing them, to avoid duplication.
How time flies! We have looked at some techniques for the three traditional methods of measuring a temperature and we still haven’t got to semiconductor sensors. Next time, I promise. Even though this is a blog in process, comments are invited.