Even an expertly engineered thermistor probe is useless without an accurate method of measuring its electrical resistance. Recall from part 1 of this series that a thermistor is nothing but a resistor whose resistance is a function of its temperature. Over its full operating range of -40℉ to 450℉, the thermistor in Range Oven Intelligence varies from about 1,000,000Ω to about 10Ω. A device which measures resistance is called an ohmmeter, and there are special challenges in designing an ohmmeter into RangeOI that can accurately measure such a broad spectrum of resistances. Ignoring those challenges would be like trying to measure your neighborhood with calipers or pulling out a measuring tape to measure grains of sand.

Designing Ohmmeter Electronics

Ohm’s Law is one of the most practical laws of electronics design. It says (among other things) that the voltage across a resistor is exactly equal to the product of its resistance and the current flowing through it. Symbolically, V = I x R. That law supplies the fundamental principle of Range OI’s measurement technique. To take a measurement, the Range OI electronics pump a variety of small bias currents through the thermistor, measure the thermistor’s voltage at each one, and use all that data to deduce the thermistor’s resistance from Ohm’s Law. Once the resistance is known, so is the temperature.

Identifying and minimizing the myriad sources of ohmmeter error is important for overall accuracy. Here’s a brief rundown of some of the types of error and how we get rid of them:

Bias current error - From Ohm’s Law we can see that carefully controlling the thermistor bias currents is the first step in taking an accurate reading. We use ultra-high-precision bias resistors and carefully-selected transistors to set all the bias currents.

Input voltage noise - Current through the thermistor makes a voltage we measure, but that voltage is victim to all kinds of corrupting electrical noise; it can come from other parts of the design, as far away as outer space, and many places in between. Careful design, signal integrity analysis, and certain numerical techniques obliterate this source of error in Range OI.  

Reference voltage error - An analog-to-digital converter (often ADC, for short) is a device that compares an arbitrary analog input voltage to a known reference voltage and generates a digital representation of their ratio. A high-precision, low-noise reference voltage like the one in Range OI is essential for accurate ADC operation.  

Quantization Error - The act of converting a signal from analog to digital is inherently inaccurate, because the analog signal has to be rounded to the nearest available digital representation. However, this type of error diminishes quickly as the resolution of your ADC increases. Using a 12-bit ADC in Range OI, along with carefully selecting all the bias currents, takes this error off the table almost completely.

We wrote Engineering Accuracy to show what goes into making our thermometer accurate, but we put the same amount of attention and dedication into every aspect, from the app to the packaging. We’re this meticulous about everything.