So, here is a very basic temperature sensor circuit. What I know about this I learned from Gimmy. He was my tutor, he was man at this, he is Mr. sensor. If you took his class you know. For those of you haven't, so here's our thermostat over here I can barely lift temperature. All right. We build a voltage divider with that and we pick a fixed resistor, we stack it on top and 10K Ohm can be typical with this it doesn't have to be, might be 5K or something but in this cartoon circuit, I picked 10K. This is applied to this systems VDD, this could be 2.5 volts or 3.3 or five volts if it's TTL, whatever the voltage are, just say it's 3.3. Then this resistance here changes based on the temperature. So, I'm calling the voltage on this node Vtherm. Okay. If your application requires it and I'm saying it does for the purpose of our course because we're going to talk about filtering. We want to filter this voltage that's here because there might be noise on it and we want to filter out that noise. So, we're going to run it through a filter to remove noise and we're going to want this to be a low-pass filter. So, maybe we want it to pass everything up to 500 hertz or everything up to 1,000 hertz or anything up to 10 kilohertz or something. They'll be some cutoff frequency when we look at some examples coming up and we want to filter some higher frequency noise out of this. We want this filter to effectively have negligible voltage drop because we're going to feed that voltage, this is the point that we're going to measure. So, whenever this voltage is and it changes as a function of this, the resistance of the thermostat here feed it into an ADD converter. Okay. ADD converters typically have very low impedance so we want to buffer the input to the ADD converter so, there's probably be a unity gain amplifier on the output of this like I got upcoming slide that shows, using an app amp as a unity gain follower to isolate and re-drive that voltage to deal with the low input impedance on the ADD converter. Critical to all of this is a what's called a precision voltage reference and you feed the precision voltage reference into the ADD converter and the ADD converter will sample that voltage on its input and it will convert it to binary. Depending on the ADD converter, you might get eight bits out or 12 or 16 or 20. The width, the number of bits here is the resolution. How many bits of resolution do I have? I'm imagining that its value is an unsigned binary value so, when you read all ones, whether it take better 20 bits or however many bits you have, when it's all ones, the input voltage is equal to the reference voltage, Vref. For those of you who've taken Jamie's class, if I say something that's not right, please speak up because this isn't an area where I'm an expert and I worked in. Although interestingly, I have been spending quite a bit of time working on temperature measurements for incited drives to report to the host system. I've learned some aura about measuring temperatures here recently with the on-the-job training. So, I cleared away some of the clutter. One of the things Jamie wanted to point out is that the these voltage references that you buy get very expensive as the tolerance on the parts go up. He said, just have your students go to digit key and have lookup of precision voltage reference and I didn't go to digit key I just made these numbers up. So, here's a 0.1 percent tolerance part, maybe $93. It's a very precise part. You move to a 0.05 percent tolerance part and the price may drop to $15. So there can be a big jump in as the tolerance goes down they get cheaper and cheaper and cheaper. So, a question you have to ask as a designer of these systems is, what's the accuracy and what's the precision that I need to report. Usually, that will come back from your requirements specifications.
