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How does one go about accurately setting reference voltages at various points in a given circuit (e.g. various threshold voltages). Obviously we can have a potential divider from the main power supply but that generally tends to fluctuate a lot, not to mention noise from other parts of the circuit.

I would assume a sort of single voltage reference from which we can derive the rest of the references. Does one use potential dividers after that? Or are there any smarter methods?

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3 Answers 3

One reference can be used, or several, it depends what you need.

If you don't care much about the ratio between references, several might be easier or cheaper.

On the other hand, on a recent design I did, the reference was more than $100 in small quantity, so it was advantageous to generate several other precision reference voltages from the master reference (high precision resistor networks are expensive, but not that expensive). You can invert a reference voltage to generate a negative, or generate a reference current, which can level-level shift the reference voltage to another supply rail.

You can use a voltage divider from a reference and buffer the resulting voltage, if required (but pay attention to maximum capacitive loading if you're using an op-amp, and decouple the load if necessary). There are also "rail splitter" chips that can be useful (though they're generally not high precision).

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So using normal resistors for getting the reference voltage is something usual? Will the resistor mismatch errors, temperature etc give problems? –  midnightBlue Jun 16 at 15:20
    
You have to do the math. For some applications 5% resistors or 1% resistors are just fine (they're pretty stable), for others you need the 0.01% 0.2ppm types that cost a small fortune. Sometimes all you need are stable parts (eg. Susumu thin film parts) and you can calibrate out the initial tolerance. –  Spehro Pefhany Jun 16 at 15:26
    
Assuming the resistors are matched quite well then, would the temperature or anything else affect them? I would guess that the temperature rise would affect both in the same if they are nearby and thus giving the same ratio? –  midnightBlue Jun 16 at 15:30
    
There's lots of things that can affect them- moisture (load life), drift, differences in temperature coefficient (temperature rise won't likely be the same, but you don't want them self-heating much anyway). You really have to do an error budget and do the math. –  Spehro Pefhany Jun 16 at 15:35
    
That must be a hell of a voltage reference. Even an LTZ1000 costs less. –  starblue Jun 16 at 18:04

One smarter solution over the use of resistor dividers is to apply a shunt regulator. A part such as a TLV431 can be biased from a higher voltage supply to produce an accurate reference voltage using a couple of resistors. An additional resistor is used to limit the overall current through the shunt regulator.

If you are familiar with the operation of zener diodes you can think of the shunt regulator as being an nice accurate zener. And then two resistors allow you to set its voltage.

For multiple references in one project you could use multiple shunt regulators or apply voltage dividers across one shunt regulator. Today's shunt references are in small packages and something like the TLV431 is rather low cost so using multiple units on one board is usually not a big deal.

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So if I understand correctly, you'd mean that there would be one of e.g. TLV431 everywhere we want a stable reference voltage and since its so cheap and small , we can do so. –  midnightBlue Jun 16 at 15:28
    
That is kind of what I am suggesting. You will have to look into just how accurate of reference you need in your circuits. A TLV431 is a good general purpose low cost solution for many applications. However if you are trying to build some high accuracy 20bit A/D setup or some such then the part I suggested may not be suitable. You can find references that cover a big span of cost range with the highest cost units being specially designed units with thermal and voltage stabilization. –  Michael Karas Jun 16 at 15:34

For more precision use, there are dedicated "Voltage Reference" chips. These are like a small linear voltage regulator, but favouring precision over current. They can typically provide only a few milliamps, but stability and accuracy can go down to the milli- (or even micro- if you pay enough) volt level.

With voltage dividers, even taking an accurate reference as the source, you are at the mercy of both the accuracy of the resistors used, the temperature stability of the resistors, and even the ratio of resistors to the input impedance of whatever you're feeding the reference voltages into.

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