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I need a cheap, somewhat accurate (~0.5%) voltage reference for some DACs. At first I was going to use a LDO voltage regulator (a TC1223 specifically) for this, it seems to fit the bill looking at the datasheet. Then I saw there is a separate category of ICs called voltage references rather than voltage regulators. But from what I can tell the voltage references with the same initial accuracy as the regulator I mentioned above, costs more, and also requires one or more external resistors (at least the shunt diode reference types).

So I was wondering what the difference between regulators and references are, and whether or not I can make do with a regulator for my needs or if I should get a reference, regardless of the higher price for seemingly similar specs. Thanks.

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What DAC are you using? – Andy aka Mar 7 at 20:28
    
DAC084S085 plus the ones integrated in ATSAM4S2B – GrixM Mar 7 at 20:39

A voltage regulator is designed to take a variable voltage in (say, 2-5v), and output a constant voltage (say, 3.3v). Now, voltage regulators are typically used to power a circuit, which means they will have a current output of a few hundred mA or more, generally speaking. In order to keep cost, size, etc down, the output tolerance on voltage regulators are (again, generally) a few 10s or 100s of mV.

For example, the RG71055 voltage regulator has a minimum output voltage of 5.2v, and a maximum of 5.8v, with a target output voltage of 5.5v, and can source 30mA. That's about a 5% voltage tolerance, assuming I number crunched correctly.

On the flip side, a voltage reference is designed to take a variable voltage, and deliver EXACTLY the rated output voltage. For example, the LT1790 can supply 5v with a tolerance of 0.1%, which is a 50x improvement over the RG71055. However, the LT1790 can only source 5mA max, which is 6x less than the RG71055. A voltage reference is used when you need to know that this line is exactly a certain voltage (in other words, really tight tolerances). On Digikey, you can get a voltage reference with 0.01% tolerance. With voltage regulators, you'd be lucky to get one with a 1% tolerance.

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Generally (though there will be exceptions) references have better specifications than regulators. Included in those specifications are ...

temperature stability
input voltage stability
output load stability

(stability is a big thing with references!)

output noise

... as well as initial accuracy. Though you will often find that some references have various grades, and better initial accuracy is available in the higher grades, at a cost!

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I would also expect that a voltage reference would have a much lower output current capability than a voltage regulator. – Peter Bennett Mar 7 at 17:53
    
Of course, that goes without saying. The output current of a voltage reference is (approximately) zero, the things are not designed to supply power. – Neil_UK Mar 7 at 17:58

The general concept and circuit is the same between voltage references. The difference is how the details are implemented.

There are two topologies, series or shunt, and an LDO or voltage reference are made in both topologies. Does it matter which one you use, no. I've used some voltage references for a power rail to increase the stability of analog electronics. I've also used LDO's for a reference, when I didn't want to add a reference to the board and I didn't need accuracy\stability. Make sure you pay attention to your load and what your requirements are.

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The differences are voltage references are designed with these characteristics in mind, to minimize voltage error.

  • Voltage Noise
  • Long Term Stability
  • Temperature Drift
  • Thermal Hysteresis

This usually always increases the cost of voltage references vs an LDO. Voltage references have lower current sourcing capabilities in a tradoff for other characteristics.

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One additional (to your other answers) factor -- some regulators have a minimum current below which they're not specced to perform. This is similar to and may in fact be higher than the maximum current available from a reference. Thus if you need a voltage for reference (i.e. you're drawing next-to-no current from it), you need a voltage reference. (For a regulator I based this on LM317, which is probably close to a worst case being both an old design and adjustable)

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Voltage regulators are designed for powering devices with a constant voltage. They can either source or sink significant currents, but their output voltage is not particularly exact; it may drift significantly as the load or temperature changes, or it may even be dependent on external resistors to "program" the regulator.

An example of a voltage regulator is an LM7805, which will output 5V ±250 mV (5%), and can source up to 2A with an appropriate heat sink. No temperature coefficient is specified.

Voltage references have much more precise and stable output, but cannot source or sink any significant amount of current. They're typically used in precision analog circuitry, e.g. with ADCs.

An example of a voltage reference is the TI REF02, which will output 5V ± 10 mV (0.2%), but can only source about 10 mA. It has a temperature coefficient of 10 ppm/°C. (That is, if its temperature changes by 1°C, its output will not change by more than 10 ppm, or 0.001%.)


For your application, the TC1223 will not be sufficient. It is designed as a voltage regulator, and has an output voltage specified within a ±2.5% range. (The "VR±0.5%" specification is a red herring; this is a "typical" value, and is not guaranteed!) You will need a voltage reference here; if you find the external resistors a hassle, use a series reference instead of a shunt diode.

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I assume that another difference might be thermal and/or over-current shut-down that LDOs usually have is another difference, not yet mentioned in any of the answers. Implemented through some on-chip varistor or similar? – Lundin Mar 8 at 10:26

You have to decide what is more important to you, accuracy or low price? If you need accuracy, then a reference should be used. If low price is required, then a voltage regulator would be your best bet.

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Not just that. Adding a voltage reference also means adding more components to the design. If there's no physical space left on the board then the option of adding another component may jeopardize the intended design specification (e.g. distance tolerances, rf interference). – captcha Mar 10 at 23:19

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