How to create precision reference 2.5 voltage

Question

Can I create a precision reference voltage of 2.5V using 7805 regulator and ordinary resistors(10% error)/diodes or do I need to rely on specific reference voltage generator?

Answer 1

A precision voltage reference is everything that a 7805 and resistive divider isn't.

Precision isn't all about how accurate the voltage is, but how accurate the voltage remains over time.

An precision voltage reference:

• Has a precise voltage - the 7805 isn't very precise at all.
• Is low noise - By comparison the 7805 has massive amounts of ripple.
• Has a low temperature coefficient - 10% resistors drift massively in resistance at different temperatures.

So if you want precision then a 7805 and some resistors are really not the way to go.

Answer 2

Yes, you can. It all depends on how much error you can tolerate. Since you didn't quantify "precision", it is a meaninless term and can only be ignored.

Obviously the error of a reference derived from a 7805 regulator is all the errors added up. Find the worst case output voltage range of the 7805, the worst case resistances of your resistor divider, and from those calculate the worst case range of your nominal 2.5 V reference. Keep in mind that the worst case for each of these is over the full temperature range you want your device to operate over.

Also consider the loading on a resistor divider. The output impedance of that will be the parallel combination of the two resistors. For example, if you use two 10 kΩ resistors, then the output impedance will be 5 kΩ. If that is feeding something that, for example, has 100 kΩ input impedance, then that adds another 120 mV drop to your reference. Even if the output was a perfect 2.5 V unloaded, you would only have 2.38 V when in use. Lower resistances in the divider decrease this effect at the expense of more current.

However, the easiest way to get a 2.5 V reference is to use a 2.5 V reference chip. These are intended for the application, and will have much better accuracy than a 7805 followed by a divider with two off the shelf resistors.

Answer 3

Although it's not designed as such, a 7805 (the L version rather less so) contains a bandgap reference and a divider made from matched resistors so its actually not a horrible reference provided you don't draw much current from it and the source is reasonably well filtered- for many purposes it's actually okay, especially if you calibrate out the initial inaccuracy (which may eliminate other system errors). It's not a 3ppm/degree C precision reference but plenty good enough for measurement of humidity, ambient comfort temperature and similar, so it really depends on your requirements.

Edit: here is the typical temperature stability of the TI 78M05

/Edit

The problem with a divider (and this would apply with a precision 5V reference as well) is that many ADCs have a requirement for a low source impedance so the divider could affect accuracy, sometimes in subtle ways.

For modest accuracy, a TL431 shunt reference may be a better choice, or any number of precision series or shunt 2.5V references.

Answer 4

If you want a precision reference voltage of 2.5V, you are better off buying a purpose-built AD584 precision voltage reference. You can pick one up packaged on a circuit board with filtering capacitors and jumpers and even a "calibration certificate" (in quotes because it is not officially traceable to a reference standard, and in many cases appears not to be even an actual measurement of the device) from Amazon for under $20. Some of the units come with the AD584LH, which is specified as ±2.5 mV at 25ºC (0.1%) and 10ppm/ºC deviation from there. Lots more real-world information available here courtesy of Mark Hennessy.