1V reference - why do we need anything else? - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 20, 2016, 05:00:05 pm
content: I was reading an Analog Devices datasheet for their 1.0 V Precision Low Noise Shunt Voltage Reference and came upon an interesting sample circuit. By combining this 1V ref with an AD8605 op-amp as shown on page 8 of the datasheet (http://www.analog.com/media/en/technical-documentation/data-sheets/ADR510.pdf), we can get any reference voltage >=1V.So, the question is:Why do I need any other reference or multiple references? I can just use the 1V ref/precision low-noise op-amp (trimmed for offset, of course) combo to create any reference voltage that is stable and low-noise. Not only that, but I am no longer limited to common ref voltages steppings (i.e., 2.048, 2.5, 5, 10). I can create any arbitrary voltage I want (>=1 V) by setting the op-amp gain via (low tempco/low drift) resistors.All of this sounds too good to be true - I am clearly missing something, or all of these other fixed voltage references would not exist.

author: ajb
author_rank: Super Contributor
timestamp: October 20, 2016, 05:18:43 pm
content: Why would I want to put a reference, opamp, and a couple of resistors on a board if I can just use a reference of the proper voltage instead? The reference plus the op amp will also have additional drift terms that will need to be considered, so you can't necessarily just trim it once and be done with it. You could use a zero-drift op amp to help with that, but of course that will cost more.For unusual reference values, there are adjustable references available, the TL431 being a classic example.

author: Fungus
author_rank: Super Contributor
timestamp: October 20, 2016, 05:21:24 pm
content: Cost, board space, drift, etc.If you can do it with a single component then it makes everything easier.

author: SharpEars
author_rank: Unknown
timestamp: October 20, 2016, 05:28:20 pm
content: Quote from: ajb on October 20, 2016, 05:18:43 pmTypical Temperature Drift (TL431B) characteristic, making these devices excellentreplacements for Zener diodes in many applications,– 6 mV (C Temp)A device with a 6 mV drift is a reference?

author: Marco
author_rank: Super Contributor
timestamp: October 20, 2016, 05:54:22 pm
content: Quote from: SharpEars on October 20, 2016, 05:00:05 pmGenerally for low voltage measurement 1 reference is all you want.As for why you would want to use something else than a 1V reference. For the non zener references the output voltage is proportional to the value of an internal resistor or ratio of internal resistors, which gives poorer long term stability than the zener based references. For very high voltages the non-linearity of resistor dividers becomes a factor as well and they become so large that the temperature drift becomes hard to control, so you might want a reference directly at the higher voltage.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 20, 2016, 06:12:36 pm
content: Quote from: SharpEars on October 20, 2016, 05:00:05 pmAnd how much money are you going to have to pay on high precision resistors and a zero drift op-amp to make that higher voltage and still meet the same accuracy, drift, and noise specs? Why would you go through that trouble when you could spend half as much on a reference that already runs at the voltage you need?

author: zapta
author_rank: Super Contributor
timestamp: October 20, 2016, 06:20:10 pm
content: In software we call it 'syntactic sugar'. That is, language features that are not really required but they make life easier.One can argue that a 3V reference voltage is a syntactic sugar because we can build one around a 1V reference.

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2016, 01:23:40 am
content: Quote from: SharpEars on October 20, 2016, 05:00:05 pmThe ADR510 is not the best or even good in every characteristic; it has higher noise and more drift than an inexpensive higher voltage reference and using a gain stage will just make the noise worse unless a lot of extra complexity is added.Where the ADR510 will be useful is in low voltage medium accuracy systems which cannot support a higher precision high voltage reference.Check out the almost 40 year old LM10 which includes a 200mV (!) reference and operational amplifier useful down to a supply voltage of about 1.2 volts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2016, 06:32:04 am
content: Most refs, internally, are 1.24 or 2.48V with an op-amp setting the exact output voltage. So, they already solved your problem of having any voltage you want. They even put the resistors in there for you.Tim

author: Legionary
author_rank: Contributor
timestamp: October 21, 2016, 06:57:30 am
content: Smaller package requiring less considerations for board isolation, noise, temperature drift, etc. Not to mention a significantly nicer tolerance calculation.That and at bulk quantities they start costing about the same.