Alternative voltage reference?

What's the voltage reference for? (I'm guessing here, but in the schematic do you mean that R11 and R20 are to simulate your potentiometer and that you already have a 2.5V supply for V2? Do you already have 5V and 2.5V supplies already present?)

Jon

Hi Jon, Sorry for not being clear. R11 and R20 are used to simulate my potentiometer. I already have a 2.5V source and a 5V source. The section highlighted in green is the part that I was referencing.

Thanks!

What's the voltage reference for?

Jon

There are series references (essentially 3 terminal voltage regulators) and shunt references (essentially integrated zener diodes) that can be programmed with 2 or 3 resistors to produce clean, stable voltage references, for low current loads.

For example:

How about something like this? It provides a low Z output drive and only a 100uA load on the 2.5v ref in.

|----------- | +5 | 2.50v ------- | | | | |\\| | | ---|-\\ | .-. | >----------- 2.00 - 2.50v | |

Thanks, Mike! This looks like a better setup than the one with my opamp circuit.

example:

Thanks again, John!

Nothing special really. I had some old notes that I was reviewing and one of the exercise questions that was on it was to come up with this type of circuit. Simple really, but I guess it's good practice for the fall semester.

You could use an LM317 with a suitable value pot in series with one of the voltage setting resistors, calculated to give you the adjustment range you want.

Dave.

Hehe. Well, since it doesn't really need to go anywhere and it is about practice, there are a number of options:

Note that there is no use of the 2.5V supply, here. And it is NOT a precision reference and it would be simply horrible over temperature changes -- only good for some +/-10C -- as drift per temperature changes are in the area of something on the order of 5mV/K or more.

Another option would be:

Again, no precision. But better temperature drift -- on the order of

I picked something like 700uA as the desired current through the 1k pot. This was to generate about 0.7V, top to bottom of the sweep of the pot, which means you can at least trap your 0.5V in there with a little extra so you don't have to nail it precisely. In other words, your 1k pot shown will sweep a little wider range than 0.5V. And there is nothing precision about any of these, nor temperature stabilized. The 2.7k*700uA produces about 1.9V, a pedestal upon which the 0.7V range of the pot rests. So roughly speaking, we're talking about a sweep from 1.9 to 2.6 volts. Of course, you can tweek things a little with R5 in the 2nd circuit and R2 with the first, so that you get it centered okay.

For precision sweep and temperature stability, a different topology would probably be used. Of course, you can keep the opamp and use the pot as part of a series resistive divider that feeds the (+) input of the opamp and use it as a unity gain follower.

Jon

Neat. Thanks, Jon! Practice makes perfect, and I need a lot of that. A year ago, I was not very comfortable with opamps. But thanks to what I've picked up here, I'm getting used to them a little better.

Welll i would like to say that there is difference between regulators and reference circuits. i refer to using LM317, its a adjustable regulator and not a reference circuits. But i would add that its better than references made using only resistors. they are no reference circuits at all. First write down what u mean by a reference circuit. Just a voltage output is not the requirements of a reference circuit. It should first be specified like whats the stability that would be required, whats teh temperatrue co- efficient, what should be change in output voltage if the supply voltage changes etc etc.

If you wish to have a fixed reference circuit, u can try LM336 adn things like that.

well, i would like to reference circuits with just resistors are useless in real life. LM317 is good if u want some current to be fed to circuits. its not a reference but a regulator.

I hope i didnot offend anyone with my comments

with regards

manoj