creating +/- 2.5v with a uC

[mrx79]

Hi,

i'am having a bit of a design problem here.
What i want to do is just add two voltages with a non inverting summing OPV circuit. I have +5v and -5v as voltages available.
So far so good.
U1 voltage is an input voltage between 0-5v and and U2 will be an control voltage which should offset's U1 by a few mV up to +/-1v.

And now the part where i have a problem with:
U2 should be the output of a PID controller which i would like to implement in a uC (avr most likely). So the uC must output +/-1v.

I thought about another non-inv sum OPV circuit where i offset a 0-5v voltage from the uC by -1v or -2,5v, which would give me +/-2,5v max. But i'am looking for another method which could reduce the amount of OPV's in the circuit.

If somebody has a more elegant idea, i'am glad to hear it.

[dannyf]

U1....U22 should be the output

Nice circuit you have there,

So the uC must output +/-1v.

No.

If somebody has a more elegant idea, i'am glad to hear it.

A charge pump would do it. Whether it is elegant is another question.

[mrx79]

Sorry for the bad quality, but hope this will illustrate what my idea is.
U2 is the input which will be fed by the uC and should be capable of voltages between +/-1 or +/-2.5v

[Zero999]

Providing the op-amp is powered from the appropriate power supply voltage. It's possible to bias and scale the output to any range you like.

See the following link:
http://www.ti.com/lit/an/sloa097/sloa097.pdf

[jaxbird]

This will give you what you ask for, but you need to consider current draw, ripple, regulations etc.

[Zero999]

The ICL7660 is probably one of the easiest ways to give a low current negative supply but I didn't think that was the problem.

http://www.intersil.com/content/dam/Intersil/documents/icl7/icl7660s-a.pdf

[mrx79]

Hi,

the negative supply voltage of the OPVs is not my problem. My problem is, that i need to create the input voltage U2 for my circuit with a µC which can be between -2,5v ... +2,5v.

Positivie voltage range is not the problem, but how do i create the negative one's with the µC?

Thanks

[dannyf]

Easy: Vout=5v/2 - Vmcu,

where Vmcu belongs [0, 5v].

[Zero999]

Positivie voltage range is not the problem, but how do i create the negative one's with the µC?

You don't. The 0 to 5V can be fed into an op-amp circuit which subtracts 2.5V from the signal.

I posted a link to a document which shows you how to do this a couple of posts ago:
https://www.eevblog.com/forum/projects/creating-2-5v-with-a-uc/msg574786/#msg574786

[mikerj]

If you are already using a micro and either onboard or external DAC for the PID controller, could you not handle U1 with the same micro, i.e. feed it into an ADC pin and have the micro sum it to the PID output?  All you then need to do is scale the DAC output.