Can a single supply op amp be used for a summer?

--- Only when Vout is referred to Vin.

In the case where Vout is referred to 0V (your case, I believe) and Vin varies, then Vin must be added to Vout.

For example, more clearly annotating the circuit:

E1 | [R1] | +--->E2 | [R2] | E3

and solving:

(E1 - E3) * R2 E2 = ---------------- R1 + R2

for various values of E3 results in the following table with R1 set to

4, R2 set to 1, and E2 referred to E3

E1 E3 E2 VOLTS VOLTS VOLTS

------|--------|-------- 5 0 1.0 5 1 0.8 5 2 0.6 5 3 0.4 5 4 0.2 5 5 0.0

That's nonsensical for your application, which I believe refers the output of the divider to ground instead of to Vin.

Such being the case, we can redraw the circuit to include the ground reference,:

E1>----+ | [R1] | +------>E2 | [R2] | E3>----+

GND>---------->0V

rewrite the equation in order to include the ground reference,:

(E1 - E3) * R2 E2 = ---------------- + E3 R1 + R2

and wind up with:

E1 E3 E2 VOLTS VOLTS VOLTS

------|--------|-------- 5 0 1.0 5 1 1.8 5 2 2.6 5 3 3.4 5 4 4.2 5 5 5.0

Which is what you want, yes?

On 8 oct, 07:56, Andy wrote: SNIP

1-

Andy, You have been abstracting this and not seeing the forest for the trees, as the saying goes. Your best bet now is to take some resistors, a multimeter, and a power supply that you can vary the voltage output of. Once you get your fingers in a breadboard with some resistors on it and your eyes on a meter readout, it will bring you back to reality. Build the circuit. Vary the input, test the output. Even if you don=B4t have a variable power supply, you can vary the voltage somehow. Put some 1.5 volt cells in series, or put some diodes in series with the output of a six volt battery. Whatever. I just suggested a couple of kluges that will give you a low impedance voltage source. Attach your power supply=B4s positive output to the bottom of the resistor string. Then measure the output voltage with respect to ground of your power supply. Michael

(E1-

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All, I think these forums are an excellent source of information. But it should also be quite apparent that communications in these forums is often imperfect, and due to this a lot of assumptions are made. We don't have the privilege of knowing one another with respect to background and/or skill level, and any assumptions made about users in the forum are almost certainly incorrect. These forums will be most valuable if we can focus on the questions being asked, not on what we perceive to be the skill level of the asker. AG

This is not a 'Forum'. It is a Newsgroup, and part of Usenet. You are accessing it through Google's crappy HTML interface.

--
The movie \'Deliverance\' isn\'t a documentary!

--
That\'s not true, since the answers must be constructed in a manner the
querant can understand.

Because these are technical fora we often assume at least a basic level
of technical competence in the querant when we answer their questions.
Sometimes, however, as in your case, we find that that assumption was
unjustified and have to get more and more basic.

Unfortunately, instead of digging in and finding out why you\'re having
trouble or asking for help with what you specifically don\'t understand
about what was explained to you, you try to set yourself up as a judge
and excuse yourself by saying that the trouble isn\'t with you, it\'s
because the presenter isn\'t presenting the material properly, or the
presenter doesn\'t understand the circuit, or the math, or some other
nonsensical crap.

I, for one, went to a good deal of trouble to try to help you out and
was extremely polite in the bargain, since this is
sci.electronics.basics, (where there are no stupid questions) while you
decided to play your little "I know what I\'m doing and you don\'t" games.

Are you still stuck, or has what we\'ve all been telling you, so far,
sunk in? 

JF

Yes, use a summing op amp configuration. But you inputs are the scaled versions of the two signals using voltage dividers.

so

Vin ---R1+------R3---* | R2 | GND

Vcc ---R4+------R6---* | R5 | GND

(use fixed width font to view)

*'s are connected to the op amp in the summer configuration

R1 and R2 form a voltage didivder scaling down Vin. So a 4:5 ratio.

R4 and R5 scale down Vcc to 1V so a 1:5 ratio.

R3 and R6 are your "weights" for the op amp summer.

So proceede as normal.

The idea is simple. You simply add the divided signals together. 0.8*Vin +

0.2*Vcc. To do scaling you use voltage dividers(an op amp confugred as an amplifier is an "active" voltage divider).

You must choose the resistors appropriately for this to work. Not just the ratio's but the relative magnitudes.

,

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forest for

ven if you

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JF, I sincerely apologize if I offended you, or anyone for that matter. It is not in my interest to offend. However, the tone of your response is exactly what I was referring to in my previous point. This kind of misunderstanding is typical in these groups. And by the way, I did make a point of personally thanking your for your efforts. But this thread...read it. It's like we are all speaking different languages. Please excuse me, but I cannot seem to make a statement which comes across as I mean it to. I will not participate further.