Shifting a signal with an op-amp

Other Hardware General Electronics
1

Dear all,

I'm studying a little bit about electronics and right now I'm experimenting with op-amps.

So far I have read about using an op-amp in the summing non-inverter configuration to shift a signal (or to add an offset) as follows :

In this case, my first question is: How can I select the resistors appropriately?

I made the selection for the resistors based on circuits I saw on google, but I'd like to know how to do it right.

The circuit works fine with a low impedance source, but if I want to vary the input via a potentiometer I'm unbalancing the resistance at the inputs and so, the output won't be as expected as it can be seen in the following simulation:

I guess I could use another OP-AMP to buffer the signal and deliver a low impedance signal, at the moment my input is positive and that will work, but what if I'd like to operate with positive and negative inputs and shift the signals a certain amount (2~2.5V) with no amplification and a positive single supply for the OP-AMP?

So, my second question is: How should I handle those cases?

Thanks in advance!

2

Hi,
Have you tried a lower value of potentiometer R23, like 1K0?

Tom... :slight_smile:

3

Hello Brio,
The (+) op amp input is very high in resistance.
Just simplify the voltage sources, adding them up
via algebra. Then the resistors form a voltage
divider which you can calculate easily. R23 just
makes V7 variable. Does that help you?
Herb

4

There are a lot of application notes available on this, starting with this one by Maxim: https://pdfserv.maximintegrated.com/en/an/AN4836.pdf Here is a good explanation of your circuit: Level shifting a +/- 2.5V signal to 0 - 5V - Electrical Engineering Stack Exchange This is a good one by TI: http://www.ti.com/lit/an/sboa038a/sboa038a.pdf These will keep you reading for a while and some even have links for more data. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

5

Thank you very much for your replies.

Regarding the second question I just wanted to provide an example of what could happen if the resistors aren't well balanced due to a high impedance source... are there any other techniques to shift the voltage?

And going back to the first question... How do you select the resistors for the inputs and the loop?

Cheers!

6

Use an inverting summing circuit, summing to a virtual ground, then the different inputs cannot interfere with each other.

7

MarkT:
Use an inverting summing circuit, summing to a virtual ground, then the different inputs cannot interfere with each other.

Dear @MarkT, I'm not sure I'm understanding your suggestion... could you please sketch a drawing?

If I use an inverting summing circuit wouldn't it imply the use of a negative voltage to feed the op-amp?

Regards