# Need help with Op-Amp DC Offset

#### David Smith_1506015375

Joined Sep 21, 2017
2
Hi all,

I need to design a circuit to answer an EE assignment question. The main requirement is to apply a positive or negative DC offset to a potentiometer-supplied voltage. The assignment has an op-amp theme.

* DC offset must cover pot's entire output swing i.e. 5V pot output can have a -5V DC offset applied.
* DC offset must be controllable from a buffered control voltage e.g. DAC (I'm assuming buffered is to prevent an easier solution such as this: https://norcim-rc.club/Radio3_files/image012.jpg)

Interpreted schematic for question:

DC Offset control circuit:

My initial thoughts were to use two opamps - one configured as a summing amplifier and one as a differential amplifier, to control the positive and negative offsets. Implementation below.

Whilst this works, it has the following drawbacks:

* Requires two control voltages so doesn't meet requirements
* Seems overly complicated - gut feeling is this can be achieved with a single opamp.

I'm currently stuck on how to overcome these. I suspect the top stage can be removed by injecting the negative offset voltage between the bottom stage's R8 and ground, but I'm still left with two indepentant control voltages.

Any help would be much appreciated.

Kind regards,
David

Last edited:

#### crutschow

Joined Mar 14, 2008
27,718
The main requirement is to apply a positive or negative DC offset to a potentiometer-supplied voltage
DC offset must cover pot's entire output swing i.e. 5V pot output can have a -5V DC offset applied.
The meaning of those two statements is not clear.

Do you need to apply up to a ±5V offset to the 0 to +5V pot voltage?
So if you apply a +5V offset the output will be +5V to +10V with a 0 to +5V pot voltage, and a -5V offset will give a -5V to 0V output?

#### David Smith_1506015375

Joined Sep 21, 2017
2
@crutschow to clarify, the output voltage must stay within the original pot range, with anything outside of this being clipped i.e:

1) Pot voltage of 5V + DC offset of 5V = output of 5V
2) Pot voltage of 0V - DC offset of 5V = output of 0V
3) Pot voltage of 2.5V + DC offset of 2.5V = output of 5V etc.

This was confusing me also, and had to query it with my lecturer. With my current progress you can easily have excess voltage on the non-inverting input of U1B, which I guess is not good practise / unsafe?

Apologies, hope this is now clear.

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#### crutschow

Joined Mar 14, 2008
27,718
the output voltage must stay within the original pot range, with anything outside of this being clipped
The simplest way to do that is to use a rail-rail op amp powered by 5V and ground.
Then the output will be automatically clamped between 5V and 0V for any combination of inputs.
With my current progress you can easily have excess voltage on the non-inverting input of U1B, which I guess is not good practise / unsafe?
A series resistor with Schottky diodes to the +5V and ground should protect against excess voltage if that's a problem for the particular op amp you use.

#### MrAl

Joined Jun 17, 2014
8,475
Hi all,

I need to design a circuit to answer an EE assignment question. The main requirement is to apply a positive or negative DC offset to a potentiometer-supplied voltage. The assignment has an op-amp theme.

* DC offset must cover pot's entire output swing i.e. 5V pot output can have a -5V DC offset applied.
* DC offset must be controllable from a buffered control voltage e.g. DAC (I'm assuming buffered is to prevent an easier solution such as this: https://norcim-rc.club/Radio3_files/image012.jpg)

Interpreted schematic for question:

View attachment 244858

DC Offset control circuit:

View attachment 244861

My initial thoughts were to use two opamps - one configured as a summing amplifier and one as a differential amplifier, to control the positive and negative offsets. Implementation below.

View attachment 244863

Whilst this works, it has the following drawbacks:

* Requires two control voltages so doesn't meet requirements
* Seems overly complicated - gut feeling is this can be achieved with a single opamp.

I'm currently stuck on how to overcome these. I suspect the top stage can be removed by injecting the negative offset voltage between the bottom stage's R8 and ground, but I'm still left with two indepentant control voltages.

Any help would be much appreciated.

Kind regards,
David
Hello,

I think you should look into the use of the inverting input of an op amp to act as a summing junction. This makes the problem very simple. You could look up an op amp voltage summing circuit or something like that.
The basic operation is that you apply two voltages and the summing amplifier sums them and nothing can be simpler. For example, if you apply 2v and 1v they sum to 3v. If you have two arbitrary voltages V1 and V2, the output would be V1+V2.
Now in the context of this problem, one voltage would be the original 0 to 5v signal and the other voltage would be the 0 to 5v offset voltage unless you need plus and minus offset and then it would be -5v to +5v for the offset (second) voltage.
There will be some loading effect on the two pots but you can make up for that with a one-time gain adjustment.
This technique requires the use of the inverting input which of course inverts the output so an additional stage would be required to invert the output unless you could invert the inputs then you would not need that.
If you could not invert the inputs and you dont want to add an additional stage, you can use the non inverting input as a summing junction the calculations are just a little more complicated.