Adding DC offset to a 0V to 7V sinewave

[radiance]

Hi,

I want to add DC offset to a 0V to 7V sinewave using a non inverting opamp configuration something like this https://tinyurl.com/yhkw43as (simulation of my my failed attempt)
On the output of the opamp I want the sinewave to be from 7V to 15V.
Is this the way to do this?

 

[radardoug]

Yes, that should work.

 

[radiance]

Is there a way to calculate the resistor values? (opamp is on -15V & +15V supply btw)

 

[Bo Deadly]

The 10K isn't needed and it will actually skew the output. But the reason it's going flat on the top is because it's clipping the supply.

An op amp is going to put out whatever it needs to to make both inputs the same. So if both resistors were say 10K, then if you have 0V on the non-inverting input and put -1V on the inverting input, then the output will go to +1V because that is what will make the inverting input 0V.

So if you put -15 on the inverting leg, it will put out +15 but now there is no room for your sine wave. So you'll have to reduce the amplitude of your sine wave and / or offset to keep the full wave within +-15V.

 

[radiance]

... So you'll have to reduce the amplitude of your sine wave and / or offset to keep the full wave within +-15V.

Thanks, I'll try a voltage divider in front of the opamp.

 

[trashcanman]

I might be wrong but I think one problem with using a non-inverting config is you're amplifying as you offset. If you have two opamps available you can use two inverting stages and get 7Vpk. the ratio of R2 and R3 below determines the offset amount.

 

[neil.johnson]

Do you need the input impedance to be very high? If not, try a diff amp with gain.

 

[abbey road d enfer]

Hi,

I want to add DC offset to a 0V to 7V sinewave using a non inverting opamp configuration something like this https://tinyurl.com/yhkw43as (simulation of my my failed attempt)
On the output of the opamp I want the sinewave to be from 7V to 15V.
Is this the way to do this?

It depends on what type of constraint you have.
Can the sinewave source work with low-ish impedance? What degree of precision do you need? Do you want adjustability?
Can the offset be derived from the negative rail? Do you want to minimize interaction between elements?
The solution that ticks all the boxes is an inverter stage. Supposing you have a 10k FB resistor, you'd inject the sinewave with a 10k, and a negative voltage via a potentiometer/trimmer and another 10K.
Actually if you use a Fet opamp, you can substitute 220k resistors.
However, a 7Vrms sine peaks at 10V. When you add 7V dc offset, the opamp runs out of juice because it hits the positive rail. You should raise the positive rail to about 18V minimum, so you must make sure the opamp tolerates that. You may, if possible, increase the positive rail to let's say 20V, and decrease the negative rail to -5 to -10. This can be done easily by connecting a zener of 5-10V in series with the negative supply pin.

 

[Newmarket]

(opamp is on -15V & +15V supply btw)

On the output of the opamp I want the sinewave to be from 7V to 15V.

I see I crossed over with Abbey on this. I was going to raise the possibility of asymmetric voltage rails but didn't.
I hadn't considered it being 7Vrms - I don't know if this is what the OP meant but yes it would be the 'normal' AC meaning.

Whatever the circuit solution you go with - to get +15V at the output you will need a positive voltage rail >15V.
You can get "Rail to Rail" output opamps that will get very close to the rail. But these are optimised for that and generally have drawbacks for audio design.
How much headroom you need depends on the opamp itself and the output current it needs to supply to the load on its output (also frequency but assuming 'audio' frequencies that shouldn't be much of an issue.
In general, using standard opamps driving a light load, then +/-18V rated opamps will be okay and you can run them at a bit less for a safety factor. +/-17V is typical.

 

[radiance]

Thanks for all these replies! I have to say, this concept of DC offset is something I'm learning...
To give some context: I have this NE566 VCO IC that can be fed with a CV signal between 8V and 15V (3/4 VCC, vcc is +15V ) want to use this IC on an aux card for the Buchla 208, which has CV signals ranging from 0V to 13,5V.
SO I need to attenuate the 13,75 V to 7V and raise the offset so it sits between 8V and 15V.

With all info so far I should be able to achieve this . Thanks all for thinking along!

 

[abbey road d enfer]

8V and 15V (3/4 VCC, vcc is +15V

???

 

[ccaudle]

Do you have to pass DC values? If not, a coupling capacitor on the input might be easiest, you can just shift the input reference from 0V to 10.5V and have the output centered around 10.5V instead of 0V. As others have pointed you either need an op-amp that can drive to the positive rail, or you will max out the output at around 13V-14V depending on model of op-amp and how much current it needs to drive.

 

[radiance]

This CV signal send from the Buchla 208 to the aux card will be used both by the NE566 and also to set the sequencer step count. I was under the impression that when using an opamp it would also act as a buffer stage...

 

[radiance]

???

Input can be between 3/4 VCC and VCC.

 

[radiance]

https://www.egr.msu.edu/eceshop/Parts_Inventory/datasheets/lm566.pdf

 

[abbey road d enfer]

NE566 VCO IC that can be fed with a CV signal between 8V and 15V (3/4 VCC, vcc is +15V )

3/4 of 15 is not 8.

 

[abbey road d enfer]

I was under the impression that when using an opamp it would also act as a buffer stage...

It's not "an impression", it's actually true.

 

[Bo Deadly]

That's a pretty typical synth style offset / scale circuit like this one from electro-music.com:


But in your case you're going to want trimmers to adjust the amplitude and offset. And you'll want to add resistors in series with both so that the trimming is much more accurate. And because you're offset is going be centered between 11.25V and 15V (13.125) you can put the offset trim between ground and -15. Also, 100K values are a little high. Back in the 70's that might have been appropriate (probably not) but a rail-to-rail op amp (like TLV2172) with 10K would work dandy. And if you don't care about or don't want the CV being inverted, you can drop the second amp.

 

[radiance]

3/4 of 15 is not 8.

Yes, my bad...this (3/4 VCC) is what's in the datasheet. However, I did measure a CV signal going into the NE566 and discovered it worked well down to 8V. Sorry for the confusion!

 

[radiance]

That's a pretty typical synth style offset / scale circuit like this one from electro-music.com:

View attachment 84426
But in your case you're going to want trimmers to adjust the amplitude and offset. And you'll want to add resistors in series with both so that the trimming is much more accurate. And because you're offset is going be centered between 11.25V and 15V (13.125) you can put the offset trim between ground and -15. Also, 100K values are a little high. Back in the 70's that might have been appropriate (probably not) but a TL082 with 10K would work dandy. And if you don't care about or don't want the CV being inverted, you can drop the second amp.

Thanks for this!