# voltage adder

Discussion in 'General Electronics Chat' started by cmoxey, Jun 6, 2008.

1. ### cmoxey Thread Starter New Member

Jun 6, 2008
3
0
Hello,
I have a project that I am working on that needs some help. What i have is a oscillating voltage varying from .2 volts to .9 volts. I would like to be able to add in a voltage of .2 to .9 volts... I know that this is possible, but I am just not that versed in the circuit design.

Thanks in advance

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Perhaps you can sketch and post a rough diagram of the signal in and the signal out.

hgmjr

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Take a look at this chapter in this site's E-book:
http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
It describes averaging and summing circuits using opamps. If your AC input is very low in frequency or has a DC level component, you'll need to use something like an opamp summer/summing circuit.

If your AC input is a moderate to high frequency and you don't care about a DC level component, you could use something like the attached circuit.

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4. ### cmoxey Thread Starter New Member

Jun 6, 2008
3
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thanks for the info... I guess I need to give you a little more info... What I have is a dc voltage that varies between .2 and .9 volts.. and I would like to be able to ad a user specified voltage somewhere between 0 and .9 volts dc to be able to bring the original voltage up a little bit. Here is a picture of what I am looking for... Thanks

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5. ### SgtWookie Expert

Jul 17, 2007
22,182
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Attached is one possibility, in simplified form.

R1 and R2 set the gain for the op amp at 3. Gain = (R1+R2)/R1 = (10k+20k)/10k = 30/10 = 3/1 = 3
R3 and R4 are the summing resistors.
Note that R4 actually represents two resistors; slightly less than 10k to Vee, and slightly more than 10k to Vcc. The adjustment will be quite sensitive. You may wish to use a 21-turn trim pot, or perhaps a 1k pot with 9.5k resistors on either side of it.

The precision of the resistors is important.

You will need a bipolar supply, or a rail-to-rail input/output op amp in order to use this circuit.
The LF353 op amp is roughly equivalent to a TL072 low-noise JFET-input dual op amp.
The TL082 might also be used, but it will add more noise to the signal.
All three of these opamps are basically audio-spectrum amplifiers.
You haven't mentioned the frequency you're dealing with.

• ###### ACoffset.PNG
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Last edited: Jun 6, 2008
6. ### cmoxey Thread Starter New Member

Jun 6, 2008
3
0
Thanks,
I will try that and see how it works.