Summing offset

[dr.power]

Hi guys,

I have got a question.
I have an altenate wave going to an inverting op amp for amplification by factor of say 30. Now I need to Sum the said alternating signal by some amount of DC offset. Suppose that the alternating signal is "Vm*sinwt, so I want to get "Vdc + Vm*sinwt" at the output of the op-amp. I know that I can use a second op-amp as a SUMMING op-amp so that add the dc offset which is generated by a pot? But Do I HAVE TO use the second op-amp as the summing op-amp? IS'nt it right If a mix the pot to the first op-amp somehow to get
"Vdc + Vm*sinwt"? It will help to avoide using a second op-amp as summing, But I am not sure if it is effective just like using the second op-amp? Of course I am able to add a second op-amp as the summing if there is any problem with the other method I am taliking about.

I myself thinking of using just the said inverting op-amp and use its noninverting pin to add the said dc offset. Any idea plz?

Thanks in advance

 

[alec_t]

But Do I HAVE TO use the second op-amp as the summing op-amp?

No. You can add the offset at the first. Do you want the DC offset amplified by 30 also?

 

[dr.power]

No. You can add the offset at the first. Do you want the DC offset amplified by 30 also?

Hi Alec.
No I do not want to Dc offset to be amplified.
We can just use a NON inverting op-amp instead of an inverting one and use a cap in seriese with "Ri" of the op amp so that let the op-amp to have a gain of 1 for DC values, But then I do not know how to add the dc offset to the op-amp...

 

[crutschow]

Two ways to add a DC offset:

1) Use an inverting op amp with a second input resistor in parallel with the first. If you make its value the same as the feedback resistor, the gain from that input will be -1 for any input signal, DC or AC.

2) Use an inverting op amp with a capacitor in series with Ri, as you noted. Input the DC at the plus (+) input which has a gain of +1 for DC.