DC offset & ampl. of AC signal with single supply op-amp

[AbusiveDog]

Hi I have the following situation:

3.3v line
GND line
input signal of +/- 250 mV
(gain of 12-13 needed to feed into A/D)
op-amp supports 2.7V+ supply and single supply

The signal comes from a microphone, is connected to 3.3v via a resistor and has a cap in series toward the output. ( **broken link removed** view the second diagram )

Offset null seems to do absolutely nothing, I think you always need a dual supply for the offset null to work.

I searched the forums but nothing seemed to help me
what's the best way to offset the signal before amplification?

coolio, thanks in advance, and don't mind my nickname, I'm neither abusive nor a dog.

 

[betwixt]

Re: DC offset & ampl. of AC signal with single supply op

You could try holding the input pin of the amplifier at half supply voltage by connecting it to equal value resistors (47K for example) between ground and the supply line. It really depends on the rest of the amplifier configuration.

Brian.

 

[LvW]

Re: DC offset & ampl. of AC signal with single supply op

As BETWIXT has mentioned, the classical way for signal amplification using only one single supply voltage is to bias the non-inv. input with 0.5*Vsupply.
However, it is important that the closed loop gain for DC is unity!
Therefore, you need two capacitors: One to couple the input signal into the pos. input node and the 2nd one to connect the voltage divider of the feedback path to ground.
Forget everything about offset correction!!

 

[AbusiveDog]

reply to BETWIXT: Yeah, that's what I tried, basic V divider. but, as far as i know, AC signal sees any DC as basic gnd and it's just effectively 47k||47k to ground.
I am using the op-amp as a non-inverting amplifier(I want to use the high input resistance of the op-amp to my advantage).

I made a voltage divider on the non-inv line of an op-amp to pull the input up and tried to amplify it using normal inv amplifier setting. that didn't work.

Added after 19 minutes:

ok, I tried LvWs recommendation...
strangely enough I get a little under 3V DC out, no matter what input I have (signal generated sine wave or mic.)
maybe my values are too high? :
2 x 100k for V divider
1 uF for both caps
560k and 47k for amplification.

 

[LvW]

Re: DC offset & ampl. of AC signal with single supply op

strangely enough I get a little under 3V DC out, no matter what input I have (signal generated sine wave or mic.)

Of course, you get a dc voltage at the output.
That´s the only goal of the bias circuitry!
For a single supply you have to select an operating point in the middle between Vsupply and ground. Thus, you also need a cap for output coupling.

 

[AbusiveDog]

Re: DC offset & ampl. of AC signal with single supply op

Hi, after giving the op-amp a rest and working on other project parts I got myself an MCP601 Op-Amp, It is specifically made for pre-ADC amplifications.
I am using this circuit:

http://www.ee.nmt.edu/~thomas/ee321/op-amp-std-ninv.html
(first Non-inverting Summer )
where
V1=input, Vin
V2=3.3V
R1=100k
R2=10k
R3=2.2k
R4=47k
and 3.3V / 0V on the supply rails.

according to my calculations I should be getting Vo=10.5Vin+1.65 , ie. exactly what I want.
but it just clips right up to 3.3v.

I don't really understand what you guys mean with output coupling... I want an AC signal on the output with an offset of 1.65V and amplification of around 10 - putting a capacitor would remove the DC and so with any offset i am trying to get.

I honestly am lost now, I have no clue how to proceed. This looked like the perfect solution, but it doesn't work .

 

[frankliner]

Your gain must be <= 1.65/.25 = 6.6 otherwise you will clip the signals.