opamp output offset problem

[seyyah]

I have a circuit like in the attachment. What i want to do is adding 2.5V offset to the input signal. But the rms value of the output is a bit lower than input but there is no clamping on the signal. What may be the problem? My 2.5V reference? or other thing. I use this opamp:
**broken link removed**

 

[Karthikeya]

seyyah
you are using a single supply opamp so you have to take care about that. what type of signal is the input? is there a dc level at the input? if you want the same rms level at the output then give no dc offset...that should work

regards

 

[seyyah]

Sorry, the problem wasn't at the configuration of the opamp. It was the sensor's configuration. I changed it and didn't check. Now it is ok.

 

[instruite]

Just a piece of advice
As I can see from your circuit Opamp is dual opamp
So inspite of directly connecting the voltage divider to the input of opamp (for 2.5V reference) use the second opamp as buffer in between voltage divider and the input signal (It will help you avoid errors due to resistor tolerances and lodaing effect)

 

[seyyah]

Like this? If so thank you for your suggestion. I have also another question. With a low frequency signal the output of the signal is good with the previous (resistor divider voltage reference one) configuration but at a 10 khz signal output is noisy. What may be the reason of this?

 

[instruite]

Yeah that is the configuration I suggested.
Regarding your noise problem you didnt mentioned if your new circuit (with opamp at reference voltage side) works OK at 10Khz
Normally while interfacing sensors we use filter at the input side of the circuit (point where sesnor is connected) and also a low pass filter at the output depending on the required operating range of frequency.

 

[seyyah]

I must try for noise problem... What if i use a voltage reference (e.g. TL431) instead of this configuration? Then i may use the opamp for another job.

 

[noiseless]

what I don't understand is what is advantage the second over the first?

noise will also couple from the power to the input