remove dc offset from signal going into an op amp

Jon Bean1

Other Parts Discussed in Thread: OPA2625, THS4131

I have a signal that varies between 19V and 17V @ 250kHz. The signal operates in a burst pattern where it outputs 2048 values then stops for a few ms. I want to input this signal into an op amp and remove the dc level. I have a circuit that has a dc blocking capacitor between the signal and the op amp input. The op amp is set as a buffer, and also has a resistor to ground. So I have a high pass filter on the input of the buffer. The issue I have is that I am finding that the circuit takes a long time to settle if I select values that maintain the signal integrity. If I then change the values to speed up the settling time the signal starts to degrade. Are there any other options that will maintain the signal integrity whilst giving a fast settling time? I am open to using a completely different circuit. Thanks

over 5 years ago

0 Ed Walker over 5 years ago

Prodigy 5 points

Hello Jon,
What is the part number of the op amp being used?

0 Jon Bean1 over 5 years ago in reply to Ed Walker

Expert 1120 points

Hi Ed

Its the OPA2625

Thanks

Jon

0 kai klaas69 over 5 years ago in reply to Jon Bean1

Guru 140200 points

Hi Jon,

can you show us a scope plot of the signal?

Kai

0 Michael Steffes over 5 years ago in reply to kai klaas69

Guru 48395 points

Hello Jon, would you consider a differential output. FDA's can remove DC offsets easily in a DC coupled signal path.

0 Michael Steffes over 5 years ago in reply to Michael Steffes

Guru 48395 points

And actually Jon, you would use a part like the THS4131 to level shift your input to 0V centered and then only use one side of the output as a single ended in to single out DC adjustment.

Would have to set up for a gain of 2 to get a net gain of 1 if only using one side of output.

0 Jon Bean1 over 5 years ago in reply to Michael Steffes

Expert 1120 points

Hi Michael

Can you explain how I would connect the op amp.

Thanks

Jon

0 kai klaas69 over 5 years ago in reply to Jon Bean1

Guru 140200 points

Hi Jon,

again, can you show a scope plot of your signal?

Kai

0 Michael Steffes over 5 years ago in reply to Jon Bean1

Guru 48395 points

here you go Jon, see if this makes any sense,

essentially using the FDA common mode loop to do a DC coupled level shift. It produces a differential output, but you can just use one side - works well it seems. Don't know your load, but match them on each output side.

DC coupled level shift of 250kHz square wave from 18V centered to 0V centered.docx

0 Jon Bean1 over 5 years ago in reply to kai klaas69

Expert 1120 points

Hi Kai

Thanks for your reply. Having looked at Michael's solution I think that should perform as I want.

Jon

0 Michael Steffes over 5 years ago in reply to Jon Bean1

Guru 48395 points

Pretty cool yeah?? I had not done this previously, but can't let those common mode control loops go to waste - if you swings are not as known as you implied, added work here could adjust for input CM and Vpp variations.

0 Jon Bean1 over 5 years ago in reply to Michael Steffes

Expert 1120 points

Thanks for the solution Michael. The output comes from a sensor which has a max output of 19V and min of 17V. So I think it should work. I will get some parts and prototype it.

Jon

0 Michael Steffes over 5 years ago in reply to Jon Bean1

Guru 48395 points

did you want to end up bipolar around ground? easy to adjust up if you want to with the CM control input.

0 Michael Steffes over 5 years ago in reply to Michael Steffes

Guru 48395 points

Also, I had set this up to produce a bipolar swing around that 18V midscale - if the nature of the sensor is that you are interested in excursion above 17, simply change the reference voltage into the other side to 17V, etc.

0 kai klaas69 over 5 years ago in reply to Michael Steffes

Guru 140200 points

Hi Michael,

but this method can only remove the DC offset, if you know the DC offset and if it remains constant. What with a pulse train of changing duty cycle and by this changing DC content?

Kai

0 Michael Steffes over 5 years ago in reply to kai klaas69

Guru 48395 points

So yes, it is a bit deterministic - but your implication of issues with changing duty cycle is back to an AC coupled idea. This DC coupled approach will not have any issue with duty cycle changes.

I normally worry a bit about using one side of an FDA output at higher F as the single sided HD is pretty hideous on HD2 - only gets good with the other side matching it and cancelling out. However, at this low frequency, probably will be acceptable. Just need to keep each side matched from an impedance perspective - which means whatever the source looks like impedance wise, try to match that on the non-signal side.

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remove dc offset from signal going into an op amp