Sonsivri

Hi All,
What is the best way to remove the DC offset from a signal? (15-20V DC (not constant and will be in this range), 1 VPP signal overlayed on top of it) I am trying to get the 1VPP out of it.

Regards,
odsk

Tons of different ways to do it..  If you're feeding a Micro-Controller (PIC, AVR, etc), probably the easiest way is to resistor divide it (say max is 21V - divide by 4.2 gives you 0-5V).. Read divided voltage into A/D and then subtract the offset in software.

That's just one example.  Tell us what you want to 'do' with the signal and I'm sure myself or someone else can help you further.

PPtk

Simplest way to do this is capacitive coupling but depending on the nature of useful signal it could be useless. In case of video signal (1Vpp) it can be used.

frist: friquency of ac signal ? if high enough, use a capacitor  ;D ;D ;D
second: if very low friquency driff, use op-amp cmrr mode to clamp DC, but u need a referance voltage.

Thanks Gents for your replies,

Frequency is 40K. the signal contains digital data as bits sent serially and I am going to use a PIC 16f877a to try and read this data.
I am favoring more the way PilotPTK mentioned with a small addition:
1 digitize the incoming signal
2 smoothing on the signal with an LP filter, and read it
3 do 1-2 (to remove the offset dynamically).

regards,
odsk

Opto Isolator may be extremely useful here.

Thanks,
 but How? I thought about that before but if I get the voltage+signal to the diode how it will make the on/off for the signal to pass.
odsk

Hi. You can use HCPL7840 analog opto-couplers.

Best Regards.

If offset is consistent enough, a zener diode in series with the LED portion of the Opto Isolator would do the job.

Use capacitive coupling and clamper:
http://www.tpub.com/content/neets/14181/css/14181_214.htm
http://www.tpub.com/content/neets/14181/css/14181_215.htm

Opamp subtraction might be another option. It can be done with a single opamp, near infinite input resistance and offset to be subtracted can be varied by a trimpot or something.

Regards...

if frequency bandwilth not too big maybe use high frequency transformer, that little trans always in modem nearby phone jack.
trans + differantial pair opamp could do the job.

I think first you have to isolate your circuit from line there are three ways:
1- Transformer isolation : Like the one used in dialup modem or ISDN Line
2- Opto isolation: Like CPC series from CLARE Co.
3- Capacitance isolation:

but in your application I think transformer is the best try products from MidCom Company or Pulse.
www.midcom-inc.com
www.pulseeng.com

Capacitive coupling sounds fine for the application, as far as I understand it, and it will be by far the easiest solution to implement.

If its digital, couple it to half your digital supply voltage (like 2.5V for a 5V supply) then just feed it into a schmiddt trigger buffer, or a comparator if you need a specific amount of hysteresis.

Thanks all for your help.
I built a circuit that have a capacitor coupling with  Lm2901 comparator and the extracted signal look fantastic  :).
Thanks again for your help.
ODSK

If the signal can not allow distortion, you also can use a op amp form a subtracter to extracted it .

Is this anything different than what I suggested

http://www.sonsivri.com/forum/index.php?topic=27008.msg91046#msg91046

or are u seconding my suggestion?

Regards...

You may try this circuit:
U1 will remove the DC offset, but inverts the signal
U2 will restore the right polarity
Good luck

Hi there, correct me if I am wrong.
You have an analog signal that has 1Vpp DC offset. You digitalize this signal through a ADC and you process the values in serial stream. Assuming that I've got this right, assuming the conversion resolution is given (lets say 8bit) and the offset symmetric you could "remove" this offset mathematically through your MCU processes. Ofcourse I don't have more info about your circuit or the way you process the data but if you can do that it saves you extra components without compromising the accuracy of your conversion.
So the main idea. If your ADC provides a negative value you can add the mathematical absolute value of the offset divided by 2 (0.5V digitalized depending on your resolution) on the measured value and have the true value w/o the offset. Vise versa on positive value you can substract that absolute value etc etc.
It will require more code and I don't know what's your flexibility on that but if it could be implemented in your case:
1) Saves space on PCB
2) Requires less components
3) It's more accurate (if offset doesn't change)

As I see my info is kinda outdated but may be in use somehow :)

Best regards,
Spyros


#Edit# Mistakenly I quoted wrong msg. Sorry about that :/

oh, sorry, I didn't read all reply before I post...so my post isn't repeat your suggestion.
just same think.