Add and remove DC Bias from a signal

Thread Starter

Dollarday

Joined Jan 25, 2012
32
Hi all :)

Quick question: I need to add and then remove DC bias to a data signal.

To remove the DC bias is easy (I plan to use a high pass filter consisting of a resistor and capacitor)

However, how do I add the DC bias to the signal?

I did a bit of reading and the following circuit was found:
http://www.ecircuitcenter.com/circuits/opsum/opsum.1.gif
(The picture is from the level shifter from http://www.ecircuitcenter.com/circuits/opsum/opsum.htm)

However, this is described as a inverting summer,which would invert my data signal (Which I do not want)

I would also like to work from a single supply OP-AMP, so I want to keep my signal above ground. Any suggestions?

Regards,
Dollarday
 

ramancini8

Joined Jul 18, 2012
473
Be careful, the non-inverting inputs interact because they work into an infinite impedance, while inverting inputs do not interact because they work into a virtual ground. See "Op Amps for Everyone"; this book is on the Texas Instruments web site.
 

crutschow

Joined Mar 14, 2008
34,073
You can also add a DC bias by using a series capacitor and a resistive divider to the supply voltage at the cap output. The equivalent resistor and capacitor values just have to be selected so they don't attenuate the frequencies of interest.
 

ramancini8

Joined Jul 18, 2012
473
The two inverting op amp solution works well, but insure the op amps are not in a feedback loop if they are both the same type. In the non-inverting summer shown in the previous post the positive op amp input is assumed to be infinity and the signal and offset inputs are assumed to be driven by zero impedance sources. So, when considering the signal input, R4 is in parallel with R5, and the parallel combination forms the lower resistor in a voltage divider with R3. Considering the offset input, R3 is in parallel with R5. You can write two equations for this situation and solve for exact resistor values, but changes will require more calculations.
 

AnalogKid

Joined Aug 1, 2013
10,950
If the source of the signal is a relatively low impedance such as an opamp or gate output, and the destination is a relatively high impedance such as an opamp or gate input, then you can add DC bias with a coupling capacitor and two resistors. This is what is happening in post #9.

ak
 
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