Adding DC offset to filter

[stickjam]

Probably a really dumb question, but my head's not operating too clear on cold meds. 

Given the 12dB/oct inverting filter as shown on the upper right of this illustration:

http://www.discovercircuits.com/PDF-FILES/LowPassFilt1.pdf

...what is the best way to add a -5V DC offset to the resulting output without affecting gain and filter characteristics and without adding another op-amp? 

Resistor to the inverting input (what value?) from 5VDC (+ or -?)?    or -5V on the non-inverting input?


Thanks!

-Bob

 

[JohnRoberts]

If the input is DC coupled and coming from 0V, connect your filter + input to -2.5V.  If the input is cap coupled, connect the + input to -5V.

JR

 

[abbey road d enfer]

Adding a res in the inverting input would change the loop gain of the opamp; as a result, the frequency and slope would be altered.
You can reference the non-inverting input to a negative voltage. Since the DC loop gain is 2, you should inject -2.5V.

 

[stickjam]

Thanks.  Sounds like the non-inverting input injection is the way to go.  That would take away possible interaction, as there are a ton of filters that need to be offset that way.  Ninety-one to be exact.  The bias voltage makes things click on the X-5.

b3groover knows what I'm 'a talkin' 'bout!    8)  ;D