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Op Amp Full Wave rectifier with Hi Pass question
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alexg:
Hello, I am building this circuit I found on net and it works fine. It is a full wave rectifier without diodes based on single supply opAmps, works great and I like it, however I want to add a hi-pass filter so that I can reject certain frequencies from the signal (signal is anywhere from 500Hz to 10kHz). I am not solid on what frequency to reject yet but for first build I want to remove anything below 5kHz.
I found many examples of how to implement hi-pass filter for single op-amp whether inverting on non-inverting input and it seems straight forward as shown on illustration#2 (left - inverting, right non-inverting versions) however full wave rectifier circuit I have uses both at the same time and I am not sure how to make that happen in this case, can someone push in right direction please?

In full wave rectifier circuit I am using following values for passives:
R1 = 20k
R2, R3 = 100k

Thank you.



Benta:
First of all, your filter will have to be before the rectifier, otherwise you'll get really weird results.

Second, why only a first order filter? If you have the opamp anyway, second order is free (except for an extra couple of passives).
I'd suggest a MFB filter, they are usually very well behaved.

rstofer:
It seems to me that your filter assumes dual supplies.  That is, I don't see any attempt to bias the input up to half of Vcc before the signal adds or subtracts through the capacitor.  There are a number of ways to create the required virtual ground:

http://www.schematica.com/active_filter_resources/op_amp_virtual_ground_circuits.html
alexg:

--- Quote from: Benta on June 25, 2018, 02:50:49 pm ---First of all, your filter will have to be before the rectifier, otherwise you'll get really weird results.

Second, why only a first order filter? If you have the opamp anyway, second order is free (except for an extra couple of passives).
I'd suggest a MFB filter, they are usually very well behaved.

--- End quote ---

Thanks, well, I am still a noob, so some things you mention are not straight forward understood for me while it makes sense, will be researching things you mention, I did however already tried today to add one more op before rectifier and used high pass filter then send filtered signal into rectifier and it seems to work so far, will play with it more and start optimizing.
alexg:

--- Quote from: rstofer on June 25, 2018, 11:56:25 pm ---It seems to me that your filter assumes dual supplies.  That is, I don't see any attempt to bias the input up to half of Vcc before the signal adds or subtracts through the capacitor.  There are a number of ways to create the required virtual ground:

http://www.schematica.com/active_filter_resources/op_amp_virtual_ground_circuits.html

--- End quote ---
if you talking about two high pass filter schematics on second illustration of original post, those are just quick picks from internet, I did not even think of that until I put a test circuit on protoboard and realized that I am only seeing half wave so ten I biased it to half Vcc and so far so good.
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