Op Amp Full Wave rectifier with Hi Pass question - Page 1
Forum section: electronics / beginners

author: alexg
author_rank: Regular Contributor
timestamp: June 25, 2018, 02:37:49 pm
content: 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 = 20kR2, R3 = 100kThank you.

author: Benta
author_rank: Super Contributor
timestamp: June 25, 2018, 02:50:49 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: June 25, 2018, 11:56:25 pm
content: 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

author: alexg
author_rank: Regular Contributor
timestamp: June 26, 2018, 03:17:07 am
content: Quote from: Benta on June 25, 2018, 02:50:49 pmThanks, 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.

author: alexg
author_rank: Regular Contributor
timestamp: June 26, 2018, 03:20:59 am
content: Quote from: rstofer on June 25, 2018, 11:56:25 pmif 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.

author: Audioguru
author_rank: Super Contributor
timestamp: June 26, 2018, 03:23:09 am
content: I do not see how the circuit you found can work properly.You do not need a virtual ground if a single supply opamp (maybe a lousy old LM358?) is used in a circuit like this one that works:

author: alexg
author_rank: Regular Contributor
timestamp: June 26, 2018, 03:37:46 am
content: Quote from: Audioguru on June 26, 2018, 03:23:09 amIt just does, its a single supply circuit so diode is not required.

author: Audioguru
author_rank: Super Contributor
timestamp: June 26, 2018, 03:52:05 am
content: Quote from: alexg on June 26, 2018, 03:37:46 amYour schematic has no opamp part number.In the circuit I posted of course the diode is needed. When the input is positive then the inverting opamp output is 0V. Then the output of the second opamp is positive only when the 200k resistors feed its (+) input and the diode prevents the 0V from passing to the (+) input of the second opamp.The circuit you posted allows the (+) input of the first opamp to be more negative than its negative supply voltage (0V) which is not allowed. Also the second opamp has the same voltage on both its inputs frequently giving an output that could be anything.

author: alexg
author_rank: Regular Contributor
timestamp: June 26, 2018, 04:02:11 am
content: Quote from: Audioguru on June 26, 2018, 03:52:05 amThe op amp I am using is MIC862 here is the datasheet:http://ww1.microchip.com/downloads/en/DeviceDoc/20005836A.pdf, If you can suggest any changes to the schematic that will make it better considering that particular op amp that would be awesome. So far a protoboard test works okay.

author: Audioguru
author_rank: Super Contributor
timestamp: June 26, 2018, 04:35:43 am
content: Like all opamps, the opamp you are using has an absolute maximum negative input of only -0.3V when its negative supply (Vv-) is 0V.In the circuit I posted, the inputs NEVER go negative (unless they have a very high level) so use this opamp in the circuit I posted.

author: alexg
author_rank: Regular Contributor
timestamp: July 02, 2018, 04:22:12 pm
content: Quote from: Audioguru on June 26, 2018, 04:35:43 amI am just curious what effect will this have on the op amp of you let negative input go below -0.3V when supply is 0? It seems to work but does it mean that I am slowly killing it with every impulse below -0.3V ? I know datasheet says "Exposure to maximum rating conditions for extended periods may affect device reliability." and I understand that but I am curious if I am just lucky to have circuit that works fine even beyond max ratings or I am slowly killing op amp?

author: Benta
author_rank: Super Contributor
timestamp: July 02, 2018, 05:03:00 pm
content: Actually, I've seen this kind of opamp rectifier before. Certain amps phase invert when the input goes below zero. IIRC, is was the CA3130.

author: Audioguru
author_rank: Super Contributor
timestamp: July 02, 2018, 10:13:16 pm
content: Your input voltage is exceeding the maximum allowed input voltage so of course you are damaging the opamp.

author: alexg
author_rank: Regular Contributor
timestamp: July 22, 2018, 10:00:05 pm
content: I built this circuit using following surface mount components:Two resistors, 100k 0.5% bothDiode: NSVRB521S30T1G datasheet:http://www.onsemi.com/pub/Collateral/RB521S30T1-D.PDFOp Amp: MIC 862 datasheet:http://www.microchip.com/mymicrochip/filehandler.aspx?ddocname=en580715Op amp's power pin is decoupled with 10uF and 0.1uF and whole thing is powered with clean Lab power supply 3.3VI get weird results, the wave does get rectified, however positive half of the wave has voltage drop somehow, I cannot figure out where does it come from, do I have a bad Op Amp choice, did it matter that I used 100k resistors in stead of 200k? there is not much else going on in this circuit, need help.At the illustration with 1kHz sine wave on input, it comes form waveform generator, directly into "input" in original schematic.

author: oPossum
author_rank: Super Contributor
timestamp: July 23, 2018, 03:14:09 am
content: Schottky diodes have high reverse leakage. Try a silicon diode like 1N4148 or 1N914 instead.

author: alexg
author_rank: Regular Contributor
timestamp: July 23, 2018, 02:18:03 pm
content: Thank you, placed an order at Digikey and will test as soon as it arrives later this week.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: July 23, 2018, 06:04:29 pm
content: This is best known precision rectifier:5 Resistors, 2 diodes, 2 OpAmp, nothing less.http://www.eeeguide.com/precision-full-wave-rectifiers/https://www.allaboutcircuits.com/worksheets/precise-diode-circuits/Did you Search for "precision rectifier" here on Forum ???

author: alexg
author_rank: Regular Contributor
timestamp: July 23, 2018, 06:19:58 pm
content: Quote from: JacobPilsen on July 23, 2018, 06:04:29 pmWill this rectifier work with single supply?

author: JacobPilsen
author_rank: Regular Contributor
timestamp: July 24, 2018, 05:05:03 pm
content: Sorry, i was unfortunately overlooked, that single supply is demanded.If you change your mind, I would recomend using voltage inverter, not an artificial ground.Quote from: alexg on July 22, 2018, 10:00:05 pmOne half of the wave is amplified by first stage, while second half of the wave has voltage drop across the second resistor.

author: alexg
author_rank: Regular Contributor
timestamp: July 26, 2018, 03:12:01 pm
content: Quote from: oPossum on July 23, 2018, 03:14:09 amWorked like a charm, thank you.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 03:32:18 pm
content: Quote from: alexg on June 26, 2018, 03:37:46 amThe diode is required, otherwise the op-amp's lower output transistor will turn on, taking the output voltage to zero.Quote from: alexg on July 02, 2018, 04:22:12 pmQuote from: Audioguru on July 02, 2018, 10:13:16 pmThat depends on the op-amp. The LM358/324 has protection diodes and if the input current is limited to a safe level, with series resistors, as is the case with the schematic posted previously, then nothing bad will happen. The input diodes will simply conduct, causing the input to sit at 0.6.-0.3 was specified on the data sheet because, that's the maximum negative voltage the inputs can be connected to, over the entire temperature range, without causing a significant current to flow through the protection diodes.

author: alexg
author_rank: Regular Contributor
timestamp: July 27, 2018, 02:05:58 am
content: Quote from: alexg on July 26, 2018, 03:12:01 pmWell, if I heat the board from 25degrees to about 40degrees, upper half-wave is shorter again I suppose because diode's reverse current leakage increases, oops.

author: oPossum
author_rank: Super Contributor
timestamp: July 27, 2018, 04:06:02 pm
content: I think changing the 200k resistors to a lower value will reduce that problem. There will always be some small difference between the two halves of the sine wave because a perfect diode does not exist and that circuit is sensitive to reverse leakage.

author: alexg
author_rank: Regular Contributor
timestamp: July 27, 2018, 04:22:44 pm
content: Quote from: oPossum on July 27, 2018, 04:06:02 pmAt the time of measurement those resistors were 20k already so I suppose I will have to deal with waveform difference at higher temperatures.