Active low pass filter - Weird output - Page 1
Forum section: electronics / beginners

author: Tomer
author_rank: Regular Contributor
timestamp: April 11, 2018, 08:09:03 pm
content: Hi All and thank you for your help.I am trying to implement an active Low pass filter using Opamp LM358N.To achieve that i am following the attached schematics (Scheme.jpg the area with red border).I used different values for the RC filter:330K820PfThat will give me a FC of ~585HzOther values are the same as in schematics.I am feeding the circuit using 1Khz Sine signal with an Amplitude of 1V (Picture 1)I am providing 4.5VDC (Picture 2)I am scoping a very weird output (Picture 3)I must be missing something, maybe with grounds ? I am attaching pictures of the BreadBoard (Pictures 4,5 and 6)I also swapped OpAmps thinking i might damage it during play, but the result is the same.* My potentiometer is 10K Linear.Link to the pictures:https://photos.app.goo.gl/7goatPJql7Tm6ykW2Any help will be greatly appreciatedThank you !Tom

author: jmelson
author_rank: Super Contributor
timestamp: April 11, 2018, 08:55:09 pm
content: Quote from: Tomer on April 11, 2018, 08:09:03 pmIt looks like you got a high-pass filter instead. It may be a slight change in the circuit is all that separates high-pass from low-pass. So,check the circuit carefully.Jon

author: Tomer
author_rank: Regular Contributor
timestamp: April 11, 2018, 09:42:55 pm
content: Hi, Thank you for your replay.I have resistor following by a capacitor to ground. That should be a RC low pass filter ? no ?

author: Circlotron
author_rank: Super Contributor
timestamp: April 11, 2018, 09:58:27 pm
content: You only have a single 4.5vdc power rail. You need a spit supply with equal voltages above and below ground, say 9 volts total.

author: mrkev
author_rank: Regular Contributor
timestamp: April 11, 2018, 10:24:50 pm
content: It looks like you have inverting and non-inverting input of the top amp swapped, probably combined with some power issue as Circlotron wrote.

author: Tomer
author_rank: Regular Contributor
timestamp: April 11, 2018, 10:37:26 pm
content: Thank you for your comments.1) I bumped the power supply to 9VDC but i did not noticed any change.Quote from: Circlotron on April 11, 2018, 09:58:27 pmShould i do something else here ? in my power supply i have 3 outputs "Red +" / "Black -" / "green Ground"2)Quote from: mrkev on April 11, 2018, 10:24:50 pmCan you further explain i am not sure that i understand.Thank youTom

author: mrkev
author_rank: Regular Contributor
timestamp: April 11, 2018, 10:46:17 pm
content: You can do something like two 1k resistors as a divider between VCC and VEE, making the virtual GND.To the inverting/non-inverting input swap, it is really simple. The top amp should be connected the same way as the bottom one, but you have "-" on the filter and "+" on the 10k/1k divider (should be the other way around).

author: Audioguru
author_rank: Super Contributor
timestamp: April 11, 2018, 11:03:49 pm
content: Your very simple filters are not active filters. Instead they are simply an RC passive filter feeding an opamp.Since your power supply is +4.5V and 0V then the inputs of all the opamps must be biased at half the supply voltage so that the inputs and outputs can swing up and down.It is obvious that you have the inputs of the lowpass filter opamp connected backwards.

author: Audioguru
author_rank: Super Contributor
timestamp: April 11, 2018, 11:17:07 pm
content: Since you have a single positive supply then the input, output and feedback resistor to ground should have series coupling capacitors to block the input bias voltage that you must add.Also the negative of your power supply must connect to the circuit ground.Instead you can use a positive supply and a negative supply that was replied about then the opamp inputs can be biased at 0V like you show. Circuit ground is connected between the positive and negative supplies where they join together.

author: Tomer
author_rank: Regular Contributor
timestamp: April 12, 2018, 03:06:00 pm
content: Guys thank you for all the comments.I understood that the original scheme had the + and - of the OpAmps signals the wrong way, i also confirmed it with a book that i have.I apply the change on the breadboard and the output seems to be a bit better now. on the scope.I am not sure that i understand how to feed the OpAmp correctly.Is it possible to supply the OpAmp + and - using my AgilentE3610A, i have 3 outputs "Red +" / "Black -" / "green Ground".Currently the + is connected to Pin8 and the - is connected to Pin4I am feeding with a 1Khz sine , 10mVIn the scope i can see the following pichttps://photos.app.goo.gl/a2Ql0dguGaJqn6Ki1Thank you again for the time and helpTom

author: Audioguru
author_rank: Super Contributor
timestamp: April 12, 2018, 05:01:30 pm
content: Only one of the opamps had its inputs connected backwards.The input and output of an opamp must be able to swing up and down. Your opamps have the (+) input biased at 0V (circuit ground) then the opamp input will be damaged when the input signal goes negative and the output cannot produce the negative swing of the signal resulting in no output or severe distortion. Opamps with the input biased at 0V must have a positive supplyand a negative supply.Since you do not have a negative supply then the (+) input of each opamp must be biased at half the supply voltage so that the inputs and outputs can swing up and down.Here is a schematic of an opamp with a single positive supply with its input biased at half the supply voltage, and an opamp with a positive and a negative supply and also with its input biased at half the supply voltage that is 0V:

author: Audioguru
author_rank: Super Contributor
timestamp: April 12, 2018, 05:11:43 pm
content: Here is the biasing when the opamp is non-inverting like your opamps:

author: Tomer
author_rank: Regular Contributor
timestamp: April 13, 2018, 04:27:08 pm
content: Thank you so much for investing so much time in detailed responses !It took me some time but with your help and additional reading i think i understood it correctly now.I created a voltage divider on my breadboard supplying + and - to the OpAmp and my scope output looks good.Thank youTom