Opamp bandpass filter too noisy - Page 1
Forum section: electronics / beginners

author: dusan
author_rank: Regular Contributor
timestamp: December 09, 2021, 08:23:46 am
content: I have band pass filter made from opamp (picture in attachment), the peak is around 650Hz, the problem is that it is too noisy. I hear a lot of white noise in headphones (maybe it's not pure white noise, it has some hollowness to it, like a wind). To reduce the possibility the noise is in the input I replaced U1 with 1k resistor and the noise is still there. I also try different opamps (741, lm258, jrc4558, ne5532p, op27g) and also tried polyester caps instead of ceramics. I also tried 2 different headphone amps. It's still there so I think the noise is by design, not bad part. How can I make bandpass filter less noisy?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2021, 09:10:51 am
content: The filter has quite some gain (~500), at least if you count from after the initial divider. I assume the divider (R1,R6) part is just there for tests. If not, this should be the main point for the noise, making tghe circuit high noise by design.For the lower frequencies the OPs input also sees an increasing impedance. So current noise from the OP can contribute at low frequencies. This could be bad for the NE5532 and OP27. The TL071, 741 and LM258 are rather high noise to start with.With so much gain the LM258 is also a bit on the slow side.Holowness / wind like soud may indicate a possible EMI problem, like picking up some RF signal. This should however depend on the OP used.I don't know the MFB filter in detail. The high gain may be needed to get a high Q. Instead of a 2nd order high Q fitler one may consider a 4th order lower Q fitler.

author: iMo
author_rank: Super Contributor
timestamp: December 09, 2021, 09:17:31 am
content: Where your filter's R and C values come from?PS: try with this tool:http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htmYour values show large gain (28dB) and possible ringing (I do assume it is a CW filter)..

author: dusan
author_rank: Regular Contributor
timestamp: December 09, 2021, 10:34:10 am
content: Ok, so which opamp should I use to get least amount of noise?

author: jmelson
author_rank: Super Contributor
timestamp: December 09, 2021, 04:34:47 pm
content: Quote from: dusan on December 09, 2021, 10:34:10 amMy favorite, all around op amp is the OP275. For really low noise, I use the AD706, which should have enough bandwidth for your 650 Hz filter.Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2021, 04:52:34 pm
content: The Circuit is not ideal for low noise. It may be easier to change the circuit, to have less divider at the input than to use a lower noise OP. The OP27 is already quite low in noise. An OPA209 would be slightly lower noise, but that is about the end there. There are OPs with lower voltage noise, but there the current noise than starts to become a problem.It is a bit unusual to have such a high Q BW filter for audio. So it is hard to tell if a different filter (e.g. 4th order and lower Q) may be better suited).Due to the large gain for the OP, the OP needs to be relatively fast. So 500 kHz GBW would be about the minimum for the circuit.

author: KT88
author_rank: Frequent Contributor
timestamp: December 09, 2021, 04:59:33 pm
content: @dusan: The culprit is indeed your circuit. While your signal gain is 30x, the noise gain is 669x. The reason:R6 is part of the noise gain - it doesn't contribute to the sigal gain but amplifies the input noise of the amplifier (resistor noise is part of it too). NG=(Rf/Rg)+1 with Rg= R1IIR6.Look up noise gain as it requires some explanation how it works.Edit: ...this depends on the frequency of course...CheersAndreas

author: Terry Bites
author_rank: Super Contributor
timestamp: December 09, 2021, 05:34:59 pm
content: At 650Hz your phase margin is only 6 degrees- right on the edge of oscillation.Use this toolhttps://www.ti.com/design-resources/design-tools-simulation/filter-designer.html

author: Terry Bites
author_rank: Super Contributor
timestamp: December 10, 2021, 02:16:59 pm
content: Horses for courses. You can go very low noise if you can afford it.Theres always a trade off betwen price and performance. seehttps://www.analog.com/media/en/reference-design-documentation/design-notes/dn355f.pdf

author: Sengcid
author_rank: Regular Contributor
timestamp: December 12, 2021, 05:00:07 pm
content: KT88 is correct. You can simulate the noise gain by setting U1 (main input) to 0V and apply a signal to the non-inverting input.Also, it is borderline stable as with any high Q filter. Have a look at a state variable filter - more opamps but probably more suitable.

author: KT88
author_rank: Frequent Contributor
timestamp: December 13, 2021, 12:11:31 am
content: A good resource is the Analog Devices Filter Wizzard:https://tools.analog.com/en/filterwizard/.It allows you to plot the noise besides many other parameters and creates an appropriate circuit.If you prefer a different brand then you could pick an amplifier that has similar specs.As a rule of thumb the GBW of the amplifier should be about 100x of your upper corner frequency or more, otherwise you will get distortion. If you have to attenuate your signal you will get better resultes if it is done after the filter for obvious reasons...CheersAndreas