OPA376 low pass filter circuit question versus OPA205 - Page 1
Forum section: electronics / beginners

author: Overspeed
author_rank: Frequent Contributor
timestamp: March 27, 2025, 07:38:34 am
content: HelloI use OPA 2025 ( 205( low pass filter circuit , I have located a similar Low Pass circuit ( same frequency ) with an OPA376 I dont understand the circuit at the input + as they connect to half V+ valueMost of the filter are signal input and ground I thinkCan someone can explain this particular circuit ? as V+ is not mandatory a low noise voltageRegardsOS

author: coppercone2
author_rank: Super Contributor
timestamp: March 27, 2025, 07:49:15 am
content: I think it has something to do with offset. If you put a single supply chip on AC it does not like that. If it has AC coupled through anything it will be around zero and then you get clipped. But I don't know why the point you circled is not groundedjust put another rail and forget about that!Maybe they mean if you want to adjust the bias at that point without causing any disturbance to the signal offset? But then again, its linked on the schematic.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2025, 08:08:00 am
content: The OPA379 is an inverting filter. So the output DC signal is inverted. In this example the V+/2 is a kind of virtual ground and should also be used for other parts of the circuit. They could have better used a dual supply and have the non inv input at ground, though this an unusual configulation with an OP-amp like OPA379.

author: tszaboo
author_rank: Super Contributor
timestamp: March 27, 2025, 08:45:25 am
content: So if your Opamp circuit is working with Bipolar power supply, + and -, you can connect the input of an inverting amplifier to ground. If your circuit is single supply, + and ground, you cannot do this. Obviously the output should be negative with any positive input, but it cannot be, because you don't have negative supply. Then you make a reference voltage, V+/2 and reference all signals to that.More info on this:https://www.ti.com/lit/an/sloa030a/sloa030a.pdf

author: Overspeed
author_rank: Frequent Contributor
timestamp: March 27, 2025, 09:06:47 am
content: HelloMain concern on a filter is the noise , by using V+ that will add noise in the signal so that strangeAnd there is no offset explanation in the manufacturer doc as V+ value is not specified.RegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: March 27, 2025, 09:15:49 am
content: Quote from: tszaboo on March 27, 2025, 08:45:25 amHelloThanks for explanation , that sound logical regarding voltage , but strange regarding the noise result and thanks for the linkI will try with bipolar supply as OPA376 is cheap and compare noise with the 2205RegardsOS

author: Terry Bites
author_rank: Super Contributor
timestamp: March 27, 2025, 04:58:57 pm
content: The lowpass bandwith directly affects the circuit noise.The required opamp specs can be ascertained from that.A lower spec part may well meet your needs.Expense does guarantee performance.Do the maths. Ther are a lot of on line noise calculators for opamp circuits.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2025, 05:44:49 pm
content: The circuit shown are not specific to the OP-amps. Both circuits would also work with the other amplifiers.The OPA376 is a FET input type with much more 1/f noise than the OPA205. So they are not that comparable.If cost is an issue there is the OPA202 as a low cost precision amplifier.There is anyway the noise from the resistors, that may swamp the amplifier noise.For the sallen key LP filter it still helps to have a reasonable fast amplifier, as there can be some loading effect that bypasses the amplifier via the feedback path. Here a faster amplifiers can have lower output impedance that can make a difference. Depending on the use case, I would consider going 3 rd order with an addition passive RC part against MHz range signal getting to the output.