About Instrumental amplifier noise suppressing  - Page 1
Forum section: electronics / beginners

author: sezus
author_rank: Newbie
timestamp: September 19, 2023, 03:31:52 pm
content: Hello everyone, I am new here. Previously i asked about some problem with instrumentation amplifier I am building. I have a question about suppressing the noise of the signal using instrumental amplifier. I want to use instrumental amplifier circuit for amplifying the underwater sound signal from the hydrophone. Underwater environment is very noisy, so i thought it would be great idea to use instrumental amplifier. Of course after preamplification i will use bypass filter, but if it is possible to somehow suppress the noise using instrumental amplifier, it would be great.I dont know how should i connect the output of the hydrophone to the amplifier, should i just connect + to V1 and - to V2? Will it help to filter the signal from the hydrophone?Sorry for stupid questions

author: Stephen_Pejak
author_rank: Newbie
timestamp: September 19, 2023, 03:38:44 pm
content: Instrumental amplifier will amplify whatever signal comes into it. Consider using bandpass filter both before and after your amplifier. Also add some 100nF and 10uF capacitors in parallel with power supply(both for each op-amp and close to the op amp). This will help filter some electrical noice as well.

author: sezus
author_rank: Newbie
timestamp: September 19, 2023, 03:50:41 pm
content: Thank you for your fast and useful reply. So if it will not filter the noise then there is no need to use instrumental amplifier? Is it better just to use a single noninverting opamp?Did I understand correctly, if the signal put into the instrumentation amplifier initially has noise, then we will not be able to filter it, but if we put a clean signal and then it became noisy inside the circuit, then we can filter it out?

author: Terry Bites
author_rank: Super Contributor
timestamp: September 19, 2023, 08:04:58 pm
content: What kind of noise do you want to lower?Intrinsic circuit noise or external noise?That three opamp thing doesnt work well in practice.For RF interference use a common mode choke and some bypass caps. See attached.C1=C3 C2=10xC1Anyway as a rough and ready approach....Match the signal source impedance to an opamp. (Tedious)Rn=Vn/In see data sheets.Use a similar Rn resistor value similar for setting the gain. Same order of magintude is good enough.See a

author: sezus
author_rank: Newbie
timestamp: September 20, 2023, 10:17:45 am
content: Thank you for your useful reply. i have some questions about impedance matching. Why do I need to match the impedance?Do I understand correctly that a transformer is needed for impedance matching? How to choose the right transformer?Thank you in advance.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 20, 2023, 12:17:14 pm
content: No. The transformer is not a matching ciruit, it rejects "common mode" RF interference from external sources.A real input signal will have a a source impedance (resistance) just as a battery has internal resistance.This resistance generates noise. Vn=E=√4⋅R⋅k⋅T⋅ΔF or Vn=7.43√R⋅T⋅ΔF . k is Boltzmann's constant, ΔF is the bandwith you want to measure over. Say20Hz-20KHz for audio.You can use matching transformers to lower the R seen by the INAmp inputs. You'll most often ecounter this in RF matching. Transformers were often used in microphone amps but not so much these days.The noise can be calculated as they are for any other opamp circuit.https://www.coilcraft.com/en-us/edu/series/a-guide-to-understanding-common-mode-chokes/