How to reduce the noise of the instrumental amplifier? - Page 1
Forum section: electronics / beginners

author: sezus
author_rank: Newbie
timestamp: September 16, 2023, 10:24:07 am
content: Hello everyone, I am new to electronics. I want to ask for a help. I am constructing a preamplifier for a hydrophone. I decided to make an instrumental amplifier with 20db gain. I am using three OP27 operational amplifiers. I want to make signal analysis so i need to gain the signal without any noise from the preamplifier. After the preamplifier i will put bypass filter.I wanted to ask some questions about my schematics. I want to add some bypass capacitors. Do i need to add only ceramic capacitors or also electrolytic?Do i need to add capacitors on input and output of the schematics?Do i need to add any extra capacitors to reduce the noise?Thank you in advance.

author: MrAl
author_rank: Super Contributor
timestamp: September 16, 2023, 10:39:57 am
content: Quote from: sezus on September 16, 2023, 10:24:07 amReduce resistor values. Megohm values are way too high. Max maybe 100k.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: September 16, 2023, 10:50:03 am
content: Quote from: MrAl on September 16, 2023, 10:39:57 amYes.Inputs have to be referenced to ground, add two 100k from inputs to ground,so the bias-currents are drained.Yes, you need decoupling-/filter-caps. Use different sizes 1n, 100n and 1u and different materials (ceramic, foil, electrolytic).Good luck

author: sezus
author_rank: Newbie
timestamp: September 16, 2023, 10:57:05 am
content: Thank you for your answer. I understood everything about capacitors, but i couldn't get you about input references to the ground. Can you please explain more?

author: Circlotron
author_rank: Super Contributor
timestamp: September 16, 2023, 11:20:39 am
content: Quote from: sezus on September 16, 2023, 10:57:05 amPut a 100K resistor from V1 to ground and another 100K resistor from V2 to ground.

author: Circlotron
author_rank: Super Contributor
timestamp: September 16, 2023, 11:23:16 am
content: What is the maximum frequency you need to analyze? You could put a low pass filter at the output of your amplifier to reduce any noise above this frequency.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 16, 2023, 12:00:54 pm
content: Opamp inputs always need a DC path to GND. Opamps always have a bias (leakage) current, and if that current does not have a path to GND, then the inputs will saturate.And also, it's more common to lower the resistors in the amplification stage by about two orders of magnitude.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 16, 2023, 01:19:44 pm
content: A diy inst amp is usually a bad idea. It looks fine in a text book but its not very practical, without expensive precision resistors it won't work very well.The resistors you'd need will cost far more than a premium ready made amp. eg An AD8429 or a INA849 @ 1 nV/√Hz ! I'm a That fanboy. Check out the THAT 1510/1512The input coupling and bias arangements will depend on the kind of hydrophone are you're using. Not all types need a diff amp.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2023, 01:47:48 pm
content: The right amplifier depends on the source impedance. In the circuit shown the current noise if the OP27 in combination with the 1 M Ohm resistor makes it very noisy. A similar effect can happen at the input side. If you don't need a differential input the instrumental amplifier is not ideal as it has 2 amplifiers and the noise adds up. A simper one sided amplifier could be lower noise.

author: sezus
author_rank: Newbie
timestamp: September 19, 2023, 03:43:59 pm
content: Thank you for your reply. As i know i can use differential input for reducing the noise of the signal. Is it possible somehow to "filter" the signal from the hydrophone using differential input? Underwater environment is very noisy so the signals which i will receive using hydrophone will have a lot of noise. Can i filter this noise using instrumental amplifier or only by using a filter?Sorry if this question is stupid

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2023, 04:28:48 pm
content: A differential amplifier can suppress common mode signals, but it does not suppress or filter noise. In many cases a differential amplifier is more noisy and if not needed one may be better of with a more normal ground referenced amplifier.An important point is to know the source impedance and the frequency range of interest. Underwater sound can cover a large frequency range (e.g. sub 1 Hz to 10s of MHz), depending on the transducer and source.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 20, 2023, 12:28:10 pm
content: Quote from: sezus on September 19, 2023, 03:43:59 pmThis is a bit of a weird question, probably based on a misunderstanding. An instrumentation amplifier can suppress a lot of common mode noise (signals), but that is something completely different.Quote from: sezus on September 19, 2023, 03:43:59 pmThis confirms the misunderstanding assumption. Your underwater environment is very likely extremely quiet. As long as there is no electric eel having an appetite for your gadget, there will be very little electrical noise around your gadget. There may be all sorts of acoustical noise under water, but for your hydrophone it is all just signal. Neither the hydrophone nor the instrumentation amplifier is able to distinguish the acoustical noise from the acoustical signal you want to capture.Compare this with a normal household, where the wiring in the walls creates a lot of mains hum. Just by touching the tip of my oscilloscope probe with a finger, it picks up 12V of 50Hz noise, and If I for example touch an isolated mains wire, that can easily go to 40Vac or more, because of capacitive coupling though the cables insulation.

author: sezus
author_rank: Newbie
timestamp: September 20, 2023, 12:58:50 pm
content: Thank you for clearing everything up, you were very helpful!