AC µV amplification - gain x1000 - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: September 22, 2022, 07:21:29 pm
content: I am looking to make a circuit to feed 0-50µV AC into a downstream which requires more like 0-50mV.Obviously at simulation level, a 2-stage op-amp circuit works, with ca. x32 gains.However in practical terms with a 10kHz sine wave in it tracks perfectly once above 30µV in, but below that exhibits progressive 'noise' with a floor with input shorted of ca. 11mV, not unexpectedly perhaps for a breadboarded trial with an LF353... I've decoupled between stages as well as on input and output.What I'm curious about are:1. With proper PCB design and a shielded case what might I expect to see in lowering of residual noise?2. Am I barking up the wrong tree using 2-stages and op-amps?3. Should I be looking at instrumentation amps?4. And/or when it comes to making it, an opamp like the LT1007/37?5. Or even a discrete FET input stage?6, The final design will be battery powered (dual 4.5V rails) - are my current lab PSUs likely to be injecting noise?Sorry for so many questions - not used to working at this sort of signal level.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 22, 2022, 07:29:54 pm
content: Remember that the signal to noise figure is set at the first stage. You can only make it worse after that! What's the necessary bandwidth? You might use a 3 or 4 pole lowpass filter to knock out noise above the passband. What's the impedance? That will determine what type of opamp you should use for best performance.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2022, 07:41:56 pm
content: The layout is usually not responsible for noise. The layout may influence interference and maybe DC dritf (from thermal effects) and a tendency for ringing.The PSU can inject noise, especially if the layout is poor. This would usually be more limited frequencies (e.g. SMPs switching frequency or resonances that cause ringing). So there is a way to tell appart noise and many types of interference. Usually OP-amps are not that critical with the supply.The LF353 is not especially low noise. Which type / kind is better also depends on the source impedance.For the 11 mV noise, it depends on the bandwidth. It may help to limit the bandwidth to what is actually needed. How much depends on the use and how critical the noise is.2 stages are perfectly OK. If may just work with 1 stage but this already requires a rather fast (high GBW) amplifier. The LT1037 may be just fast enough for single stage.

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2022, 12:49:26 am
content: Availability is a bit of an issue these days, I was just checking Mouser for audio op amps and found the OPA1677 is in stock at $1.23, is JFET, and has 4.5nv/(Hz^0.5) or better than 5 times the noise performance of the LF353. Might be worth a look if you need JFET inputs, otherwise for best noise performance go with a bipolar input op amp. e.g. the OPA1612, bipolar input, has 1.1nV/(Hz^0.5)

author: alm
author_rank: Super Contributor
timestamp: September 23, 2022, 08:59:26 am
content: Don't forget about current noise. That's why Kleinstein was asking about source impedance. Source impedance (Rs) determines the impact of current noise (In): In*Rs. The higher the source impedance, the more important current noise becomes. You should balance current and voltage noise depending on the source impedance. SeeThis AD tutorial on opamp noise.For example, the OPA1612 has better voltage noise than OPA1677, but current noise is a couple of orders of magnitude worse.

author: Marco
author_rank: Super Contributor
timestamp: September 23, 2022, 09:41:20 am
content: How close are you to the rails? Is the output loaded by a resistor?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 23, 2022, 09:56:43 am
content: Please post a schematic with what you already did, the goal specs, and the destination of the circuit.Without a schematic of what you already did, and without numeric specs goals, you won't get many helpful answers, only generic advises.

author: jonpaul
author_rank: Super Contributor
timestamp: September 23, 2022, 11:10:19 am
content: This is similar to a microphone preamplifier.Depending on the source ( bal, unbal, Zo) normally a transformer at the input for impedance matching, voltage step-up and CMRR.You should define the source much betterJon

author: Marco
author_rank: Super Contributor
timestamp: September 23, 2022, 12:37:55 pm
content: Crossover distortion seems the most likely culprit, load down the output more and/or to the negative rail instead of circuit ground if applicable.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: September 23, 2022, 02:15:13 pm
content: Helpful replies thank you all.I will now formulate my circuit more carefully and then revert back with results, details and schematic etc. once I have done a more thorough analysis so as not to waste people's time.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 23, 2022, 02:22:29 pm
content: See Dave on this subject.Shielding can only reduce interference, noise picked up from the environment, not the circuit its self.A low levels its going to be needed. Use a groundplane on the PCB if you make one. Put your projects it in a diecast box.You will need clean power supplies.I'd be looking at low noise audio amps eg Have a read of the LT1115 datsheet.You can parallel up opamps to get lower noise. The individual noise contributions are not correlated so they tend to cancel.Using more opamps has it limits, the law of dimishing returns applies.https://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdfThis pdf from TI has some good advice on gain distribution.http://dicks-website.eu/noisecalculator/index.htmlhttps://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html