Electronics > Beginners
Strange step in amplified signal (with a differential opamp circuit)
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petert:

--- Quote from: Zero999 on August 29, 2019, 09:17:48 pm ---
--- Quote from: petert on August 29, 2019, 01:03:37 pm ---The last post is currently over my head, I have to admit.
--- End quote ---
I can explain in more detail if you like.

--- End quote ---
Thank you, I will probably go back to the more low level details once I am more familiar with the higher level issues.

I experimented more with various (also lower) resistor values and it seems that the sine wave is clean, but the added noise remains, when chaining the two opamps.
It also happens at low frequencies of 4 kHz and less, so it is most likely due to the chaining of the two opamps, and picking up some noise along the way, since the single opamp amplifier did not have these issues.

Adding capacitors (220µF) around the power supply did not change anything either.

So I suppose the easiest way is to find an opamp that can provide the necessary gain in one stage.

For future reference, a link explaining some of the datasheet terminology: http://www.learnabout-electronics.org/Amplifiers/amplifiers64.php

Other useful op-amp links (also to prevent oscillation, second one in German):
https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/gain-equations.php
https://web.archive.org/web/20130828131053/http://elektronikwissen.net/opamp/9-opamp-wissen.html

Av (Voltage Amplification), dB and other terminology in context:
http://www.learnabout-electronics.org/Amplifiers/amplifiers13.php
magic:
Time to learn some noise calculations :)

LM358 doesn't specify noise, but the "improved" LM358B does and its performance is probably similar.
You use gain of 100 per stage, so full gain bandwidth is 10kHz because of 1MHz GBW.
Input noise density of LM358B is 40nV/rtHz. Multiply this by square root of bandwidth and we get 4000nV RMS noise from DC to 10kHz.
Multiply this by the magic number 7 and we get peak to peak value of noise which is not exceeded for over 99.9% of time. That's 28µV.
Furthermore, for various reasons, noise density increases at lower frequencies. The datasheet quotes a total of 8µV p-p in the range 0.1~10Hz.

Looking at the screenshots, your input signal appears to have amplitude comparable with LM358's input noise. So it's not very surprising to see noise roughly equal to the signal at the output too.

You need a lower noise opamp. Of the cheap ones, NE5532 has almost 10x less noise. A cascade of two NE5534 would offer some marginal further improvement. OPA2209/OPA2210 cuts it in half and LT1028/LT1128 halves the noise again if very low resistances are used throughout the circuit. Price increases accordingly.

To get 10000x gain at 100kHz in one stage it takes a 1GHz opamp. I'm not saying such things don't exist, but they tend to be pricey and noisy too.

And really, LM358 is a crap chip. It shouldn't be used unless you need the particular combination of low price, moderately low power consumption and the ability to pass signals down to the negative rail. Oftentimes it can be upgraded with something like RC4558, NE5532 or TLC/TS272. Or with more expensive parts. Sadly, it seems that there is no "LM358 done right", with tolerable input noise, better GBW, no crossover distortion and working down to the negative rail. And still cheap :)
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