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Topic: Too much noise when reading Amplified Analog Signals (Read 1 time) previous topic - next topic


Dec 12, 2012, 08:27 pm Last Edit: Dec 12, 2012, 08:30 pm by dc42 Reason: 1
At the very least, you need to add a resistor between pin 3 of IC1 and ground to supply its input bias current. Try 1K. The blogspot article doesn't allow for Vin being capacitively coupled.
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Before trying different amplifiers, try and reduce the resistors for IC2 by a factor of 10. I've used 741 before and found that at even 1,000 Hz frequency the large resistors make the amplified signal noisy. You have a scope so you can compare how your signal from the first stage looks like with two sets of resistors. If you do 47K and 1K then you will get the same amplification but hopefully less noise.

Yeah, this is the first thing I noticed too.

Also, the gain of the 1st stage = 48, which seems pretty high, especially when OP has given no
indication of what sort of signals are coming from the guitar ckt. If the gain is too high, the
output of the 1st stage will be saturating, and even noise will be triggering the 2nd stage.

Also, the 1st stage is DC-coupled, so any DC-offset out of the guitar ckt will be multipled by 48.
Probably needs to be AC-coupled.

This sort of thing needs a systems-level design, meaning amplifier design needs to be selected
in conjunction with actual [measured] signal levels.


When everything is cooperating, this circuit can generate periodic pulses with the same frequenncy as the input signal's  fundamental frequency.

No it will not do that. It will pick out the strongest harmonic in the waveform not the fundamental.
The thing is that a guitar has some of the higher harmonics louder than the fundamental for part of its waveform.

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