Measuring very very small voltage fluctuations?

[Magician]

I didn't say it won't, It will works with dual power line +V, GND, -V. Next, as it outputs AC, you will have add one more cap at the output, and two resistors to create DC offset required for arduino ADC be capable to process negative halfwave.
 

[MushfiqSarker]

Since it seems as if you want fluctuation on a sine wave which peak to peak is the same, you can ignore the DC biasing on the Op Amp.

If you connect the Op Amp to VCC and GND with a non-inverting amplifier setup where the positive input is GND, you will get a half wave rectified output.

Thus if you are sending it a 0.005 sine wave it will get amplified to a 0.005*(gain) half wave and then you can read in the half wave via your ADC.

This takes out the trouble of DC biasing.

Mushfiq

[Grumpy_Mike]

Yes but you have to have a much more expensive rail to rail op amp. And then you need to protect he input of the op amp to prevent damage when the input goes negitave.

[MikeX]

Grumpy_Mike, why won't something like an LM124 work? If it brings the voltage up above 2.5, the Arduino should see that as high, and it should be good enough, no?

[Grumpy_Mike]

the Arduino should see that as high,

I thought you wanted to measure the fluctuations not just detect them? After all it says measuring in the title.

[MikeX]

If I can count the frequency of the wave, it will be a good start. 

[Grumpy_Mike]

You can measure just the frequency better using a comparator like an LM339, but you don't say what you expect it to be. My guess is that it is either at mains or it is random so it won't get you very far.

However I thought you said:-

as it's a sign wave. I would like to capture the wave with enough resolution to plot it graphically

[MushfiqSarker]

Yes but you have to have a much more expensive rail to rail op amp. And then you need to protect he input of the op amp to prevent damage when the input goes negitave.

A expensive Op Amp is not required. I am using an LMV321 from Texas Instruments right now to do this exact thing with AC 60Hz signals. The voltage inputs range from 20mV to 1V with a 5x gain.

It is working for me. The input will not go negative due to there being no negative voltage present for it to swing too.

[MikeX]

I too think a regular op-amp should be ok in my low frequency problem. Comparator would be the right tool to detect the frequency indeed, but eventually I'd like to get the wave sampled.

So which direction should I look at if an op-amp like LM124 is the heart of the circuit? Should I look at a non-inverting configuration, with the inverting side at ground, then feed the signal through a capacitor into the non-inverting side?

[MushfiqSarker]

MikeX,

When deciding on op amps, you need to check the Common Mode Input Voltage Range. This is a direct quote from Note 10 on the datasheet:

The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3V (at 25°C). The upper end of the common-mode voltage range is V+ ? 1.5V (at 25°C), but either or both inputs can go to +32V without damage (+26V for LM2902), independent of the magnitude of V+.

This range determines how much you can swing safely. V+ is your postive input voltage. Thus, you can swing 0 to (V+)-1.5.

Keep that in mind.

[MikeX]

If my DC wave is between 1.000v and 1.005v, then if I run that through a capacitor, then my swing will be 0.0025v around 0, well within the limits. Am I understanding it right?

[MikeX]

Would something like this work with an LM124 considering how small the swing is? Noise?

[Grumpy_Mike]

If you are feeding the op amp with a single ended supply then no the circuit will not work.
The +ve input will sit at 0V and as it is not a rail to rail output amp, nothing much will happen when the input goes slightly positave.

Use another resistor from the +ve input to +5V to bias the input to a mid rail point.

[MikeX]

Biased like this?

[Grumpy_Mike]

Yes.