How i can measure and view AC signal under 5 milliVolt

nima_1981 · May 2, 2014

Hi
I have a RIGOL oscilloscope and i can view ac signal and measure that voltage by micro Controller (atmega32)
But when that signal become under 5 milli volt my oscilloscope can not show that signal and also i can not read that signal by Micro Controller , Please Help me to View and measure that signal .
Did You have any Deferential Micro Volt meter Schematic ??

Audioguru · May 3, 2014

Why don't you use a low noise preamp to amplify the signal so it can be seen on the 'scope and measured by the microcontroller??

KlausST · May 3, 2014

Hi,

amplifying is one solution, but if your signal is noisy then you also amplify the noise.

To get around this:
You my use analog filters (lpf, hpf) combined with the amplifierc to get rid of some noise.

If your frequency is known you can use mathematical solution in software like filters, correlation, dft to get reliable values of very small signals.

Hope this helps.
Klaus

Edit1: some spelling

poorren · May 3, 2014

what's purpose to measure that signal, to check time domain waveform or check else?

if that is for waveform, the low noise amplifier is the best way.

nima_1981 · May 4, 2014

poorren said:
what's purpose to measure that signal, to check time domain waveform or check else?

if that is for waveform, the low noise amplifier is the best way.

Hi guys
Thank you for reply only i want measure RMS that Signal . but i can not best solution's for that i use ad620 but that is not liner . did you have any solution for measure that signal ??
and that small signal you see it its deferential voltage across the resistor , this is milli ohm meter or micro ohm meter .
and about that noise i think that's only error of my oscilloscope .

eemert · May 4, 2014

You can use LT1028 as well, there is an example called "Low Noise, Wide Bandwidth Instrumentation Amplifier" in the data sheet. I guess this would work fine to measure a differential signal.

nima_1981 · May 5, 2014

eemert said:
You can use LT1028 as well, there is an example called "Low Noise, Wide Bandwidth Instrumentation Amplifier" in the data sheet. I guess this would work fine to measure a differential signal.

Hi emmert
is that shematic ??

1) How i can change that gain (its 1000) and what is that formula ?
2) I can not find lt1208 in our country , can you say to me another part number that same as Lt1028
thank you

KlausST · May 5, 2014

Hi,

Lt1028, yes. I've used them, they are very low noise.
You say milliohm shunt, then you need an opamp with low voltage noise, usually with bipolar input stage.
(Not with fet input stage), A good layout design is also important when you amplify that small signals.
Lt1028 has about 0.85 nV / sqr(Hz) and a very low 1/f corner frequency.

There are many opamps with low voltage noise. TI, ST, AD....
Do you have access to distributors like farnell or digikey in your country?

Klaus

- - - Updated - - -

Hi,

One more thing.

Maybe one side of your shunt is connected to gnd, then you can use a one opamp solution instead of the 3 opamp instrumentation amplifier.

Klaus

nima_1981 · May 5, 2014

And what about formula of gain in that solution?

eemert · May 5, 2014

Hi nima_1981,

I suggest you to look up Linear Technology Design Note 355 called "Op Amp Selection Guide for Optimum Noise Performance", i think this document is very helpfull.

About the gain, I can not say the formula directly by looking up, most plausible solution is the testing the circuit in LTspice. (software is free of charge)

KlausST · May 5, 2014

nima_1981 said:
And what about formula of gain in that solution?

You want me to do your homework?

Look for "noninverting opamp circuit".

I'm sure you can solve it by your own,
and be proud of you.

Good luck
Klaus

nima_1981 · May 7, 2014

please help me for design a industrial amplifier with op amp with 3 gain 1- 10 - 100
as i say i found this but i can not change gain i need 3 gain also i can not find how i can change tha gain of this structure .

thanks

Audioguru · May 7, 2014

nima_1981 said:
please help me for design a industrial amplifier with op amp with 3 gain 1- 10 - 100
as i say i found this but i can not change gain i need 3 gain also i can not find how i can change tha gain of this structure .

It is an ordinary instrumentation amplifier that is shown in books, in school and in Google.
I found this one in Google where they tell you how to simply change the gain:

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