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Topic: Confusion of OPAMP (Read 652 times) previous topic - next topic

james98786

Hi everyone,

I get confused about OPAMP.

If a signal with noise is amplified by OPAMP, will signal-noise ratio increase?

What's the difference between these two results?
1. Send amplified signal from OPAMP to Arduino
2. Send signal to Arduino and write a program to multiply a factor

Help me !

Thanks !

terryking228

Hi,
Quote
If a signal with noise is amplified by OPAMP, will signal-noise ratio increase?
The output of the OPAMP will be the original signal (with whatever signal-to-noise ratio there was)
But: In some situations the OPAMP might add a small amount of noise.

Quote
Send signal to Arduino and write a program to multiply a factor
Now you have another factor: The arduino Digital Input has only 1024 values. If you are using only a few of the lower values, then the results will have large steps in value.  And since that is not the original signal, it is New Noise.

What range of voltages do you need to measure?
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

DVDdoug

#2
Feb 24, 2017, 12:15 am Last Edit: Feb 24, 2017, 12:16 am by DVDdoug
Any "active" circuit is going to add some  noise and degrade your signal-to-noise ratio.

Op-amps by themselves aren't particularly  noisy and usually most of the the noise comes from the power supply, or electromagnetic noise gets picked-up by the wires and that noise gets amplified.   

And, there are low-noise op-amps if a regular op-amp isn't quiet enough.  (That won't help if the noise is coming from the power supply or somewhere else.)

It also depends on the gain.    A buffer amplifier (with a voltage gain of 1.00) isn't going to add much noise.   But a microphone preamp with a gain of 60dB (1000) usually has some audible background hiss. 

With higher-gain amplifiers, the problem is often that you have a low-level signal (otherwise you wouldn't need a lot of gain) and because of the weak signal, whatever you're feeding-into the amplifier has a poor signal-to-noise ratio to begin with.   The amplifier amplifies the noise and the signal together, making the noise more noticeable.


james98786

I get confused that why we use OPAMP when signal and noise are amplified together.

So amplifying a signal by OPAMP is for reducing the requirement of ADC ?

By the way, I remember the frequency response of OPAMP is single pole model.

Won't it operate like a low pass filter ?

Where is noise you said from ?

Thanks !

allanhurst

An opamp will amplify anything you feed it, within it's frequency range. And it's response is generally a 2-pole lowpass approximation.

The universe gives you noise in many ways. A common one is thermal noise, given by Boltzman's law.

About -174dBm/ Hz at 300 K .

Allan


Jiggy-Ninja

I get confused that why we use OPAMP when signal and noise are amplified together.
Dynamic range. Op Amps add noise, but converting an analog signal to digital values also adds noise, called quantization noise. You want the analog signal to cover the full extent of the ADC's input range in order to minimize quantization noise.

So yes, the op amp circuit adds noise, but the intent is that the reduction in quantization noise will more than make up for that.
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MarkT

#6
Feb 24, 2017, 12:29 pm Last Edit: Feb 24, 2017, 12:31 pm by MarkT
Noise is fundamental, it can never be removed unless you know statistics on what the signal should be
like (for instance a bandwidth limit).

Any device with resistance will add noise, and each source of such noise is independent so the noise
power adds.  Active devices also add noise by other mechanisms.

When a signal is close to the "noise floor", then amplifying it can add significantly more noise, degrading
the S/N ratio.

when a signal is well above the noise floor, amplifying is usually pretty faithful, only multiplying the
existing noise in the signal, leaving the S/N ratio intact.

Most of the time you will be working with signals well above the noise floor - the noise originates in
some sort of sensor that is inherently noisy, so most opamps will be OK.

For very low level signals you might need to select a low noise opamp.  Good noise performance for
an opamp is something at or below 5nV/sqrt(Hz).

For a 10kHz bandwidth thats 0.5uV rms noise - pretty small basically.  Random opamp might be
an order of magnitude or two worse.

[ put another way - you have to do the maths ]
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MrAl

Hi everyone,

I get confused about OPAMP.

If a signal with noise is amplified by OPAMP, will signal-noise ratio increase?

What's the difference between these two results?
1. Send amplified signal from OPAMP to Arduino
2. Send signal to Arduino and write a program to multiply a factor

Help me !

Thanks !
Hi,

Op amps can add noise but there are special low noise versions that are less noisy than others.  These are usually the FET input types but some bipolar types are low noise also.  But it does depend a lot on your application what kind of op amp you want to use.  In audio pre amp work for example you want a low noise op amp, and that is if nothing else in the circuit is going to add much noise too.
Stay away from very high value resistors too.  Probably 100k or less.

If you are going to digitize using an ADC you enter a new world of noise however.  You have factors like how often you sample, or should i say how often you CAN sample given the max sample rate of the ADC.  I think with the Uno type the max sample rate is about 15kHz or around there (check specs), so you may find it works for speech but not for high quality music.  The type of filtering when you go to DAC also plays a role in the total output noise.

james98786

OK, I think I got it.

Thanks everyone !!

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