How to measure noise from a current source

[kehindekennet]

I'm asked to get the noise signal from the constant current source I built with a 741 Op-Amp. I know I have to read it from the scope, but how do I go about the connections to the scope, and what other procedures must I follow to read the noise contribution? Pls refer to the figure provided in the link below to see the circuit.

http://www.freeimagehosting.net/n92gg

 

[yungman]

I looked at the circuit quickly, I don't quite understand the circuit nor would I do it this way to get a constant current source. If you use resistors to simulate a current source, the output impedance is way too low unless you use very high value resistors which would limit you to very low current capability. Also, it only work if the current injection point is ground referenced. If the point is moving, all bets are off. It is much easier to make a current source with an op-amp and a MOSFET.

But be that as it may. To measure the noise, you terminate the output with a resistor and measure the noise. You can use a RF noise meter for measurement. It is possible to measure with the scope, but it is not accurate because it is hard to determine the exact amplitude. I did a lot of noise measurement with scope just because I did not have the noise meter. I had to make judgement what is the amplitude as all you see is a fuzzy line on the screen!

The difficult part is you likely have to put amplifier stages to amplify the signal before you can use the noise meter and the the amplifier themselves produce noise. With the noise meter, it's easier, you put a resistor at the input of the amplifier chain, measure the noise. Then connect the current source and see the increase of the noise. Then you do calculation and get the noise contribution of the current source alone. Remember, noise are root mean square, not direct addition.

 

[kehindekennet]

Additional Details
I am sorry if I am speaking unprofessionally. How do I get to know the frequency range of the measured noise? I made some amendments to the circuit, and have attached it in the link below, together with the output I got from the scope. How do I know the noise frequency range? I hope the scope output I attached is correct, because I am a novice as per the use of the Agilent scope. Thanks.

http://imageshack.us/photo/my-images/585/justrw.jpg/

 

[yungman]

Current noise is wide band to very high frequency. We mostly talk about to the frequency limited to the frequency of interest.

The frequency of the current noise of your circuit is the limit of the frequency response of your circuit. Say, it your amp has a -3dB frequency at 1MHz, then you only need to worry about noise slightly higher than 1MHz and that's it. Say 2MHz or even less.

I cannot read you graph because some PC stuff is running and checking my computer when I open the link and I don't like it!

 

[alphysicist]

There are a few steps you can follow to measure the noise from a current source using a 741 Op-Amp. First, make sure all connections are secure and that the circuit is functioning properly. Next, you will need to connect the output of the current source to the input of the oscilloscope using a BNC cable. The oscilloscope should be set to AC coupling and the input impedance should be set to 1MΩ. You can then adjust the time scale and voltage scale to get a clear view of the noise signal.

To accurately measure the noise contribution, you will need to subtract the DC signal from the total signal. This can be done by using the "Math" function on the oscilloscope and selecting "A-B." The A channel should be set to the output of the current source and the B channel should be set to a stable DC voltage source of the same magnitude as the current source.

Once you have subtracted the DC signal, you will be left with only the noise signal. You can then use the cursors on the oscilloscope to measure the peak-to-peak amplitude of the noise signal. This value can be used to calculate the root mean square (RMS) noise voltage using the formula RMS = (peak-to-peak amplitude)/2√2. This will give you a measure of the noise contribution from the current source.

It is important to note that the noise from the 741 Op-Amp itself will also contribute to the overall noise signal. To measure this, you can repeat the same steps above but with the input of the oscilloscope connected to a stable DC voltage source instead of the output of the current source. The difference between the two measurements will give you the noise contribution from the Op-Amp.

In addition to these steps, it is important to ensure that the circuit is in a quiet and stable environment to minimize external sources of noise. It may also be helpful to take multiple measurements and average them to get a more accurate result.

I hope this helps guide you in measuring the noise from your current source. Good luck with your experiments!