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| Question about preamp for measuring noise. |
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| matiasro:
How are you? I am writing from Argentina. My oscilloscope is a Rigol 1052 and HP3478A multimeter. I am a DIYer and audio enthusiast, although I have done many successful projects in the field instrumentation i am a newbie. I make this query as I begin to measure noise levels at regulators and power supplies. The noise level of my Rigol is very high for this and I know I should build a preamplifier to amplify the signal to achieve and exceed the measured noise level of the instrumental. I believe that for this task I need to use an amplifier with a factor equal to or greater than 1000x and low noise. I was looking for information on this and there is so much material with words associated with other issues hampering my search. On the other hand I saw few people carried out projects for this purpose. Someone could tell me if you use any preamplifier with your multimeter or oscilloscope? It would also be important if you know of a working circuit to achieve my goal. I send a hug from Argentina and from already thank you very much. Matias |
| tekfan:
Measuring noise with a digital oscilloscope is very hard. Not only is there analog noise in the front end but there is also digital noise that is made by the AD converters. Even if you do build a sensitive preamp to amplify the noise it will still be hard to measure on the Rigol. This is what analog scopes do best. If you will do these measurments often you may want to find a used analog scope. If measuring noise below several mV then you will have to be very careful about ground loops and exposed conductors. If you use unshielded wire you will get meaningless readings. Your best bet is to measure the noise with your HP3478A set to AC volts. That will give you a true RMS reading of what the noise is from 20Hz to 100KHz. If you can build a preamplifier so that you can use the 30VAC range on your multimeter you will get 20Hz to 300KHz frequency range. If all you're doing is audio then the direct measurment without the preamplifier should be enough. The good thing about using a multimeter is that it is floating and doesnt introduce ground loops to the measurment. If the device you are trying to measure has a low impedance output you won't even need shielded wires. |
| Tube_Dude:
--- Quote from: tekfan on March 16, 2012, 01:10:31 pm ---The good thing about using a multimeter is that it is floating and doesnt introduce ground loops to the measurment. --- End quote --- Thats the great superiority of handheld multimeters in workshop, because working with batteries, they are inherently free of mains induced ground loops and noise. 100% agree with the use of a analog scope for low noise level inspection, and even for Audio work in general. ;) Cheers |
| jahonen:
I'm not sure if the preamp is a bad idea, I once build one with 80 dB gain and was able to get very meaningful results with it. I didn't even have any external shielding although I first thought that I absolutely need one (but incidentally, PCB was a 6 layer one, because I put it on a free space on DUT PCB panel which would have been discarded otherwise :P). I used it to characterize the output noise of a low-noise voltage regulator. I also had a lowpass filter on preamp output to get meaningful and defined noise bandwidth. Preamp is built around ADI microphone preamp SSM2019 (40 dB gain) and a post-amplifier with AD797 opamp (also 40 dB gain). Worked like a charm with our DSO at work. Even slight bending of the DUT PCB was clearly visible due to minuscule reference voltage change. I can post the schematic if anybody is interested. Regards, Janne |
| matiasro:
Thank you all for the answers. They have been very useful to me. For now I'll settle for being able to measure the noise level of a simple circuit with a LM317 for example. I know it's hard to find people who have done some preamp to measure these signals, so any input is very helpful for me. Janne, I want to see that circuit did not understand if the PCB was 6 layers or would have been ideal if they had 6 layers. My English is not very good and sometimes translators do not do a good job. Could you clarify that point?. Of course thanks and waiting for new contributions. A hug. Matias. |
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