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27th April 2010, 08:37 PM  #631 
diyAudio Moderator
Join Date: May 2008
Location: Toronto

Here Jan, this is the subject. See A2 feedback resistors. I wish there were guarantees, but practically... ???

27th April 2010, 08:51 PM  #632 
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Join Date: May 2008
Location: Toronto

Sure one can argue that using precision resistors the gain is guaranteed, but that's not sufficient to me. Also, setting the gain by watching the input and output signal on a scope or measuring it with a voltmeter is as precise as the instrument that measures it.
Here's what I have in mind. Use a microcontroller and a highly stable precision reference like LTC6655, to generate a 1.25V pp sine wave. Feed this into a voltage divider using two precision resistors like 10k series 10R, to obtain a signal of 1.25mV pp. Feed this into the 60dB amplifier and compare the output to the original 1.25V output from the microcontroller. In fact one could use an analog dial to search for the null. Build this so that it is a switchable mode CalibrateAmplify. This same setup can be used to calibrate the AC voltmeter too, being in effect a precise AC voltage generator. I think I worked out the details, that's why I was asking if people would be interested in a thread about it. Anyway, I think I'll just open a thread when I got it working. 
27th April 2010, 09:16 PM  #633 
diyAudio Member

Statistics again  since you're making an RMS measurement (the square root of the sum of the squares of device and amplifier noise) once the amplifier is 70% less noisy than the device under test, you are at +/ 2 sigma.

28th April 2010, 12:56 AM  #634 
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Join Date: May 2008
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Thanks again Jack, but I tell you, I'll leave this to you guys that are more experienced. To me little makes sense, messages are cryptic, I don't know. Are you talking about +2 standard deviation? Well, I skimmed the article and will go over it again in more detail.
I'll start another thread about what I mentioned before, and will pull back here because I'm getting nowhere. Good luck with your noise measurements. 
28th April 2010, 01:17 AM  #635 
diyAudio Member

The 95% confidence level  I just wanted to point out that you don't have to get down to femtoAmperes or picoVolts for measurements which are relevant. The SSM2019 amplifier is fine, so are the ones you describe.

28th April 2010, 06:56 AM  #636  
diyAudio Member

Quote:
I wouldn't worry about that. jd
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28th April 2010, 07:02 AM  #637  
diyAudio Member

Quote:
I saw this post after my previous post. So, why the var resistor? Why not calculate the total gain for 60dB and set it there? With those opamps you probably get better than 0.1% without any adjustment. jd
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I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason  Daniel Dennett Check out Linear Audio Vol 7! 

28th April 2010, 10:11 AM  #638 
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Join Date: Jul 2004
Location: Scottish Borders

because 3k16:100r twice, gives a gain of 1063 times or +60.53dB.
The resistor ratio for the second stage is 29k67:100r for close to +60dB gain and only if all the gain setting resistors are 0.1% tolerance. Use 1% and dial in the gain with the VR. One can then swap it to fixed resistors, for longer term stability.
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28th April 2010, 10:20 AM  #639  
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Quote:
Within 0.1% of course. jd
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I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason  Daniel Dennett Check out Linear Audio Vol 7! 

28th April 2010, 10:25 AM  #640 
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Join Date: Jul 2004
Location: Scottish Borders

and the difficulty as Iko has pointed out is how to determine the final gain that has been created.
Using +0.1% resistors in two gain stages will give 60+0.04dB accuracy. That's good enough for me. But how do we measure/confirm it? Using 1% resistors gives a tolerance of ~+0.4dB.
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