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Old 27th April 2010, 08:37 PM   #631
iko is offline iko  Canada
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Here Jan, this is the subject. See A2 feedback resistors. I wish there were guarantees, but practically... ???
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Old 27th April 2010, 08:51 PM   #632
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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 micro-controller and a highly stable precision reference like LTC6655, to generate a 1.25V p-p sine wave. Feed this into a voltage divider using two precision resistors like 10k series 10R, to obtain a signal of 1.25mV p-p. Feed this into the 60dB amplifier and compare the output to the original 1.25V output from the micro-controller. In fact one could use an analog dial to search for the null. Build this so that it is a switchable mode Calibrate-Amplify. 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.
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Old 27th April 2010, 09:16 PM   #633
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Originally Posted by ikoflexer View Post
Of course the assumption here is that the amplifier self-noise is significantly lower than the noise to be measured.
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.
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Old 28th April 2010, 12:56 AM   #634
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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.
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Old 28th April 2010, 01:17 AM   #635
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The 95% confidence level -- I just wanted to point out that you don't have to get down to femto-Amperes or pico-Volts for measurements which are relevant. The SSM2019 amplifier is fine, so are the ones you describe.
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Old 28th April 2010, 06:56 AM   #636
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Originally Posted by ikoflexer View Post
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 micro-controller and a highly stable precision reference like LTC6655, to generate a 1.25V p-p sine wave. Feed this into a voltage divider using two precision resistors like 10k series 10R, to obtain a signal of 1.25mV p-p. Feed this into the 60dB amplifier and compare the output to the original 1.25V output from the micro-controller. In fact one could use an analog dial to search for the null. Build this so that it is a switchable mode Calibrate-Amplify. 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.
I'm sure you can get it to work, but I don't follow the importance of super-accurate gain numbers. So the nominal 60dB gain is really 59dB, or 61dB, so what?
I wouldn't worry about that.

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Old 28th April 2010, 07:02 AM   #637
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Here Jan, this is the subject. See A2 feedback resistors. I wish there were guarantees, but practically... ???
Iko,

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.

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Old 28th April 2010, 10:11 AM   #638
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So, why the var resistor?
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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Old 28th April 2010, 10:20 AM   #639
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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.
I see. You could use a next-higher value for the stage 2 feedback resistor and calculate a parallel resistor to get to exact 60dB....
Within 0.1% of course.

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Old 28th April 2010, 10:25 AM   #640
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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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