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Ultra low drift GAIN
Ultra low drift GAIN
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Old 9th August 2020, 10:52 PM   #31
mchambin is offline mchambin  France
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The only way to go one step beyond is to hire an IC designer to integrate my stuff. ;-)

May be some active circuitry to sense resistor temperature differences and apply gain adjustments accordingly.
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Old 10th August 2020, 12:19 PM   #32
mchambin is offline mchambin  France
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Design is simple finally.
Resistors available at Mouser.
5ppm/K is affordable from $1.
2ppm/K is not. Over $30.

From the neat analysis in post #23.
Using 5 ppm/K resistors is the way to go.
This should give the very low drift I need, despite temperature differences up to 10℃.
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Last edited by mchambin; 10th August 2020 at 12:24 PM.
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Old 10th August 2020, 12:32 PM   #33
Conrad Hoffman is offline Conrad Hoffman  United States
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You can also build a simple bridge and compare resistors TC by clipping and immersing them in warm mineral oil. I've done this for metrology standards and you can do quite well with a few dollar bag of ordinary 1% metal films.
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Old 10th August 2020, 01:38 PM   #34
mchambin is offline mchambin  France
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Nice, a TC matching gig.
How accurate did you get ?

About TC what is the best technology ?

What about resistor stability over time ?
Thin film, thick film, through hole, SMD,,,,

What about resistor stability over time ?
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Last edited by mchambin; 10th August 2020 at 01:52 PM.
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Old 10th August 2020, 07:51 PM   #35
sgrossklass is offline sgrossklass  Germany
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OP, why do you even have these insane performance requirements in the first place? What is the system-level performance metric that necessitates them? CMRR or what?

I'm not sure why you are so worried about temperature. I would make sure on circuit level that you only need relative matching between resistors, then you can place identical value surface-mount parts onto a PCB in a smallish area (with critical matches right next to each other) and put them in a box (shield) or pot them if really necessary.

If a solution requires me to jump through major hoops, I would always double and triple check that its benefits are really worth it and that there isn't another, smarter solution that gets the same done a lot more elegantly. Sometimes you have no choice but often you do. As they say, if you've been digging yourself into a hole, stop digging.
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Old 10th August 2020, 08:57 PM   #36
mchambin is offline mchambin  France
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I thought I found the solution with Vishay PTF that has 5ppm TC.
No good, because the 1/8 watt is Rth 520 K/W
This is no better than a 25ppm TC, Rth 140 K/W

I need a better TC X Rth.
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Old 10th August 2020, 09:37 PM   #37
Conrad Hoffman is offline Conrad Hoffman  United States
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I've never gone back to see how well the resistors did, but I had built a Kelvin-Varley divider and probably got it well under 100 ppm overall. A few years ago it was still accurate. One hint- use thru-hole and put a heat sink (hemostat) on the leads when you solder them. Resistor parameters can change permanently when you solder them.

I have to agree with the above post- why in the world do you need this? Want serious gain stability? Use a ratio transformer. There have been a bunch, like Gertsch and General Radio Corp. You might even wind your own. Step up or down, then buffer.
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Old 10th August 2020, 09:42 PM   #38
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by mchambin View Post
I thought I found the solution with Vishay PTF that has 5ppm TC.
No good, because the 1/8 watt is Rth 520 K/W
This is no better than a 25ppm TC, Rth 140 K/W

I need a better TC X Rth.
That's only true when self heating is the problem. When the main problem is changing ambient temperature combined with mismatched temperature coefficients, those 5 ppm/K resistors are much better than the 25 ppm/K resistors.
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Old 12th August 2020, 01:28 PM   #39
mchambin is offline mchambin  France
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I agree this 0.001dB requirement is madness.
This is for experimental studies, I know this is beyond ordinary audio, then let's find solutions and see how far one can go.
I found arrays of 8 same independent resistors.
Vishay NOMCA
TC absolute +- 25ppm
TC tracking +- 5ppm
This could implement
gain +7 with R, 6R
gain -7 whith R, 7R
gain -1 whith 4R,4R
gain 0.2 with 5R,R
What gain accuracy could I get ?
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Old 12th August 2020, 02:36 PM   #40
jcx is offline jcx  United States
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AC response over audio frequencies is going to be a limit - check how many decades of margin is needed between amp corner frequencies and audio band limits for 100 ppm error

DC coupling would work for the lower corner but the upper limit is tough, gain intercept even 2 decades beyond 20 kHz and 100 dB excess gain @20k...

Last edited by jcx; 12th August 2020 at 02:43 PM.
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