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Old 21st July 2011, 01:31 PM   #6621
RCruz is offline RCruz  Switzerland
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Where do you use it ?
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Old 21st July 2011, 01:40 PM   #6622
AndrewT is offline AndrewT  Scotland
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If the variable resistor is genuinely a "trimmer", then precision is not required and the $32 each, I see in Google results won't be necessary.
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Old 21st July 2011, 02:07 PM   #6623
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First stage Rs ?
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Old 21st July 2011, 02:17 PM   #6624
Salas is offline Salas  Greece
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2nd
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Old 21st July 2011, 02:18 PM   #6625
Salas is offline Salas  Greece
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Quote:
Originally Posted by AndrewT View Post
If the variable resistor is genuinely a "trimmer", then precision is not required and the $32 each, I see in Google results won't be necessary.
I believe its very low ppm they sell at that price.
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Old 21st July 2011, 02:18 PM   #6626
RCruz is offline RCruz  Switzerland
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Why 2nd ?

I know you already told me about that but I can not remenber the basics.
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Old 21st July 2011, 02:24 PM   #6627
Salas is offline Salas  Greece
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Its more effective since the signal is larger, and slave to the head-amp's noise so not to introduce different noise between channels. Rs in 1st is part of its noise outcome.
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Old 21st July 2011, 02:28 PM   #6628
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Thank you Salas
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Old 21st July 2011, 03:06 PM   #6629
AndrewT is offline AndrewT  Scotland
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Default trimmer VR for Rs

Then add the trimmer in parallel to the existing Rs, or in parallel to a slightly higher Rs.

eg.
Existing Rs = 30r
Proposed range of Rs +-10%
Select 36r for Rs and 1k for VR trimmer +110r fixed resistor.
Minimum trimmed Rs = 36//[0k+110r] = 27r1
Maximum trimmed Rs = 36//[1k+110r] = 34r9
Set Vr to 70r for nominal Rs = 36//[70r+110r] = 30r

Now, let's look at the sensitivity of Rs for the VR tolerance
At maximum trimmed Rs:- VR+2% gives 1020r and trimmed Rs becomes 34r89
A 0.05% increase in trimmed Rs for a +2% tolerance on the VR value.

If Rs ~+-20% is required for a gain adjustment range, then use 36r in parallel with 75r +VR= 2k
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Last edited by AndrewT; 21st July 2011 at 03:27 PM.
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Old 21st July 2011, 03:18 PM   #6630
Salas is offline Salas  Greece
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Paralleling the trimmer to a fixed resistor is a good practice indeed. Have used it like that in many occasions, for Rs here also.
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