Easily Identify Indian Engineers

Lestraveled

Joined May 19, 2014
1,946
.......The idea of adjusting a CMRR offset, accurately and easily to within 0.1%, with a single turn variable resistor of 10% of the resistor value is nonsense.......
I made a mistake in the earlier post. I didn't trim the resistor to .1%. I trimmed it to .01% Also for fun, and you said it was nonsense, I changed to a single turn pot and trimmed it to 100K .01%.



Yes, you are absolutely right on the Jensen app note. The one you posted was the one I was trying to find.
 

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rogs

Joined Aug 28, 2009
279
You were lucky to have a 93.1K resistor to hand.....

Better not open the door and let the temperature in the room change..... That'll probably mess up your calibration!

I think I prefer to more modern approach, like the 'bootstrap' option suggested by Jensen...

Or, better still, use a transformer.......I find that the most satisfactory answer to this problem....
 

Lestraveled

Joined May 19, 2014
1,946
Rogs
I agree with you about the transformers. They are the device to use.......if you can afford them. The Jensen transformers are generally over $100 each, where as the op-amp circuit you posted would take less than $10 to build. There are much better audio op-amps out there, like the 5534.

The 93.1K resistor thing. I think National Semi used that value for the same reason I used it. It was the closest value I had on hand. Simple economics.
 

rogs

Joined Aug 28, 2009
279
........ he just implied it couldn't be done.
No, I didn't imply or say that... I said I didn't think it could be done 'easily and accurately'.

And I still think adjusting a single turn trimmer - like the one in your photo - with a rotational accuracy of better than one third of one degree (which is an accuracy better than the +/-0.15% 'adjustability' limit in the device spec) cannot honestly be described as 'easy or accurate'.....

It means the outer extremity of the trimmer slot being adjusted with an accuracy of 0.00036" (0.009mm)

You've shown it's possible. It's just not really a very good idea, IMHO....
 
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