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Old 26th Mar 2020, 9:46 am   #27
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,411
Default Re: Another DIY voltage reference...

Quote:
Originally Posted by mhennessy View Post
I first ran into this when Douglas Self published a pre-amp design back in 1996... The design called for a 50nF capacitor, so he used five 10nF 1% polystyrene types that gave 50nF at 0.48%. I was impressed!

At the same time, the Maplin catalogue had a "box-out" that explained that combining 1% resistors gave you an overall tolerance of 1%. Hmm...
Ignoring switch resistance, Maplin are correct, combining 1% resistors does give an overall tolerance of 1%. (All 1% high - overall value is 1% high. All 1% low, overall value is 1% low. So, 1% is the best that can be guaranteed.)

However, if the resistors are independent of each other (ie from different batches) then it is likely that above-tolerance in some, will combine with below-tolerance in others. So the final value is more likely to be rather better than 1%, even though it cannot be guaranteed.

This probably explains Doug Self's result, that his 10nF capacitors came from a spread of values. Of course, he could have measured each one, combined a few lows with a few highs, and got significantly better than 'worst case.'

The same would apply to voltage references.

But - returning to capacitors - not necessarily to them. I was told by a capacitor manufacturer's rep (polyester film types), that they had improved the accuracy of their process capability to the point where they could be confident of values within 2%. So, for their 100nF 10% capacitors, they set their machines to make 93nF capacitors (which would turn out between 91 and 95nF), label them 100nF 10%, and sell them saving 7% on the materials cost of a true 100nF. Everything was in spec, but for the chap who bought a boxful hoping to be able to measure them and fine one bang on 100nF, he'd be sorely disappointed!
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