SV: Re: SV: Re: [sdiy] Compensating PTC and NTC resistors

[René Schmitz]

Hi Karl and all,

karl dalen schrieb:

> So how does it works then if its just an idea? Asumed TC of zero are not
> a viable reality not even with MFR, it clearly states +/-100ppm, whare do
> you find MFR of 0 TC?. Even Tony A suggests teasing down the tempco of PTC
> by a serial resistor although he didn't say which one. You suggests
> paralleling to reduce tempco of NTC's.

Since you can't be sure wether its +100 or -100 or just somewhere 
between like exp(pi) you can dimension it like it had 0TC.

Heres what I did on the VCO3 where I had to step down the too large TC 
of the KTY81:
1k * 7900ppm/3354ppm = 2.355k -> Series resistor = 2.355k - 1k = 1.355k, 
and I so chose 1.4k

The result will be close enough. You'd be at a third order effect 
already. Compensation of the tempco compensation correction temperature 
coefficient. Before the results of that would need to be taken into 
account you'd need to eliminate all second order effects first.  (Like 
tolerances and the number of approximations that this expo/tempco math 
has.) If you knew the exact TC of all the resistors, well then I would 
suggest to go over and calulate the influence of the other stages as 
well. Input summing network, feedback resistors of CV scaling network, 
the influence of the top resistor in a base voltage divider, tempco of 
integration cap, opamp drift and so on...

My point hardly anyone would seriously, and consistently be able to do 
that. (And then an experimental approach like Ian's is better IMO.)
At some point you need to stop theorising, and get practical.

Cheers,
  René