MOTM

On 2010-05-03 12:12 -0000, peter_ivens wrote:

> A tempco with a temp. coeff. of 3500 , 1K 1%, is not so
> difficult to find. Most moduls of MOTM use a tempco of 3500 ppm
> (like the MOTM-300) but in the BOM for the 440 & 485 there is
> specified 1K 1% 3300 ppm. 

I don't think it matters much. The tempco of the circuit is
an approximation anyway, because there are other sources of error
than the transistor pair. And so is the tempco of the actual
resistor because it varies with temperature (and possibly from
device to device).

If anything, it's preferable to have a resistor with too high a
tempco because you can always reduce it by wiring an ordinary
metal film resistor in parallel (metal film resistors have tempcos
like 50 or 100 PPM/K).

Check Ian Fritz's "dial-a-tempco" page for a really refined
treatment.

> Elby design delivers a second resitor to lower the temp. coeff.
> but then you can take also a tempco with a tolerance of 5% from
> Tyco. 

Tycos are cheap but aren't they 3000 PPM/K ? The Akaneohm sound
like a much better deal. Or the Panasonic SMT if you're prepared
to some kludgery.

-- 
Andr\ufffd Majorel http://www.teaser.fr/~amajorel/