[sdiy] Tempco adjuster idea

[ASSI]

On Sunday 27 April 2003 16:40, Ian Fritz wrote:
> Yes, but, we are interested in small variations around room
> temperature. Therefore, it is the coefficient at room temperature (25
> C) that matters to us. This coefficient should be 1/298.15 K^-1 =
> 3550 ppm/K.

3354ppm/K if we are going to be picky.

> Nonsense.  If B=0, then compensation is exact for any A  (ignoring
> nonlinearities).  AT/T = constant.  You seem not to understand the
> idea of multiplicative correction.  The converter response is
> proportional to exp[a (AT+B)/kT], where a is the constant factor
> needed to obtain 1V/oct response. When B=0 the T factors cancel. This
> is how converters are compensated.

I don't?
exp[ a {AT/kT + B/kT} ] = exp[ aA/k ]*exp[ aB/kT ]
(but only if a is not temperature dependent itself).

Now, a delta Vbe circuit delivers an exact PTAT response and a diode 
linearized OTA exactly cancels the temperature coefficient of the input 
voltage. It appears that if you feed the OTA tail with a PTAT current 
and the OTA input with the CV for the expo, the overall response should 
just have the desired temperature compensation without any tempco 
resistor, which leaves you with the temperature dependence of the rest 
of the circuit.

But then again I may not really understand these things, but I'm sure 
you'll tell me where I erred.

> Why on earth do you think A should be zero?

I don't, I merely observed that you can get a slightly higher tempco 
down to where you need it with relative ease. BTW, Rene Schmitz' 
suggestion of using an NTC in the input leg of the divider is a good 
one I think: it appears you need about -150ppm/K for your 3200ppm/K 
resistors and that is certainly within range by just chosing the right 
resistor (TaN thin films should be close).

> No! Again, you seem not understand at all!  What I am suggesting is a
> circuit to electronically adjust the tempco to exactly 3550 ppm/K for
> proper compensation of the converter.

I've looked at the circuits at your site and what they do is to make 
the scale factor of the CV into the expo converter temperature 
dependent. The circuit you propose implements some general linear 
function, which has some appeal, but I don't know how stable that will 
get as you get the offset drift into your tempco. The circuit I 
outlined above does without a tempco resistor, but hinges on the OTA 
being linear enough and obviously has problems with the offset drift 
from the OTA as well.

> In what sense is this "usual"?  I've never seen it done for a VCO.
> The main temperature effect is multiplicative -- an additive
> correction won't work at all.

Sorry if I've been unclear. See above for the actual expo compensation 
I had in mind, the rest of the comment was directed at adjusting for 
the linear temperature dependencies of the rest of circuit, lumped 
together as a first-order approximation and compensated for by adding 
the correction to the linear CV input. The last opamp in your circuit 
has a similar function and can be extended with another input to do 
just that.

> I have no fundamental objection to the brute-force thermostating
> approach.  But, of course, you don't just abra-cadabra heat something
> up.  It takes heating and temperature control circuitry to do this,
> it eats up quite a bit of power, stray heat can effect other circuit
> elements, you usually end up using low-performance array transistors
> for the converter -- we've been over all this many times before.  Op
> amps are cheap, even high-quality ones.

By ovenizing I mean to put the whole circuit into a thermally 
semi-isolated canister so that the heat flow to ambient can be well 
enough controlled (the thermal equivalent to a current source: high 
dynamic resistance). This cuts down on the required power and required 
equalization time, minimizes the effects on other circuitry and allows 
to use relatively low temperature differences vs. ambient. In fact, 
linear circuits will often produce enough heat all by themselves, so 
the only requirement may be to keep the power dissipation in the 
canister constant and perhaps add some thermal mass to avoid 
temperature oscillation (and thermal oscillations are definitely 
nasty). Even if you only manage to produce a 20dB ambientRR, you can 
use much less precise means of temperature compensation and still come 
out ahead in overall tempdep. 


Achim.
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