[sdiy] Tempco adjuster idea

[ASSI]

On Sunday 27 April 2003 07:27, Ian Fritz wrote:
> The ideal tempco resistor (for compensating exponential current
> generators) has what's called a PTAT (proportional to absolute
> temperature) response. This means that the resistance is of the form
> R = AT, where A is a constant and T is absolute temperature.  This
> dependence cancels the 1/T factor in the exponential of the
> transistor I-V response function. The corresponding temperature
> coefficient (1/R)(dR/dT) is just 1/T, or 3350ppm/K at room
> temperature.

Keep in mind that this is already an approximation for the temperature 
dependence of an idealized pn junction. Once you get the diffusion 
resitances and other second order stuff in, things start to look 
different again. The rest of the circuit will have it's own idea about 
temperature dependence as well, especially noticeable if you're not 
careful with the type of resistors.

> So what if you buy some tempcos and their coefficients are different
> from this?  Since they are normally made of metals, their resistance
> will still have a linear T dependence, but they will not be PTAT.  In
> other words, their resistance can be well approximated as R = AT + B,
> where the constant B is the resistance extrapolated back to T = 0. 
> If B is positive then the coefficient is too low, and conversely if B
> is negative then it is too high.

The TKR is defined as 1/Ro*(R(T)-Ro)/(T-To), which is why you always 
need to know what Ro and To is. This is different from the above 
differential definition except around To and also different from 
(dR/dT) except for linear temperature dependence.

> So to correct for an incorrect coefficient one just needs to make a
> circuit that cancels the B term.  This looks easy to do, at least on
> paper, and the only drawback is that it takes several amplifiers
> instead of just one to condition the control voltage inputs.

No, you still have to fight both coefficients. The offsets introduced 
by B can be canceled by trimming at a single temperature. Then when 
you find that the compensation fades as you go away from that 
temperature, that means that A is incorrect. You can trim A towards 
zero rather easily with passive components, but not the other way 
around. BTW, Conrad has a bunch of 3900ppm/K temp sensors from Heraeus 
that are almost reasonably priced.

[...quad opamp circuit...]
Also make sure that the damn circuit does not oscillate with the the 
gains so low... The tempco resistor compensates the actual converter, 
what you are trying equates to compensating the CV itself if I 
understood correctly. If so, I'd suggest it's far easier to produce a 
PTAT signal with a bandgap and then mix that with the VC in the usual 
way.

> I hope to try this idea out in the near future so that I can use
> the  3200ppm/K units that KRL sold me.

Heat up your expo converter to 40°C for the trim and save the opamps 
for something more useful. :-)


Achim.
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