[sdiy] Tempco adjuster idea

[Magnus Danielson]

From: ASSI <Stromeko at compuserve.de>
Subject: Re: [sdiy] Tempco adjuster idea
Date: Sun, 27 Apr 2003 13:24:13 +0200

> 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.

Things like bulk resistance can be handled separately by fortunately much smaller
alterations to the curcuit. However, the bulk resistance compensation usually seen
out there only handles part of the bulk resistance problem, as I have pointed out
before.

So, handling the temperature compensation which comes from the fundamental expression
kT (which is a quantum mechanic energylevel) is still a valid study. Several of the
other effects we can and should handle separately. You can choose a low rBE component
like the MAT-02 or you may make more or less elaborative compensation schemes.

> > 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.

True linear curve may not exist really, but it may be accurate enought in the range
we are discussing.

However, the tempco-less discussions we had before seemed very interesting and with
the amount of hardware used to compensate the curve of tempcos we are not too far off
in hardware to start with.

> > 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.

Why do I start to consider ovenized oscillators as a fair deal?

> > 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. :-)

My point. Ovenized oscillators is a known trick of the trade and when done properly
you can acheive *really* stable temperature, enought to make that ~ 3300 ppm/C dampen
down to way below other sources of drift. This is done for crystal oscillators on a
regular basis.

Cheers,
Magnus - with ovenized crystal oscillators constantly heated in the room