[sdiy] Dial-a-tempco

[Ian Fritz]

At 01:45 AM 5/28/2003, Czech Martin wrote:
>It's not entirely clear what you mean, do you have a schematic
>of said circuit?

Not on line yet, but it is very simple.  Imagine 3 opamps.  The first is an 
inverting (unity gain) summer to add up all the control inputs.  Call its 
output Vin.  The other two amps work in parallel and both are driven by 
Vin.  The first of these is an inverter with the tempco resistor in the 
feedback path.  Its output is proportional to Vin*(AT+B).  The second 
parallel amp is a standard "sign changer" circuit (see the Op Amp Cookbook, 
for example).  Its output is proportional to Vin*C  where C is tunable from 
-1 to +1.  The outputs of the parallel amps are combined passively at the 
base of the converter transistor.  The sign changer is adjusted so that C 
cancels B, leaving an exact PTAT voltage.

>And: why is your frequency measurement error +- 2Hz?

Ooops, busted!  I'm still using a simple homebuilt counter from the EN 
days.  It counts zero crossings for 1 sec.  It can be switched to 10 sec 
counting, but that's too boring, even for me.

   Ian