reference

Grant Richter grichter at execpc.com
Wed Nov 10 05:25:59 CET 1999


> > > Zener diodes.
> > 

In the feedback loop of an op-amp this is quite a good
reference. The largest variation in a zener will be the
dynamic resistance (the change in zener voltage from
least current to maximum). By isolating a zener in the
feedback loop of an op-amp the current flow is held
constant. If you consider the positive terminal of an
op-amp connected to ground, and a zener diode
in the negative feedback loop with a resistor to -V.

The op-amp output will servo to whatever value will make
the virtual ground (negative input) the same as
the positive input (ground). That would be the zener voltage
with current controlled by the resistor to -V. If you
make this a resistor and a trimpot (with a value of
10% of the resistor) you can adjust the output
to an exact voltage.

This produces a reference with a temperature coefficient
of (as I recall) about 300ppm per degree C and is
quite inexpensive in relation to a dedicated voltage
reference.

This is also the basis for most integrated references
who rely on stabilizing the temperature characteristic
of the band-gap source (perhaps a zener) so they can
have guaranteed specs like 10 ppm per year.




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