[volt-nuts] LTZ1000 resistors, lower collector currents
john at devereux.me.uk
Mon Nov 7 09:45:11 UTC 2011
I was looking at the LTZ1000 datasheet circuit again (page 7 of
One of the problems is all the expensive resistors. R1, R4, R4 all need
to be Vishay bulk metal foil types as far as I can see. Even R2 and R3
should be, given the long term drift specs of other available resistor
types. And they are awkward values.
I found the LT5400 quad matched resistor network:
This seems to have very good ratio drift specs - an order of magnitude
better than any resistor I can find. And it is only ~$5. I thought it
could replace the R4/R5 divider, replacing 2 or 3 $15 parts.
Best I could come up with is this circuit:
To get the right operating temperature (~45-55'C) I had to use quite a
high collector resistor R3. And the LT5400 impedance is 10x higher than
the datasheet 1k/13k combination. I don't know if there are some bad
effects. Of course I don't have a proper LTZ1000 model so this might all
need adjusting for a real part.
- what is the effect of a higher impedance circuit around Q2.
- Is there any reason to make R2 the same as R3? That is, does the Q1
operating point need to be the same as Q2?
- Does anyone know any good sources for "ratio set" networks or
precision resistors in general? I know digikey, farnell, mouser.
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