[volt-nuts] LTZ1000 resistors, lower collector currents
m1k3k1 at hotmail.com
Mon Nov 7 18:18:28 UTC 2011
> From: john at devereux.me.uk
> To: volt-nuts at febo.com
> Date: Mon, 7 Nov 2011 09:45:11 +0000
> Subject: [volt-nuts] LTZ1000 resistors, lower collector currents
> Hi all,
> 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?
As far as I can tell there is no real need for these two currents to be the same, I have been doing some modelling of this and the exact current is almost an arbitary decision. It does mean a different voltage at balance, about 60mV per decade current scal.
> - Does anyone know any good sources for "ratio set" networks or
> precision resistors in general? I know digikey, farnell, mouser.
> John Devereux
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