[volt-nuts] LTZ1000 resistors, lower collector currents

[m k]

> 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
> <http://cds.linear.com/docs/Datasheet/1000afc.pdf>).
> 
> 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:
> 
> <http://www.linear.com/product/LT5400>
> 
> 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:
> 
> <http://ee.devereux.me.uk/LTZ1000-1.png>
> <http://ee.devereux.me.uk/ltz1000-1.asc>
> 
> 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?
Hi John,
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.
> 
> Thanks,
> 
> John
> 
> -- 
> 
> John Devereux
> 
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