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? - Does anyone know any good sources for "ratio set" networks or precision resistors in general? I know digikey, farnell, mouser. Thanks, John -- John Devereux

>- Does anyone know any good sources for "ratio set" networks or >  precision resistors in general? I know digikey, farnell, mouser. I've never used them, so I can't speak for their prices (maybe someone else here can), but Texas Components (http://www.texascomponents.com/) appears to be willing to sell you individual custom-value foil resistors, dividers, or networks. Randy.

Randy Scott <scottr9@yahoo.com> writes: >>- Does anyone know any good sources for "ratio set" networks or >>  precision resistors in general? I know digikey, farnell, mouser. > > I've never used them, so I can't speak for their prices (maybe someone > > else here can), but Texas Components (http://www.texascomponents.com/) > appears to be willing to sell you individual custom-value foil resistors, > dividers, or networks. > > Randy. Thanks Randy, I had not heard of these. They seem to be resellers / customizers of Vishay parts. Vishay themselves do claim to offer special values in prototype quantities but I have assumed they would still be too expensive (compared to the equivalent made up of individual stocked parts). Perhaps I should actually ask them. -- John Devereux

Randy Scott wrote: >> - Does anyone know any good sources for "ratio set" networks or >> precision resistors in general? I know digikey, farnell, mouser. > > I've never used them, so I can't speak for their prices (maybe someone > > else here can), but Texas Components (http://www.texascomponents.com/) > appears to be willing to sell you individual custom-value foil resistors, > dividers, or networks. I have purchased many sets of precision Vishay resistors from Texas Components - great company to work with, excellent products. -- Bob Smither, Ph.D. smither@c-c-i.com ========================================================================= Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishment inflicted. -- Eighth Amendment to the Constitution of the united States ========================================================================= Circuit Concepts, Inc. 281-331-2744

> From: john@devereux.me.uk > To: volt-nuts@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 > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there.

Hi John, Vishay also claim that special values are the same cost as standard. They basically build / laser zap to order. I've not ordered special foil resistors from them but have had pots with odd values ans laws made without any issues in the past. Robert G8RPI. ________________________________ From: John Devereux <john@devereux.me.uk> To: volt-nuts@febo.com Cc: Discussion of precise voltage measurement <volt-nuts@febo.com> Sent: Monday, 7 November 2011, 13:21 Subject: Re: [volt-nuts] LTZ1000 resistors, lower collector currents Randy Scott <scottr9@yahoo.com> writes: >>- Does anyone know any good sources for "ratio set" networks or >>  precision resistors in general? I know digikey, farnell, mouser. > > I've never used them, so I can't speak for their prices (maybe someone > > else here can), but Texas Components (http://www.texascomponents.com/) > appears to be willing to sell you individual custom-value foil resistors, > dividers, or networks. > > Randy. Thanks Randy, I had not heard of these. They seem to be resellers / customizers of Vishay parts. Vishay themselves do claim to offer special values in prototype quantities but I have assumed they would still be too expensive (compared to the equivalent made up of individual stocked parts). Perhaps I should actually ask them. -- John Devereux _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.