[volt-nuts] Homebrew LTZ1000 reference?

[Bruce Griffiths]

Mike S wrote:
> At 10:47 AM 11/19/2009, gsteinba52 at aol.com wrote...
>>          Linear Technology has an application note on this very subject:
>>
>> cds.linear.com/docs/Application%20Note/an86f.pdf
>>
>> Devour the whole app note
>
> Schematic's fine for what it is, but they call for $40/ea resistors, 
> probably special order from a distributor. (VISHAY VHP-100 0.1%, ~0 
> ppm tempco, 5 of 'em) which I would like to avoid the expense of...but 
> you can get Xicon 10ppm (288-0805 series) ones for <$1 from stock.
>
> Beyond the basic app note is getting an accurate voltage (10 V?) 
> referenced to the low accuracy LTZ1000 (it's extremely stable, but not 
> very accurate). Again, the app note would have you use a custom 
> resistor array (and even that won't get you accuracy, unless special 
> ordered, custom matched to each LTZ1000 - $$$$).
One solution if one requires a 10V reference is to use a DAC for fine 
adjustment of the output of a gain stage.
Use the 7V output from the basic LTZ1000 circuit as the DAC reference.
The only problem being that the required adjustment range is about 0.7VV 
to accomodate the 7.0 -7.5V output voltage range for the LTZ1000.
To achieve 1ppm adjustment resolution a DAC with 17bit or greater 
resolution is required.

Since the DAC output need not respond rapidly to digital input changes 
one can use something like an indirect PWM DAC that doesnt require any 
precision components.
Such DACs can easily achieve >20 bit resolution and monotonicity 
together with low drift.
However this requires using a microprocessor or equivalent with a PWM 
output.
If one does add a microprocessor then one can also add a thermal 
annealing algorithm to compensate the effect of powering down the 
LTZ1000 after calibration.

Bruce