[volt-nuts] Homebrew LTZ1000 reference?

[Bruce Griffiths]

Bruce Griffiths wrote:
> 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
>
>
Another option if one prefers not to use a microprocessor is to use 
coarse and fine multiplying DACs where the range of the fine DAC is 
4LSBs of the coarse DAC.
During the lifetime of the reference only the input to the fine DAC need 
be adjusted.
If parallel input DACs like the AD7945 or equivalent are used then the 
output can be set by DIP switches or jumpers.

Bruce