[time-nuts] V standards

[Bruce Griffiths]

Richard Moore wrote:
> On Dec 2, 2008, at 12:25 AM, Bruce and PHK wrote:
>
>   
>>> <clip>
>>> There was a recent Design Idea in EDN or Electronic Design (don't  
>>> remember
>>> which one) where someone claimed 32 bit resolution out of two 16  
>>> bit dacs. That
>>>  claim is ridiculous of course due to noise and matching etc, but  
>>> you can
>>> probably get 20 - 24 bits resolution and accuracy out of a  
>>> cascaded system with
>>> some care.
>>>
>>> bye,
>>> Said
>>>
>>>       
>> If you build this circuit using the values shown in the schematic you
>> won't actually achieve 20 bits performance.
>> One of the resistor values is incorrect.
>> The resistor matching requirements aren't anywhere near as  
>> stringent as
>> one may think on first glance.
>> With some subtle alterations to the operating mode the resistor ratio
>> matching tolerances can be relaxed considerably.
>> In closed loop digital control systems the circuit noise should be at
>> least 1LSB or so to maximise performance.
>> The trick is to increase the DAC resolution until this condition is
>> achieved and not increase the noise to meet the condition.
>> 20 bit resolution performance is easy to achieve, 24 bit performance
>> requires 2 a little more work.
>>
>> Actually monotonicity to about 26 bits was claimed with noise a bit
>> below 1ppm.
>> However since the noise varies with the DAC output such claims are
>> perhaps a little too simplistic.
>> The noise can be reduced when required by using a better reference.
>> Any drift in DAC gain and offset could be compensated by a Kalman  
>> filter
>> should they become significant.
>>
>> The major advantage of such a DAC is the inherent monotonicity which
>> cannot be achieved and maintained (around coarse DAC transitions)
>> without frequent calibration when the outputs of 2 16 bit DACs are  
>> combined.
>>
>> Bruce
>>     
>
> Bruce, is there a way for non-subscribers to see this article?
>
> Dick Moore
>
>   
It should be publicly available by now, just use google with search
terms like: EDN PWM 32 bit DAC.
However there are some design errors some of which are relatively benign
others which may result in component destruction.

Bruce