[sdiy] Filter cut off DAC

ASSI Stromeko at nexgo.de
Tue May 22 14:21:49 CEST 2018

On Tuesday, May 22, 2018 11:02:19 AM CEST paula at synth.net wrote:
> Brian, would you care to give the INL and DNL figures for the 16bit DAC
> used in 1981?

It's a Burr-Brown DAC71-CSB-I (in my PDF of the Rev.3 Tech Manual it's clearly 
marked as "14bit DAC", althought there was a 16bit variant in that family.   
Can't find a data sheet for that part online.  But even if they would have 
used the latter, the 4051 multiplexing and S&H that follow the DAC would have 
removed any semblance of the extra two bits anyway.  The precision of these 
DAC was also limited by the settling time you allowed it to use, so the very 
wide multiplexing used in the Prophet probably means they used a rather high 
rate and low settling time instead of shooting for the maximum accuracy from 
the DAC.

> I somehow suspect that these days a 12bit DAC, even a reasonably priced
> one, would have a better INL and DNL than the DAC from 1981.

You really need to be careful with these specifications.  For a CV DAC the 
absolute accuracy is going to enter the picture quite prominently unless you 
can calibrate a correction curve (possible, but unlikely) so INL is more 
important than DNL, but the extra resolution from having more bits still 
qcomes in handy when you have the compute power to create smooth enough 
signals at a higher update rate.  A lot of DAC are guaranteed monotonic these 
days (resistor string), so even if the INL is only 12bit, these 16bits of 
resolution are extremely helpful.  On the other hand, for an oscillator 
linearity is the key requirement and you are usually using the full scale of 
the output span to maximize SNR, so here you might be better off looking for a 
different DAC architecture.

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