[sdiy] Consider this DAC

[ASSI]

On Saturday 13 March 2010, Antti Huovilainen wrote:
> Please correct me if I'm wrong, but a TL072 for example has 18
> nV/sqrt(Hz) voltage noise. For 20 kHz bandwidth (RC lowpass at
> 10kHz meaning settling time of less than 0.2 msec) the resulting
> RMS noise is 2.5 uV. That's quite a lot less than 1mV.

Look up the graph of noise vs. frequency to see that you've missed to 
integrate a substantial amount of noise.  The flicker noise corner is 
at around 400Hz for the TL072 and the noise figure on the first page 
is for 1kHz and a source impedance of 50Ohm (guess what?).  There's 
also this pesky little thing called offset and offset drift (with both 
temperature and common mode voltage) that will need attention.  But 
maybe I'm still missing your point.

My definition of 1mV _precision_ is that there is at most +-0.5mV 
deviation from the ideal CV across all operating points.  For a 10Vpp 
signal range that results in about 14bit precision across the whole 
system (in voltmeter speak: 4½ digits) - which is doable, but not as 
easily or cheaply as you seem to believe.

Getting back to the CV->VCO example: "merely 1mV precision" across the 
whole system (just having this for the CV is not going to cut it) 
would allow you to set up two VCO five octaves apart (say 220Hz / 
7040Hz, after calibrating the beats out with both VCO at the same 
frequency) and have them beating at less than 2.5Hz.  Not only that, 
if you reverse which VCO plays the lower and the higher frequency, you 
still get less than 2.5Hz beats.  Modulating both VCO with the same 
voltage never produces any faster beats than said 2.5Hz and 
transposing the patch four octaves lower (13.75Hz / 440Hz) produces 
less than 0.2Hz beat frequency of the 32nd harmonic of VCO1 vs. the 
fundamental of VCO2.  Lastly, if you switch that hypothetical synth 
off, let it sit for a year and switch it back on, you would get the 
same results (not exactly the same beat frequencies, mind you, but in 
the same range, perhaps after warm up).  Ian Fritz might have such 
wonderfully precisely calibrated VCO, but there surely are a lot of 
systems that would fail this test.  In other words their precision is 
worse than 14bit/1mV and yet they are still perfectly useable.


Achim.
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