[time-nuts] "Audio" DAC for GPSDO?

[Bob Camp]

Hi

There are several reasons why they don’t recommend the typical MCU DAC for control applications:

1) They are noisy at low frequency (1/f noise corner). That impacts their hitting their INL and DNL specs.
2) They have constant current leakage at DC. That makes their “center” value wander around by more than the spec’s would suggest. 
3) The major steps are trimmed for AC (high sample rate) compensation (the trim includes capacitance effects). At DC … no capacitance effects. 

Yes it’s all one big mess and the effects slop back and forth between the categories. Bottom line - they very much do not want you to measure their INL and DNL numbers on a continuous DC basis and then return the parts as being out of spec. MCU ADC’s can have some of the same issues. Even some pretty fancy outboard ADC’s only work well at DC if you put a chopper around them.

Bob

On Jun 2, 2014, at 7:22 PM, Bob Stewart <bob at evoria.net> wrote:

> Hi Poul,
> 
> I've been reviewing microchips literature and the way I read it is that the DAC isn't sensitive to staying at a fixed value.  If it's on, the FIFO is fed to the DAC.  If the FIFO is drained, then the user-settable default value is fed to the DAC.  When the output amp is turned off, it goes to a high impedance output.  I also noticed that Finput can vary from 0-45 khz.  I'm not certain what a 61db SNR would mean at DC values.  I see that the specifications are for a 15 uA load.  I assume that's not hard to meet with a typical op-amp.
> 
> It's interesting that in one paragraph they call the DAC default register a safety feature for industrial control applications, and then a few inches later a black box warns that it's not recommended for control type applications.  
> 
> 
> Bob
> 
> 
> 
> ________________________________
> From: Poul-Henning Kamp <phk at phk.freebsd.dk>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> Sent: Monday, June 2, 2014 4:08 PM
> Subject: Re: [time-nuts] "Audio" DAC for GPSDO?
> 
> 
> In message <1401742940.44103.YahooMailNeo at web142705.mail.bf1.yahoo.com>, Bob Stewart writ
> es:
> 
>> Could someone explain to me how such an audio DAC differs from a non
>> -audio DAC and why it's not suitable for this application?=A0 Is this just 
> 
>> a disclaimer from microchip to avoid liability or is there some practical
>> reason to go with a traditional DAC?
> 
> A lot of them have DC protections, so you can't leave them at a particular
> input value for very long before they go into safety mode and clamp the
> output to zero.
> 
> Your speakers love them for this, your OCXO not so much.
> 
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
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