Re: [time-nuts] "Audio" DAC for GPSDO?
Hi The DAC noise is likely “outside the loop” in a GPSDO. It’s not reduced by feedback. You can indeed find some of these parts that are noisy enough to degrade the ADEV or phase noise of an OCXO. Bob On Jun 2, 2014, at 10:13 PM, Bob Stewart wrote: > Hi Bob, > > I decided to look at Mouser for 16-bit DACs, and found the MAX541CCPA for > $12.47. On the one hand, it's an extra $12.47 for the project. On the > other, the dsPIC is a 3.3V device. I'll have to give some thought as to > whether I want to lose that much output range between the DAC and the EFC > divider which will be placed right at the OCXO. Given the flexibility of the > dsPIC33 pin remapping, I may just add it to the board but jumper around it at > first. Hmm, I could just place it right at the OCXO, since I need to > communicate with it with SPI. TBH, I don't think I have the equipment needed > to measure the noise unless it's really bad. I'm already way past my ability > to measure with my TIC daughterboard added to the VE2ZAZ board. > > This is all new territory for me. Should be fun. =) > > Bob > > > > From: Bob Camp > To: Discussion of precise time and frequency measurement > Sent: Monday, June 2, 2014 8:01 PM > Subject: Re: [time-nuts] "Audio" DAC for GPSDO? > > > 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 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 >> To: Bob Stewart ; Discussion of precise time and frequency >> measurement >> Sent: Monday, June 2, 2014 4:08 PM >> Subject: Re: [time-nuts] "Audio" DAC for GPSDO? >> >> >> In message <1401742940.44103.yahoomail...@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 >> p...@freebsd.org | TCP/IP since RFC 956 >> FreeBSD committer | BSD since 4.3-tahoe >> Never attribute to malice what can adequately be explained by incompetence. >> ___ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-b
Re: [time-nuts] "Audio" DAC for GPSDO?
Hi Bob, I decided to look at Mouser for 16-bit DACs, and found the MAX541CCPA for $12.47. On the one hand, it's an extra $12.47 for the project. On the other, the dsPIC is a 3.3V device. I'll have to give some thought as to whether I want to lose that much output range between the DAC and the EFC divider which will be placed right at the OCXO. Given the flexibility of the dsPIC33 pin remapping, I may just add it to the board but jumper around it at first. Hmm, I could just place it right at the OCXO, since I need to communicate with it with SPI. TBH, I don't think I have the equipment needed to measure the noise unless it's really bad. I'm already way past my ability to measure with my TIC daughterboard added to the VE2ZAZ board. This is all new territory for me. Should be fun. =) Bob From: Bob Camp To: Discussion of precise time and frequency measurement Sent: Monday, June 2, 2014 8:01 PM Subject: Re: [time-nuts] "Audio" DAC for GPSDO? 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 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 > To: Bob Stewart ; Discussion of precise time and frequency > measurement > Sent: Monday, June 2, 2014 4:08 PM > Subject: Re: [time-nuts] "Audio" DAC for GPSDO? > > > In message <1401742940.44103.yahoomail...@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 > p...@freebsd.org | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] "Audio" DAC for GPSDO?
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 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 > To: Bob Stewart ; Discussion of precise time and frequency > measurement > Sent: Monday, June 2, 2014 4:08 PM > Subject: Re: [time-nuts] "Audio" DAC for GPSDO? > > > In message <1401742940.44103.yahoomail...@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 > p...@freebsd.org | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] "Audio" DAC for GPSDO?
I think these kinds of DACs are meant to be clocked out at some fixed sample rate, like 44.1KHz and your software has to stuff a FIFO so there is some milliseconds of delay in the queue. Before you use these write some pseudo-code and see if you can make it work. One idea is to never write to the FIFO and change the "default" value that gets written when the FIFO is empty, that way you never mess with filling a FIFO thousands of times per second. My Arduino's PWM outputs are working well. The GPSDO has been running now for months. It's proof that you can do this with no external active components. I'm sure there must be some version of a PIC that has analog outputs that can directly drive a XO's EFC pin.If you need even one external IC, you'd be best off getting a different uP. On Mon, Jun 2, 2014 at 2:02 PM, Bob Stewart wrote: > Now that my TIC is working with Bert's board, I'm considering taking the next > step of designing a GPSDO from scratch. There are several projects I'd like > to do with a dsPIC33, so that was a natural choice. But I now understand > that it has an "audio" DAC and is not recommended for process control. 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? Is this just a disclaimer from > microchip to avoid liability or is there some practical reason to go with a > traditional DAC? > > On reading through the various datasheets, it appears to me that the concern > might be that the input data to the DAC might be interrupted, thus causing it > to go to some programmable "safe" output voltage. My initial thought was > just to control the value of the safe voltage and not bother to feed the DAC, > though I haven't really explored the idea. > > Bob - AE6RV > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] "Audio" DAC for GPSDO?
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 To: Bob Stewart ; Discussion of precise time and frequency measurement Sent: Monday, June 2, 2014 4:08 PM Subject: Re: [time-nuts] "Audio" DAC for GPSDO? In message <1401742940.44103.yahoomail...@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 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] "Audio" DAC for GPSDO?
In message <1401742940.44103.yahoomail...@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 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] "Audio" DAC for GPSDO?
Now that my TIC is working with Bert's board, I'm considering taking the next step of designing a GPSDO from scratch. There are several projects I'd like to do with a dsPIC33, so that was a natural choice. But I now understand that it has an "audio" DAC and is not recommended for process control. 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? Is this just a disclaimer from microchip to avoid liability or is there some practical reason to go with a traditional DAC? On reading through the various datasheets, it appears to me that the concern might be that the input data to the DAC might be interrupted, thus causing it to go to some programmable "safe" output voltage. My initial thought was just to control the value of the safe voltage and not bother to feed the DAC, though I haven't really explored the idea. Bob - AE6RV ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
