Why do most MCU's (even newest) have only 5-6bits D to A ?

[coppice]

Precisely!  If you are concerned about the rate of change, it's not DC.    Which was coppice's point: DC is low bandwidth, the lowest possible bandwidth.  Something that should readily be served by a low-bandwidth DAC, like filtered PWM!

PWM will cause small variable component to present, at the PWM frequency. If say you control a voltage controlled oscillator with PWM, it will cause spurs in the oscillator output below and above the oscillator frequency.  If this may be a problem depends on the application.

A low performance DAC, like the 5 and 8 bit ones people have been discussing here, usually has a horrible amount of digital noise on the output signal. Both a PWM signal and a cheap DAC need some heavy filtering to get a clean result.

[David Hess]

5B DAC can be dithered with PWM or SDM to get much higher resolutions.

Dithering does not improve linearity and linearity is what limits DAC resolution.

Non-linearity limits accuracy. It doesn't limit the resolution you can get with dithering, unless you have really gross issues like non-monotonic codes.

That is exactly the problem with the limited accuracy of integrated DACs.  Don't you think they would extend the R-2R ladder by 3 bits to get 8 bits of resolution if it resulted in only increased integral non-linearity?  The bragging return would be huge.

I found something interesting in some datasheets.  They *do* use charge redistribution DACs at higher resolutions with a sample-and-hold for each output.  Of course a 12-bit DAC with *5 counts of DNL* is not even 10-bits but that is still pretty good.  Microchip does not give a DNL specification for their integrated 8-bit R-R2 DACs or at least the ones I checked; my guess is that consistent with their other specifications, they are really 6-bits.

So yes, higher resolution integrated DACs are routinely not monotonic at their stated resolution.

[fourtytwo42]

Seems odd when a device can have a 10-bit or 12-bit A 2 D, why D 2 A is so limited!

If you are looking for a solution to your problem that does not involve pwm try an spi dac, there are many available eg https://www.microchip.com/wwwproducts/en/MCP4921 and simple to drive, most mcu's have spi.
As for the PIC16F series I still use that as a very good mixed signal part with a fast fully featured pwm for smps unlike some of the arm based parts that have poor pwm's IMOP (lacking certain features and no or little analogue integration).

[ogden]

A low performance DAC, like the 5 and 8 bit ones people have been discussing here, usually has a horrible amount of digital noise on the output signal.

Unless DAC is delta-sigma (which is not the case here with 5-6 bit DACs), it's resolution have nothing to do with digital noise on the output signal. You are free to prove me wrong by explaining what you mean by saying so.

[splin]

Dithering does not improve linearity and linearity is what limits DAC resolution.

Dithering is a well known ( [EDIT] at least by those who happen to  know it!    ) technique for improving linearity of DACs as well as ADCs.  Low levels reduce differential linearity errors by averaging over a number of nearby codes, but large scale dithering, up to 50% of full scale (or even more), can be used to reduce linearity errors over much larger ranges. Characterising its effectiveness for any specific DAC is another matter however.

[EDIT] See this paper for example: http://www.precisionmechatronicslab.com/wp-content/uploads/2017/01/J17c-Preprint2.pdf

[coppice]

A low performance DAC, like the 5 and 8 bit ones people have been discussing here, usually has a horrible amount of digital noise on the output signal.

Unless DAC is delta-sigma (which is not the case here with 5-6 bit DACs), it's resolution have nothing to do with digital noise on the output signal. You are free to prove me wrong by explaining what you mean by saying so.

By digital noise I meant the non Gaussian noise which is the result of leakage of the transitions in the digital circuitry into the analogue circuitry. To get that really low you need to commit quite a lot of silicon to isolation areas between the digital and analogue sections of the chip, which raises costs. So, in a lot of chips the isolation is pretty marginal, and leakage can change massively with temperature, the exact frequency at which the digital circuitry is being clocked, and other factors. There are some embedded sigma delta converters where the output signal (either a digitised signal from an ADC or an analogue signal from a DAC) quality can be really bad under just the right conditions. In some cases it can be a nightmare trying to track these issues down, if the converter performance looks good most of the time.

[David Hess]

Dithering does not improve linearity and linearity is what limits DAC resolution.

Dithering is a well known ( [EDIT] at least by those who happen to  know it!    ) technique for improving linearity of DACs as well as ADCs.  Low levels reduce differential linearity errors by averaging over a number of nearby codes, but large scale dithering, up to 50% of full scale (or even more), can be used to reduce linearity errors over much larger ranges. Characterising its effectiveness for any specific DAC is another matter however.

[EDIT] See this paper for example: http://www.precisionmechatronicslab.com/wp-content/uploads/2017/01/J17c-Preprint2.pdf

I wonder how practical that is in real life.  The DAC they are starting with is much better, even relatively, then what will be found integrated onto a microcontroller.

If they dithered over 100% then they could throw away the DAC and make a PWM or delta-sigma converter.  There is a reason delta-sigma converters largely dropped the use of multi-bit quantizers.

[coppice]

Dithering does not improve linearity and linearity is what limits DAC resolution.

Dithering is a well known ( [EDIT] at least by those who happen to  know it!    ) technique for improving linearity of DACs as well as ADCs.  Low levels reduce differential linearity errors by averaging over a number of nearby codes, but large scale dithering, up to 50% of full scale (or even more), can be used to reduce linearity errors over much larger ranges. Characterising its effectiveness for any specific DAC is another matter however.

[EDIT] See this paper for example: http://www.precisionmechatronicslab.com/wp-content/uploads/2017/01/J17c-Preprint2.pdf

I wonder how practical that is in real life.  The DAC they are starting with is much better, even relatively, then what will be found integrated onto a microcontroller.

If they dithered over 100% then they could throw away the DAC and make a PWM or delta-sigma converter.  There is a reason delta-sigma converters largely dropped the use of multi-bit quantizers.

Dithering really hard is just a way to use the multi-bit converter your manager told you to, while faking a sigma-delta converter out of it to get clean results.

[SuzyC]

Most replies seem to me to say a MCU chip is too noisy to fabricate a proper monotonic DAC and other schemes such as dithering and sigma-delta clever schemes are needed to attempt 8B DACs.

I don't get it. Trying to design with a 5B DAC today is like designing circuits using 20% tolerance resistors and paper capacitors. 

If 10-12B ADC is today quite common on any MCU chip, then noise problem have already been solved?

So why not at least 8B (a very much more powerful, practical HW tool)  DACs be more commonly offered, even if the output required external buffering and scaling?

Dithering requires more code, more CPU time, PWM requires filtering and creates settling time issues to attend to, while external DACs means bigger BOM and increased size of PCB cost.

[coppice]

Most replies seem to me to say a MCU chip is too noisy to fabricate a proper monotonic DAC and other schemes such as dithering and sigma-delta clever schemes are needed to attempt 8B DACs.

I don't get it. Trying to design with a 5B DAC today is like designing circuits using 20% tolerance resistors and paper capacitors. 

If 10-12B ADC is today quite common on any MCU chip, then noise problem have already been solved?

So why not at least 8B (a very much more powerful, practical HW tool)  DACs be more commonly offered, even if the output required external buffering and scaling?

Dithering requires more code, more CPU time, PWM requires filtering and creates settling time issues to attend to, while external DACs means bigger BOM and increased size of PCB cost.

There is no problem making an MCU really quiet, and putting a 16 bit DAC in it. The problem is it costs, and the market for a chip with that performance + price combination is too small for it to be viable to develop silicon for it. Find a high volume application for a device like that, and you'll find devices of that type rapidly appear.

[techman-001]

Most replies seem to me to say a MCU chip is too noisy to fabricate a proper monotonic DAC and other schemes such as dithering and sigma-delta clever schemes are needed to attempt 8B DACs.

I don't get it. Trying to design with a 5B DAC today is like designing circuits using 20% tolerance resistors and paper capacitors. 

If 10-12B ADC is today quite common on any MCU chip, then noise problem have already been solved?

So why not at least 8B (a very much more powerful, practical HW tool)  DACs be more commonly offered, even if the output required external buffering and scaling?

Dithering requires more code, more CPU time, PWM requires filtering and creates settling time issues to attend to, while external DACs means bigger BOM and increased size of PCB cost.

I wouldn't worry about it. Seeking an opinion here is just asking to get caught up in endless arguments between armchair experts that won't help a prototype appear on your desk.

The best way (imho) is to pick any modern MCU you like that has the tools you need to write and debug in the programming language you're happy with and just design your prototype.

You can buy a cheap 'development board' to test your code etc, but as far as noise is concerned, you'll need to design your own pcb designed  to minimize DAC or ADC noise. The main thing and this has been mentioned here before, is to follow good PCB noise reduction techniques, ground plane etc.

Just go and DO it or you'll be here arguing until the heat death of the universe!

[Siwastaja]

Most replies seem to me to say a MCU chip is too noisy to fabricate a proper monotonic DAC and other schemes such as dithering and sigma-delta clever schemes are needed to attempt 8B DACs.

No, noise is not limiting the analog performance of an MCU to 5 bits, unless the design is completely failed. Either you understood some comment wrong, or someone had no idea what they were talking about.

You could say that noise starts to be a concern after very roughly about 10 bits. The higher noise margin you want,  the more difficult, and expensive, it becomes, but going from 5 bits to, say, 8 bits, definitely isn't a problem defined by noise; it's something else.

I don't get it. Trying to design with a 5B DAC today is like designing circuits using 20% tolerance resistors and paper capacitors. 

The likely key is: it comes for practically free. A 12-bit DAC on that 30-year old architecture would have increased the product price, and moved it to a different product category altogether. You can come up with a lot of practical use cases for a free 5-bit DAC. So it's not a key feature, that MCU is not meant to be used for precision analog circuits; the DAC is a useful bonus.

If you need 2000 arbitrary analog voltage levels, then the 5-bit DAC is of course the wrong tool for the job. Use an external DAC. The noise characteristics will be better, too.

Sometimes you are fine with a 20% tolerance resistor (a heater, for example!); sometimes you are fine with 32 voltage levels. One use case would be setting a current limit to 32 different levels. Another would be to inject a finetune into the voltage feedback network, so if you have, say, a +12V supply, you could finetune it in 0.1V steps from from 11.0 to 14.2V.

[NorthGuy]

I don't get it. Trying to design with a 5B DAC today is like designing circuits using 20% tolerance resistors and paper capacitors. 

If 20% resistors meet your requirements, there's no difference whether you use 5% or 0.1% resistors. If you only need 3-bit DAC for your design, there's no difference whether you use 5-bit or 12-bit DAC (other than cost and such).

[SiliconWizard]

So anyway, I think you got your answer. Turns out there is still little demand for embedded DACs in MCUs, so the correct introduction to your question would have been "most MCUs don't have any DAC". Then among those that do, they are usually low-res DACs for all the reasons exposed above (noise, cost, die area, etc.) And, as I said earlier, there ARE some MCUs that have embedded DACs with 12-bit or higher. Sure the offer is small (although 12-bit DACs start being relatively common in ARM Cortex-M4+ MCUs), but the demand is probably small as well. Of course, if you're mainly looking at the low-cost MCUs, then the answer is even more obvious if you take the above into account.

To better understand why you're considering the matter, it may help to know what kind of applications you have in mind.

[splin]

So anyway, I think you got your answer. Turns out there is still little demand for embedded DACs in MCUs, so the correct introduction to your question would have been "most MCUs don't have any DAC".

"Turns out"? Do you have access to sales figures for a major MCU manufacturer or are you just speculating? I assumed you were an active engineer (who don't often have access to that sort of information) rather than a sales/marketing type. You may well be correct but it would be helpful to have some idea of how much credibility we can reasonably assign to such a claim. I personally can't think of anywhere near as many applications needing a DAC compared to an ADC but that isn't saying much!

Or are you basing it on the percentage of MCU model variants which include a DAC? That doesn't tell you anything about the relative sales of those variants - chances are, on that basis, you could could draw the same conclusion about demand for CAN bus peripherals. Sure CAN hasn't been around that long but maybe demand for a decent DAC goes hand in hand with the need for commensurate processing speed for signal processing? DMA or fast interrupt handling is also likely required for any non-trivial DAC application, also eliminating most old 8 and 12 bit MCUs. (Actually I would have loved to have had an 8048 variant with 2 x 8 bit DACS for our 1200b/s modem design back in the early '80s but they didn't exist!)

Then among those that do, they are usually low-res DACs for all the reasons exposed above (noise, cost, die area, etc.) And, as I said earlier, there ARE some MCUs that have embedded DACs with 12-bit or higher. Sure the offer is small (although 12-bit DACs start being relatively common in ARM Cortex-M4+ MCUs), but the demand is probably small as well. Of course, if you're mainly looking at the low-cost MCUs, then the answer is even more obvious if you take the above into account.

Cortex M0, STM32F0x (apart from the x0) have 12 bit DACs and I'd say they are pretty mainstream. Well I say 12 bit but they are only linear to 10 bits and there are no noise specifications so they could be even less. Still a bit of dithering, oversampling and filtering could address those problems and still maintain a much higher bandwidth than a PWM DAC whilst preserving a valuable timer for some other important use.   

Interestingly I can't find any noise specs for any STM32 DAC nor, unlike their ADCs, any questions or discussions on the 'net about it.  So perhaps you are right and nobody cares.   

Does anybody here have any real numbers for any MCU's real world DAC noise performance?

[David Hess]

There is no problem making an MCU really quiet, and putting a 16 bit DAC in it. The problem is it costs, and the market for a chip with that performance + price combination is too small for it to be viable to develop silicon for it. Find a high volume application for a device like that, and you'll find devices of that type rapidly appear.

Actually noise is a big problem.  At the 16 bit level, even discrete ADC and DACs have problems with noise.  Very few if any microcontroller converters achieve even 12 bits noise free.

[iMo]

My first 32bit board ever was the stm32L100 VL Discovery. It has got 2x12b DACs. I did some measurements with the DAC and it fit my 3.999V meter reading pretty well (even close to zero). No idea about noise.
Before that I messed a lot with pic24F and dspic33. Both have got 2x10bit or 12b DACs. I did measurements too - a crappy linearity. The datasheet states the DACs are not intended for DC precision apps but "audio".
Thus a decade back there were mcus with 12bit DACs.. 

PS: the basic mcu spec infos on "4-6bit DACs" are usually related to the DACs used with internal Comparators - they set the Comparator's threshold.

[NorthGuy]

Before that I messed a lot with pic24F and dspic33. Both have got 2x10bit or 12b DACs. I did measurements too - a crappy linearity. The datasheet states the DACs are not intended for DC precision apps but "audio".
Thus a decade back there were mcus with 12bit DACs.. 

dsPIC33FJ128GP802 (made in 2007) had a 16-bit delta-sigma audio stereo DAC. But that's totally different from small 5-bit ratiometric DACs in small PIC16s.

[ogden]

Well I say 12 bit but they are only linear to 10 bits

I do not understand this scaremongering of nonlinearities and absense of noise specs for built-in DACs. Every low cost DAC are like that - built in or not. Just opened stm32f302 datasheet, 12-bit DAC have ±4 LSB INL. Comparing to generic I2C 12bit DAC from TI, DAC121C081 having +/- 8bit INL, it does not look that bad at all

[edit] Some would say that TI have DACs with 1LSB INL. Yes. Their price is good too Some cost more than stm32f302 which is not only DAC but much more.

and there are no noise specifications so they could be even less.

String and R-2R <=12bit general use DACs usually do not have SNR specified like "24"-bit audio DACs do.

[coppice]

There is no problem making an MCU really quiet, and putting a 16 bit DAC in it. The problem is it costs, and the market for a chip with that performance + price combination is too small for it to be viable to develop silicon for it. Find a high volume application for a device like that, and you'll find devices of that type rapidly appear.

Actually noise is a big problem.  At the 16 bit level, even discrete ADC and DACs have problems with noise.  Very few if any microcontroller converters achieve even 12 bits noise free.

Its all about budgets. What you said was true a few years ago, but lots of MCUs are getting 14 to 16 bits of real performance in well laid out designs, and some are doing it at quite high (for an MCU) speeds. They are, however, limited by the cost constraints which dominate thinking in most of the MCU market. When performance is more critical than cost you can find some rather expensive MCUs that sell in relatively small numbers into niche applications which achieve very quiet analogue performance. These massively increase the resources put into power and signal isolation, and ground quality, and use differential circuitry where possible.

[iMo]

When not happy with an internal DAC (or an external DAC chip) you may still follow this Maker 
http://jayakody2000lk.blogspot.com/2020/01/24-bit-stereo-audio-dac-for-raspberry-pi.html

[iMo]

Ok, but the output of a DAC is static, the output of a PWM RC DAC is dynamic.
So why are 5-6Bit DAC's being now found in newer MCU's?
What is that  "specific purpose"?
What can a 5-6Bit DAC be used for?

As I wrote above, the specific purpose of the 4-6bit DACs in most today's MCUs is to
a) create an input voltage, or ref voltage for the internal ADC(s),
b) to create a threshold voltage for the internal Comparator(s),
c) their output could be routed to a Pin - that is not its primary purpose, rather a nice to have option, imho.
PS: read the datasheet

[David Hess]

There is no problem making an MCU really quiet, and putting a 16 bit DAC in it. The problem is it costs, and the market for a chip with that performance + price combination is too small for it to be viable to develop silicon for it. Find a high volume application for a device like that, and you'll find devices of that type rapidly appear.

Actually noise is a big problem.  At the 16 bit level, even discrete ADC and DACs have problems with noise.  Very few if any microcontroller converters achieve even 12 bits noise free.

Its all about budgets. What you said was true a few years ago, but lots of MCUs are getting 14 to 16 bits of real performance in well laid out designs, and some are doing it at quite high (for an MCU) speeds. They are, however, limited by the cost constraints which dominate thinking in most of the MCU market. When performance is more critical than cost you can find some rather expensive MCUs that sell in relatively small numbers into niche applications which achieve very quiet analogue performance. These massively increase the resources put into power and signal isolation, and ground quality, and use differential circuitry where possible.

Since designers have so much trouble getting 16 bit performance out of discrete 16 bit converters under ideal conditions, I would require proof of that.  Now if you mean 16 bit performance out of delta-sigma audio converters and DC instrumentation converters, I might believe that if the external signal conditioning is excluded, and I am dubious about the audio case.

[aheid]

I got a STM32F303 on a "bluepill" form factor board. How would I go about measuring the DAC performance? Got access to oscilloscope and 5.5 digit bench dmm.

[ogden]

I got a STM32F303 on a "bluepill" form factor board. How would I go about measuring the DAC performance? Got access to oscilloscope and 5.5 digit bench dmm.

You need firmware that can be controlled from PC (though USB CDC serial or just UART), then some test scripting software running on PC which steps through every stm32 DAC code and DMM reading. Why you want to measure DAC performance? Do not trust datasheet or what?