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

[coppice]

What?  Why you say so? Sigma-delta DAC is as DC-accurate as PWM DAC, properly designed PWM/PDM DAC is linear enough to be used for DC voltage standard such as Keithley 263 Calibrator.

Have you tried looking at the DC specs for actual sigma-delta ADCs and DACs? The linearity is usually superb, but there is usually a fairly big DC offset that wanders around quite a bit with temperature. This issue can be tamed, and there are instrumentation sigma-delta converters which do achieve good DC results, but it costs and isn't usually done.

[schmitt trigger]

I do have knowledge from involvement in the systems design of MCUs. Systems designers get a lot of casual "wouldn't it be nice if there was a cheap low performance DAC, perhaps to DC bias the op-amps on the MCU die into range" type input from customer visits. However, when you get to hard questions like "would this DAC actually make you buy a shed load of parts at a reasonable price?" you are left with a few hard cases where specific DAC performance is needed for a specific application. In those cases, if the volume justifies the NRE, you will find there is a device (or a small cluster of related devices which use the same die) that contains a suitable DAC. Its gain, linearity, speed, etc. will have been tailored for a specific high volume customer need. That's why you'll find a small sprinkling of 10 and 12 bit DACs in some MCU ranges. If you look at the complete peripheral mix on this chips, and you have some suitable applications knowledge, you might be able to identify the application the chip was designed for.

I think that this is the most sensible answer so far.

If a microcontroller company, any company, determines that they can produce a particular feature reliably, at a price point which will have mass-market appeal and their NRE recouped in a reasonable time, then said feature sooner or later will get incorporated into the microcontrollers.

Simple as that. The market's "invisible hand".

[ogden]

Have you tried looking at the DC specs for actual sigma-delta ADCs and DACs?

I did even better than that. - Wrote previous post.

The linearity is usually superb, but there is usually a fairly big DC offset that wanders around quite a bit with temperature.

Widely known SD ADC (&DAC) problem is presense of DC idle tones, the closer to midrange, the lower freq of the tones, but what is DC offset wandering? Citation needed. Pointers to datasheet figures showing that will be helpful as well.

[coppice]

Have you tried looking at the DC specs for actual sigma-delta ADCs and DACs?

I did even better than that. - Wrote previous post.

The linearity is usually superb, but there is usually a fairly big DC offset that wanders around quite a bit with temperature.

Widely known SD ADC (&DAC) problem is presense of DC idle tones, the closer to midrange, the lower freq of the tones, but what is DC offset wandering? Citation needed. Pointers to datasheet figures showing that will be helpful as well.

You shouldn't see idle tones from a modern sigma delta ADC, but if you zero the input do you get zero for the digital result? In most devices the offset you get is quite significant, and moves around quite a lot as the temperature changes. The datasheets usually specify the size of the offset properly, but they don't usually tell you its likely to wander quite a bit with temperature, and you can't just calibrate it away. The offset you are seeing is not from the digital processing. Its from the front end. If its a switched cap front end its usually better than if its a continuous time front end. I think most of the converters you'll find on MCUs have switched cap front ends. Sigma delta DACs are subject to the same issues are ADCs.

[ogden]

You shouldn't see idle tones from a modern sigma delta ADC

They are still there, just suppressed below noise floor with dithering. Actually good question what comes first - noise floor spec or level of idle tone spurs.

but if you zero the input do you get zero for the digital result?

Perhaps most of audio ADCs drifts, for example mentioned AD1955 have 25ppm/oC gain drift. On the other hand offset error drift of LTC2400 is 0.01 ppm of VREF/°C and 0.02 ppm full scale error drift. That means not every SD ADC as you say "wanders around quite a bit with temperature". For a moment you got me worried...

[coppice]

but if you zero the input do you get zero for the digital result?

Perhaps most of audio ADCs drifts, for example mentioned AD1955 have 25ppm/oC gain drift. On the other hand offset error drift of LTC2400 is 0.01 ppm of VREF/°C and 0.02 ppm full scale error drift. That means not every SD ADC as you say "wanders around quite a bit with temperature". For a moment you got me worried...

Gain drift and DC shift are completely separate things. The gain drift of a sigma-delta converter is a combination of two things - the temperature stability of the reference, and the temperature stability of the gain of the front end modulator. In most cases the reference will dominate. In some cases you can use an external reference, and improve that figure, but not all sigma-delta converters support the use of an external reference.

[ogden]

Gain drift and DC shift are completely separate things.

Gain drift for ADC which have "floating" zero at 1/2 Vref means that zero drifts as well. Anyway please be so kind and open datasheet of LTC2400, show where it's specs confirm your words "wanders around quite a bit with temperature".

[edit] For a fun let's check TM7705, 1$/1piece 16-bit SD ADC. Zero drift: 0.5 μV/℃ typ. Note that LSB of 16-bit ADC with 2.5V reference is 38uV. Where is "wanders around" - I can't find. Citation needed.

[SuzyC]

Seems also odd to me that everyone posting here is clearly only speculating about the why's and why nots of having at least ~8-bit DACS.

Why is it that anyone  working at Microchip or any other MCU mfg fails to post any feedback to this board?

It would be nice,  once in a while, to get the facts from someone who really knows.

Kinda like a Trump Impeachment trial. 

[splin]

Gain drift and DC shift are completely separate things.

Gain drift for ADC which have "floating" zero at 1/2 Vref means that zero drifts as well. Anyway please be so kind and open datasheet of LTC2400, show where it's specs confirm your words "wanders around quite a bit with temperature".

[edit] For a fun let's check TM7705, 1$/1piece 16-bit SD ADC. Zero drift: 0.5 μV/℃ typ. Note that LSB of 16-bit ADC with 2.5V reference is 38uV. Where is "wanders around" - I can't find. Citation needed.

The LTC2400 auto calibrates zero and gain every conversion.
TM7705 has a chopping input to elimate zero drift. I believe it is an AD7792 clone or at least very similar which also autozeros every conversion.

It's easy with an ADC, requiring only an analogue mux to periodically connect the input to ground to measure offset. Much harder to do with a DAC as you'd have to output a zero code and measure the offset with an ADC - whilst maintaining the signal output using a sample and hold. If you had an ADC that outperforms the DAC then you could measure the DAC error directly but that would likely be an expensive solution.

I'd suggest that neither solution would be feasible for an MCU DAC peripheral. Minimising zero drift would have to be in the analogue domain.

Comparing ADC and DAC

[Jan Audio]

Suzy try at : microchip.com/forums
I think it is a good marketing strategy, i bought chips because it had a DAC ( i did not see it was useless 5-bits )

[coppice]

The LTC2400 auto calibrates zero and gain every conversion.
TM7705 has a chopping input to elimate zero drift. I believe it is an AD7792 clone or at least very similar which also autozeros every conversion.

It's easy with an ADC, requiring only an analogue mux to periodically connect the input to ground to measure offset.

Its actually quite hard to do this if you want high accuracy. Take a multi-input ADC on a well laid out board with a good ground. Connect all the ADC inputs together, and scan around sampling them all. In most cases they don't quite agree, because of small DC offsets through the mux, which are a little different on each mux channel. It takes really careful design, and additional die area, to completely drive those offsets from the design.

[coppice]

Seems also odd to me that everyone posting here is clearly only speculating about the why's and why nots of having at least ~8-bit DACS.

Why is it that anyone  working at Microchip or any other MCU mfg fails to post any feedback to this board?

It would be nice,  once in a while, to get the facts from someone who really knows.

Kinda like a Trump Impeachment trial.

You definitely do get input from people in the know. However, most people don't wish to advertise their current or former employer's names. Every vendor has haters, sometimes for good reasons, and sometimes for bad. These people have a nasty tendency to lay into anyone they can associate with a vendor they hate.

[ogden]

The LTC2400 auto calibrates zero and gain every conversion.
TM7705 has a chopping input to elimate zero drift.

Right. That's my point that SD ADC's (that I know) aimed at low freq/DC applications do not "wander around quite a bit with temperature". Existence of zero calibration in those ADC's actually strengthens my argument.

Comparing ADC and DAC

Fair point. There are not that many "non-audio" SD DAC's to start with. One I know, DAC1220 have 1ppm/oC zero offset drift specs. Still do not qualify as "wander quite a bit".

[JPortici]

Before calling them useless, dacs are not only to be used for high res signal generators. Altough 5 bits can be adequate for sine waves if properly filtered

From what i gathered the 5bit dac is there because it was initially used as threshold for the internal comparators and for that role it is adequate.
Add a buffer, route to a pin and you can put DAC output on marketing material

In more advanced parts like the dsPIC33C there are multiple 12bit DACs but only one DACOUT pin. AGAIN, the DAC serves as a setpoint for the internal comparators and other analog blocks (in this case for motor control or Digital SMPS) and not for signal generators.

[Howardlong]

I'll also add that most DACs on PICs are simple resistor networks and lack any buffering, so to use them externally you typically need to buffer the outputs. Equally, many of those chips with DACs also have on chip opamps that can be configured internally as buffers for the DACs internally (this doesn't apply to the dsPIC33C which has its own DAC buffer for its DACOUT pin, and the opamps can only be configured externally once enabled).

[mikerj]

Seems also odd to me that everyone posting here is clearly only speculating about the why's and why nots of having at least ~8-bit DACS.

Why don't you demonstrate why all these people are wrong, and why including 8 bit DACs really wouldn't impose any cost penalty for manufacturers.  So far you have continued to insist they should be provided, and rejected all explanations without providing anything to support your own position. 

Someone with a modicum of critical thinking skills would deduce that if 8 bit DACs were effectively free, and would give manufacturers a competitive advantage by including them, then it would be happening.  Do you think this is some kind of conspiracy by "big microcontroller"?

[SiliconWizard]

Seems also odd to me that everyone posting here is clearly only speculating about the why's and why nots of having at least ~8-bit DACS.

Why don't you demonstrate why all these people are wrong, and why including 8 bit DACs really wouldn't impose any cost penalty for manufacturers.  So far you have continued to insist they should be provided, and rejected all explanations without providing anything to support your own position. 

Yup.

And, we still don't have any answer as to the applications where the OP would need 8 bit DACs (or higher), and that couldn't be fulfilled with the existing offer (as I said, probably just for cost reasons, and the OP still doesn't want to listen to the reasons why DACs are costly) or alternatively implemented with just PWM or SDM.

Additionally, we also don't know whether the 8-bit would just be for the numbers or for a real accuracy need. Point is, a crap (read: simple and inexpensive) 8-bit DAC will have no more than 5-6 enob anyway. Would the OP still want an MCU that avertises an 8-bit DAC with that kind of characteristics, so they can feed the peripheral programmatically a full 8-bit word?

As you also mentioned, proper output buffering is also an issue and has a cost. Unless the MCU also embeds opamps usable for this (with low enough offset and RRO - which is not that cheap either) (of course you'll be expecting the whole 0V-Vref range!), you'd often need an additional buffer IC anyway - so you can as well use an external DAC for the same part count.

As I said earlier, it ultimately looks like a battle for low-cost, just assuming that semiconductor vendors are crooks. Well, not just crooks, but also clueless apparently, since some seem to think it's not just a market/cost reason, but just that they don't want to "innovate".

Why don't you design your own MCUs all with good embedded DACs, targetting the sub-dollar price tag (because above that, you can definitely find some MCUs with decent DACs), and see how that goes?

[David Hess]

Seems also odd to me that everyone posting here is clearly only speculating about the why's and why nots of having at least ~8-bit DACS.

It is more than speculation.  We *know* that the common digital CMOS processes (1) being used are not suitable for better DACs at any price.  Even their built in references are horrible.  In my earlier posts, I described some of the problems.

(1) Common packaging technologies are also not suitable for analog precision.  Analog ICs use a encapsulation inside the package which prevents stress from being applied to the semiconductor die.