need high resolution PWM

[tatus1969]

For an instrument that I am desigining, I need a PWM at 450kHz with good linearity and resolution (equivalent to at least 14 bits). At the moment, I have a 16-bit DAC with 900kHz update rate and output filtering. The PWM is then generated in the classic way by a triangle generator and a comparator. I am also opting for an additional feedback loop for linearization (cancel out comparator deadtime). The circuit is then used to create a mix of DC and sinusodial AC amplitudes from DC to 50kHz.

Using a DAC to convert the digital information to analog, and then to use triangle+comparator to then make it "digital" again adds more cost and complexity to the solution that I feel would be necessary. But a pure digital / counter based approach isn't possible either. I wonder if there are affordable chips dedicated to this task, does anyone know? I need good linearity and resolution. Precision gain and frequency flatness aren't important, that is controlled by other means.

[NorthGuy]

14-bit at 450kHz is about 120 ps. I know newer dsPIC33C can do 250 ps resolution (according to the data sheet), just one bit short of what you want.

[iMo]

STM32F334 can do 217ps.

[PCB.Wiz]

For an instrument that I am desigining, I need a PWM at 450kHz with good linearity and resolution (equivalent to at least 14 bits).
.... But a pure digital / counter based approach isn't possible either. I wonder if there are affordable chips dedicated to this task, does anyone know?

 (1/450k)/2^14 = 135.63ps
Infineon XMC parts specify 150ps PWMs

I think TI has some 180ps DSPs too....

[tatus1969]

Thanks for your answers, especially the STM32F334 was new to me as I didn't know that ST also has devices with high resolution timers now.

I should have mentioned that I already have an STM32 in the design (STM32L4R9). I need that one because the application requires significant processing power (need for on-the-fly floating point DFT with two channels simultaneously) and I also have two sigma-delta ADCs that this micro can directly support.

The DAC/comparator solution will be around 5..10€ and gives me 16 bits on paper, but I expect that a full digital solution would have a better phase noise performance. I don't want to add another micro or DSP though, so I'll likely stick with my current solution. I was hoping for a magic block like "SPI in" "PWM out", but I'm not aware of such a thing.

The task is quite challenging: I need to superimpose a sinusodial AC in the millivolts range on a 0..30V DC voltage. I'm using PWM because I also need high output current. And the frequency analysis also requires phase coherency, which is why I am using the microcontroller to shape the waveform.

EDIT: just had a look at XMC4400, they also have sigma delta modulators, run at 120MHz, and are comparable or better in price. As I am an STM32 "native", I had started with that but I guess that I need to reconsider now...

[NorthGuy]

You don't really need floating point for DFT.

IMHO, instead of using many MCUs stitching them together through I2C, SPI or alike, it's usually better to chose one MCU which does everything you need. Even if you're an STM prisoner, you should be able to find a single STM to do all your tasks.

[MT]

Thanks for your answers, especially the STM32F334 was new to me as I didn't know that ST also has devices with high resolution timers now.

I should have mentioned that I already have an STM32 in the design (STM32L4R9). I need that one because the application requires significant processing power (need for on-the-fly floating point DFT with two channels simultaneously) and I also have two sigma-delta ADCs that this micro can directly support.

The DAC/comparator solution will be around 5..10€ and gives me 16 bits on paper, but I expect that a full digital solution would have a better phase noise performance. I don't want to add another micro or DSP though, so I'll likely stick with my current solution. I was hoping for a magic block like "SPI in" "PWM out", but I'm not aware of such a thing.

The task is quite challenging: I need to superimpose a sinusodial AC in the millivolts range on a 0..30V DC voltage. I'm using PWM because I also need high output current. And the frequency analysis also requires phase coherency, which is why I am using the microcontroller to shape the waveform.

EDIT: just had a look at XMC4400, they also have sigma delta modulators, run at 120MHz, and are comparable or better in price. As I am an STM32 "native", I had started with that but I guess that I need to reconsider now...

There is also the STM32H743 and H750 etc who have the computing power and HRTIM1 but runs at max 400Mhz instead of F334 4.6Ghz.

[tatus1969]

You don't really need floating point for DFT.

I'm using the Goerzel method on up to 32768 samples, which is fast but numerically not very stable. I may try integer arithmetic, but that part is working very well and other things need more attention right now.

IMHO, instead of using many MCUs stitching them together through I2C, SPI or alike, it's usually better to chose one MCU which does everything you need.

That's definitely my goal. And multiple MCUs and possible other logic like CPLDs or FPGAs make the field firmware update procedure much more complicated.

I'll stick with the analog solution for now, as it has another important advantage. The feedback loop that I have mentioned is not only for linearization, but it also provides power supply ripple rejection. In a fully digital solution, I'd have to emulate this in the microcontroller as well, and I'm not sure if I could achieve a comparable performance.

Even if you're an STM prisoner, you should be able to find a single STM to do all your tasks.

Not a prisoner, but getting acquainted with new toolchains and architectures takes time that I can't use to concentrate on my real task: designing this instrument. I'd rather go the second best route if that saves me time.

[tatus1969]

There is also the STM32H743 and H750 etc who have the computing power and HRTIM1 but runs at max 400Mhz instead of F334 4.6Ghz.

That timer is too slow at 2.5ns, and these chips only seem to have a single sigma delta demodulator input...

[nctnico]

For an instrument that I am desigining, I need a PWM at 450kHz with good linearity and resolution (equivalent to at least 14 bits). At the moment, I have a 16-bit DAC with 900kHz update rate and output filtering. The PWM is then generated in the classic way by a triangle generator and a comparator. I am also opting for an additional feedback loop for linearization (cancel out comparator deadtime). The circuit is then used to create a mix of DC and sinusodial AC amplitudes from DC to 50kHz.

Why aren't you using a class-D audio amplifier chip?
http://www.ti.com/audio-ic/amplifiers/speaker-amplifiers/overview.html
This one for example (a completely random pick) can deliver over 30W, has 111dB of dynamic range (around 18 bit) and a bandwidth of way over 50kHz.
http://www.ti.com/product/TAS5825M

You'll have to look for one where you can disable the DC offset fault detection.

[PCB.Wiz]

I was hoping for a magic block like "SPI in" "PWM out", but I'm not aware of such a thing.

The  Class D amplifier mentioned above, is probably the nearest to Serial IN, (i2s) to PWM out.

The task is quite challenging: I need to superimpose a sinusodial AC in the millivolts range on a 0..30V DC voltage. I'm using PWM because I also need high output current.

What range of AC (mV, Hz) do you need to superimpose on the DC, and at what current ?

Another approach could be to take a standard SMPS, which has a ramp/comparator built in, and control the DC and AC sides separately.
A high resolution DAC sets DC levels, and then a scaled modulator DAC can set the AC modulation.
If the AC is always small, you can use a DC SMPS for the main DC control and then a linear regulator (which has low voltage drop & thus tolerable? power loss) for the AC modulate.

[nctnico]

The method of PCB.Wiz also seems like a good approach to me. Generate the DC level with a regular power supply and provide the AC part using a seperate path. A transformer could to the job and this also provides a way to step the AC voltage down. For example: using a class-D amplifier to drive a 10 to 1 transformer gives a lot of dynamic range for small signal levels. A suitable transformer would be a gap-less one intended for building a forward converter. These are standard off-the-shelve parts from Coilcraft or Wurth (and probably others as well).

[ali_asadzadeh]

16Bit SPI @ 50MHz would do that for you, you can use up to 8 words as your PWM duty cycle, also it's free in your STM32.

Also you can use circular DMA to free update the PWM

[OwO]

Does the output have to be fixed-frequency PWM or is delta-sigma modulation acceptable too? It's very easy to implement on a STM32, you just need to compute the "bitstream" in chunks and use the DMA to stream the bit values onto a GPIO. You can easily get modest dynamic range this way, for example 16bit equivalent @ 1MHz toggle rate is easily achievable.

[tatus1969]

Thanks to all your ideas, and I'm open in all directions.

Here is what the amplifier specifically needs to do:
- drive complex loads down to 1 milliOhm
- source current into load and sink current from it
- DC 0.0 to 30V
- sinusodial AC of 1mV to 5V (at least, better full range) and 1mHz to 50kHz
- current is +/- 5A

It is a device meant for DIYers, so every penny counts to keep the BOM cost in a reasonable region.

I'll tell more about this in this forum soon. At the moment I need to stay fuzzy as this something that is new to the market and I don't want our Chinese copier friends to be faster than me. It's maybe interesting to mention that such instruments exists in the professional segment, starting at USD8000.

Does the output have to be fixed-frequency PWM or is delta-sigma modulation acceptable too? 16bit equivalent @ 1MHz toggle rate is easily achievable.

That's an interesting idea, but without calculating it my intuition tells me that I can't get 16 bits for a 50kHz signal from a 1MHz bitstream. My sigma delta ADCs achieve just that but run at 20MHz.

16Bit SPI @ 50MHz would do that for you, you can use up to 8 words as your PWM duty cycle, also it's free in your STM32.
Also you can use circular DMA to free update the PWM

The SPI itself is not my problem, that's running well. Although I can't use DMA for it, because the STM32's SPI block does not allow me to insert adjustable pauses between packets. I ended up doing the (integer) waveform calculations and SPI feeding in an interrupt at 900kHz. The microcontroller is surprisingly powerful and only needs 10% of its time for all that.

Why aren't you using a class-D audio amplifier chip?

The  Class D amplifier mentioned above, is probably the nearest to Serial IN, (i2s) to PWM out.

I actually have one in service in my proof-of-concept build, a Hypex UcD180HG. These use a hysteretic approach that should theoretically have less problems driving my low impedance load. I don't see any instability, which is good, but it a) introduces significant nonlinearity at lower frequencies, and b) it is designed for a symmetrical supply. The latter causes issues when sinking load current, ending up in the need of having dissipative supply voltage limiters even when it is sourcing current. The solution needs to run from a single supply and be able to operate down to the negative rail. And all class D amps that I have looked into have classic output stages with switching deadtime, and that is a linearity killer for power stages operating in two quadrants. If my concept turns out to be working, then it will almost completely eliminate that (at the expense of higher idle current). I'm actually thinking of making a later spin off in form of a high end audio amp :-)

Another approach could be to take a standard SMPS, which has a ramp/comparator built in, and control the DC and AC sides separately.
A high resolution DAC sets DC levels, and then a scaled modulator DAC can set the AC modulation.
If the AC is always small, you can use a DC SMPS for the main DC control and then a linear regulator (which has low voltage drop & thus tolerable? power loss) for the AC modulate.

The method of PCB.Wiz also seems like a good approach to me. Generate the DC level with a regular power supply and provide the AC part using a seperate path. A transformer could to the job and this also provides a way to step the AC voltage down. For example: using a class-D amplifier to drive a 10 to 1 transformer gives a lot of dynamic range for small signal levels. A suitable transformer would be a gap-less one intended for building a forward converter. These are standard off-the-shelve parts from Coilcraft or Wurth (and probably others as well).

My requirements don't allow that, as the AC isn't always small. But I might end up with a hybrid approach if I can't get a single output stage to meet my requirements, like having a low resolution class D power stage tied to a second high resolution / low voltage "servo" class A amplifier. But that could make the solution unaffordable for its target audience.

[OwO]

Instead of a ramp gen + comparator I would at least use a feedback+integrator+hysteresis analog delta-sigma modulator which will give you better linearity at the upper and lower extremes of the signal swing, as well as killing two birds with one stone by also adding the negative feedback you need. It would also likely use less parts than a classic PWM modulator, consisting of only ~2 op-amps.

EDIT: Also don't make the mistake of thinking you can keep "clones" out by secrecy, it doesn't work that way. There are "real engineers" in China, given the idea they can engineer their own solution that costs less and performs similarly. The only way to do things is make your product competitive as well in terms of bang for buck.

[ejeffrey]

+1 on using delta sigma output over pure PWM.  For many applications that will be strictly better and the computation overhead can be very small.  At least if you are filtering anyway.  If you are trying to switch transistors that can't switch at higher frequency you have to weigh the noise budget more carefully, but often even then delta sigma is better even if it degrades your high frequency SNR slightly.

[tatus1969]

Instead of a ramp gen + comparator I would at least use a feedback+integrator+hysteresis analog delta-sigma modulator which will give you better linearity at the upper and lower extremes of the signal swing, as well as killing two birds with one stone by also adding the negative feedback you need. It would also likely use less parts than a classic PWM modulator, consisting of only ~2 op-amps.

+1 on using delta sigma output over pure PWM.  For many applications that will be strictly better and the computation overhead can be very small.  At least if you are filtering anyway.  If you are trying to switch transistors that can't switch at higher frequency you have to weigh the noise budget more carefully, but often even then delta sigma is better even if it degrades your high frequency SNR slightly.

Sounds like a good idea, I'll definitely evaluate that. The MOSFET's switching transitions will be around 20 to 50ns, and the delay will be maybe double of that. Switching action creates more heat than usual with my intended topology, and I cannot see at the moment whether PWM or sigma delta will be more beneficial here. I'll design my first attempt such that I can easily switch between them. Which sampling clock frequency would you choose? Available sigma-delta class D audio amplifiers seem to use 6 .. 10MHz.

Good SNR is especially important at low frequencies. I can easily average across multiple periods at 10kHz, but that's not an option at 1mHz :-)

Also don't make the mistake of thinking you can keep "clones" out by secrecy, it doesn't work that way. There are "real engineers" in China, given the idea they can engineer their own solution that costs less and performs similarly. The only way to do things is make your product competitive as well in terms of bang for buck.

That's exactly why I am hinding the idea at the moment. I'm an indivdual and they are many, my only chance is to use my time advantage.

[PCB.Wiz]

Thanks to all your ideas, and I'm open in all directions.

Here is what the amplifier specifically needs to do:
- drive complex loads down to 1 milliOhm
- source current into load and sink current from it
- DC 0.0 to 30V
- sinusodial AC of 1mV to 5V (at least, better full range) and 1mHz to 50kHz
- current is +/- 5A
...

The  Class D amplifier mentioned above, is probably the nearest to Serial IN, (i2s) to PWM out.

I actually have one in service in my proof-of-concept build, a Hypex UcD180HG. These use a hysteretic approach that should theoretically have less problems driving my low impedance load. I don't see any instability, which is good, but it a) introduces significant nonlinearity at lower frequencies, and b) it is designed for a symmetrical supply. The latter causes issues when sinking load current, ending up in the need of having dissipative supply voltage limiters even when it is sourcing current. The solution needs to run from a single supply and be able to operate down to the negative rail. And all class D amps that I have looked into have classic output stages with switching deadtime, and that is a linearity killer for power stages operating in two quadrants. If my concept turns out to be working, then it will almost completely eliminate that (at the expense of higher idle current). I'm actually thinking of making a later spin off in form of a high end audio amp :-)

If you need to go cleanly to 0V, a common trick for universal device programmers, was to generate a slight negative rail, of the smallest needed to reach 0V. (eg -1.2V or -2V)

If you want that power range, and low price, I'd still say Class D is the best choice, as you get to piggyback on the big volumes, but you may need to tweak/enhance designs to get DC operation.

You have also not mentioned measure/control of current, which could be harder in Class D designs.


Is your load passive (like a battery) or active (like operating electronics) ?

[tatus1969]

If you need to go cleanly to 0V, a common trick for universal device programmers, was to generate a slight negative rail, of the smallest needed to reach 0V. (eg -1.2V or -2V)

I want to avoid that if possible, but doing that would extend the usability of the device even further.

You have also not mentioned measure/control of current, which could be harder in Class D designs.

Both voltage and current are controlled via other parts of the circuit (the MCU adjusts the DAC output waveform accordingly), which is why the only task of the power amp is to provide linearity and SNR. Precise gain is not necessary.

Is your load passive (like a battery) or active (like operating electronics) ?

Passive, but complex of any kind (both reactive and capacitive).

[PCB.Wiz]

Both voltage and current are controlled via other parts of the circuit (the MCU adjusts the DAC output waveform accordingly), which is why the only task of the power amp is to provide linearity and SNR. Precise gain is not necessary.

A useful part to look at could be the TI TAS3251 - that has 24b DACs included, and i2s interface, 36V and ~300W
You would need to do a careful DC level shift on the coupling, and probably add an external ADC to verify/feedback DC offset errors.

For more discrete solutions, Audio DACs can go to above 192KHz sample rates (TI part is 96kHz) and the specs I checked give 0Hz as minimum.
Their DC may not be super-stable, but it does look like they go to DC.
 

[tatus1969]

I have to update one of the requirements: 14 bit DAC resolution is definitely not enough, I'll have to go for at least 20 bits. I was chasing noise problems in my demonstrator (using a 16 bit DAC), which made this clear to me. The picture shows the DAC value in blue, and red is the measured DC current into a 50 milliohm load (no AC amplitude). Clearly not good enough.

A useful part to look at could be the TI TAS3251 - that has 24b DACs included, and i2s interface, 36V and ~300W
You would need to do a careful DC level shift on the coupling, and probably add an external ADC to verify/feedback DC offset errors.

I'll definitely try that, as i2s naturally gives me enough resolution, and the chip seems to be DC capable. The power stage is closed loop, which is my major concern as the loop is certainly not tuned to drive an ultra low impedance load.

[Yansi]

Are you the proffesor that has written us on the st support service recently about the hrtim and GaN?

[FrankBuss]

I don't think it is a good idea to use PWM for the output. There is a reason why most good power supplies are using a linear output stage with big heat sinks.

A 20 bit DAC with 900 kHz gets really expensive, and I doubt it would help, because then you would need very accurate comparators with very low hysteresis, and a very fast switching output stage as well. And it would cost more than linear regulation with a big MOSFET and heatsink.

[tatus1969]

Are you the proffesor that has written us on the st support service recently about the hrtim and GaN?

No :-) But I have actually written you (ST) in the past, because that STM32 has problems getting the DFSDM channels in sync. Even when I start them in consecutive instruction cycles, they exhibit varying phase shift. I use two channels and need them to sample them synchronously, because I am measuring the phase between the channels. It is only 1..2 degrees at 50kHz if I remember correctly, but this is significant in this application. Haven't heard back from ST sadly, my expected annual volumes seem to be low (1k .. 10k)...

I don't think it is a good idea to use PWM for the output. There is a reason why most good power supplies are using a linear output stage with big heat sinks.

They do that to keep noise levels down, but analog noise is not my problem as you can see from the graph. On the left, you can see the analog noise which is only a few 100 microamperes. This includes the class D audio amp that is in the system, and it creates *huge* output ripple.

But try to get a linear amplifier drive a 1 milliohm load - this is a really challenging task. That's why my plan is to achieve enough "stiffness" in the H bridge itself, which should allow me to reduce feedback gain. And I should even be able to live with nonlinearity, because the system uses frequency analysis.

EDIT: And I still would need the DAC.

A 20 bit DAC with 900 kHz gets really expensive, and I doubt it would help, because then you would need very accurate comparators with very low hysteresis, and a very fast switching output stage as well.

I'll use a 24 bit I2S DAC for audio applications, their prices are reasonable. The output stage is custom and uses a new method to elimininate dead time. That method already uses 4.5ns comparators, 2€ a piece.

And it would cost more than linear regulation with a big MOSFET and heatsink.

I'm not so sure. Power dissipation would be 5A x 15V30V = 75W150W meaning substantial heat sink and probably fan. And power consumption also doubles requiring a bigger PSU brick.