Attainable PWM accuracy?

[George Herold]

On Thursday, October 17, 2019 at 4:39:22 AM UTC-4, piglet wrote:

On 16/10/2019 20:01, George Herold wrote:
I didn't know about Stephen Woodward. Has he written any books?
Other good articles?


No books that I know of. Stephen Woodward has contributed many
Ideas-for-Design to EDN/ED since at least the early 1990s. I always
remember the devastatingly clever things he'd do with PWM'd 4053 analog
switches. Reminds me of the clever tricks Jim Thompson would do with a
TL431

piglet

Thanks piglet, I found several EDN articles. What's sad is that
there are no pictures /schematics to go along with the older ones.

I read an article on diode laser thermal control that I found a
little confusing... maybe I should send him an email?
Here,
https://www.electronicdesign.com/archive/tdl-self-senses-temperature-improve-spectroscopy-accuracy
Did he ever post here?

George H.

 

[Joseph Gwinn]

On Oct 17, 2019, George Herold wrote
(in article<6790f770-d4a2-480f-888b-8d3b894cae57@googlegroups.com&gt:

On Thursday, October 17, 2019 at 4:39:22 AM UTC-4, piglet wrote:
On 16/10/2019 20:01, George Herold wrote:
I didn't know about Stephen Woodward. Has he written any books?
Other good articles?

No books that I know of. Stephen Woodward has contributed many
Ideas-for-Design to EDN/ED since at least the early 1990s. I always
remember the devastatingly clever things he'd do with PWM'd 4053 analog
switches. Reminds me of the clever tricks Jim Thompson would do with a
TL431

piglet

Thanks piglet, I found several EDN articles. What's sad is that
there are no pictures /schematics to go along with the older ones.

I read an article on diode laser thermal control that I found a
little confusing... maybe I should send him an email?
Here,
https://www.electronicdesign.com/archive/tdl-self-senses-temperature-improve-spectroscopy-accuracy
Did he ever post here?

Not that I recall, but he has published recently, so it should be possible to
contact him:

..<https://www.electronicdesign.com/analog/simple-design-equations-
thermoelectric-coolers-revisited>

I had no problem with the original link.

Joe Gwinn

 

[speff]

On Saturday, 12 October 2019 05:15:10 UTC+8, Lasse Langwadt Christensen wrote:

fredag den 11. oktober 2019 kl. 22.52.50 UTC+2 skrev John Larkin:
On Fri, 11 Oct 2019 16:17:51 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2019-10-11 15:54, Andy Bennet wrote:
On 11/10/2019 18:16, Phil Hobbs wrote:
Hi, all,

As part of the aforementioned cathodolumiescence system, I'm doing a
bias supply for multipixel photon counters (MPPCs).

MPPCs are extremely voltage sensitive--the gain of this one goes from
~0 to 2E6 between 52 and 55 volts' bias.  (See
https://www.hamamatsu.com/resources/pdf/ssd/e03_handbook_si_apd_mppc.pdf>.)


Sooo, there's a bit of pressure to keep the bias very stable, but it
doesn't have to change very often.  Accordingly, I'm tentatively
planning to use a 12 to 16-bit PWM with good filtering.  It'll
obviously have to be buffered with a tinylogic gate running from a
stiff reference supply, to prevent voltage sags inside the LPC845 MCU
from spoiling the accuracy.

The '845 can run its PWM clocks at 7.5 MHz, so that would be an output
frequency of 114 Hz to 1.8 kHz. I'd need about 100 dB of filtering to
get the output ripple on the MPPC supply down to a millivolt or so, so
that's 3 RC sections with 33 dB attenuation each, i.e. corner
frequencies of 2.5 Hz to 40 Hz (TCs of 4 to 60 ms).  Not
horrible--100k*0.68uF at 16 bits, 40k and 0.1 uF at 12 bits.

Anything else besides buffering and filtering that I haven't thought
of that might limit the accuracy?  How good can you make a PWM, anyway?

Thanks

Phil Hobbs


Rather than PWM, have a look at pulse density modulation which will ease
the filtering requirement. Not sure if this is do-able in software, but
I have used FPGA implementations of this with great success.

Sort of a delta-sigma thing? You can put an M-bit delta-sigma extension
on an N-bit PWM pretty easily using a timer interrupt. I don't think
we could get the required accuracy bit-banging something like that.

Cheers

Phil Hobbs

You can also delta-sigma the LSB of a PWM to effectively add more
bits. That basically interpolates between two PWM levels.

I think that may have filtering advantages. I don't know if a uP can
reasonably do that; we did it in an FPGA.

I guess you could even PWM the LSB of a PWM.

AND a fast and a slow PWM?

in hardware there's also the trick of inverting some or all of the bits
in the counter

The TI trick to get 180ps PWM resolution on some of their MCUs is interesting.

https://www.ti.com/lit/ug/spru924f/spru924f.pdf

Best regards,
Spehro Pefhany

 

[mpm]

On Saturday, October 12, 2019 at 2:34:01 AM UTC-4, Rick C wrote:

> It would be so nice if they just included a thousand or so LUTs in with every MCU. Then you could design the hardware you need rather than trying to fit the design into the limited hardware available.

Is there a way to "+1" this reply?
I just finished a product that required a 24-hour timer under certain conditions, while it was doing its other normal functions. No big deal I guess; a timer interrupt and some registers, with some care and attention to not spending too much time in the interrupt service routine.

But at 8-bits? 24 hours is a lot of counting!

 

On Sat, 19 Oct 2019 08:06:53 -0700 (PDT), mpm <mpmillard@aol.com>
wrote:

On Saturday, October 12, 2019 at 2:34:01 AM UTC-4, Rick C wrote:

It would be so nice if they just included a thousand or so LUTs in with every MCU. Then you could design the hardware you need rather than trying to fit the design into the limited hardware available.

Is there a way to "+1" this reply?
I just finished a product that required a 24-hour timer under certain conditions, while it was doing its other normal functions. No big deal I guess; a timer interrupt and some registers, with some care and attention to not spending too much time in the interrupt service routine.

But at 8-bits? 24 hours is a lot of counting!

We just built an 8-hour timer into an FPGA to ignore triggers after
one working day, and so prevent the customer from using our code
(un-paid-for demo code) in a real production machine. People wouldn't
like their 24/7 fab lines to shut down every 8 hours.

40 MHz clock, took a bunch of bits.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics