[sdiy] Digital delay - whine from serial RAM access

[Vinicius Brazil]

Hi Tom,
I use dsPIC33FJ128GP802 for the last 4 or 5 years in my most demanding
projects, audio and non-audio.
I made many tests with his DAC (including delay and flanger using the
internal memory) and the results were not very satisfactory.
I consulted the support from Microchip and some forums and spoke something
in the  72 dB S / N ratio.
I love this chip but has not yet reached a good result with the DAC.

Vinicius Brazil
www.vbrazilsystems.com

On Sun, May 8, 2016 at 7:49 PM, <rsdio at audiobanshee.com> wrote:

> Although you cannot create a ground plane with a strip board, you can
> maintain separate paths for analog ground and digital ground. The key is
> that all current flowing out of the power supply has to return to the same
> power supply via some ground path. Thus, you need separate ground nodes to
> avoid interference. Any digital return current that flows over analog
> ground paths will create a voltage drop due to non-zero resistance, adding
> noise to the analog signals (which are all referenced to analog ground).
>
> First off, if your strip board has multiple rails along each side, then
> you should be able to dedicate one to analog ground and one to digital
> ground.
>
> Second, if you cannot dedicate a rail to analog ground, then you can still
> run a star ground for analog that doesn't share ground paths with digital.
> Strip boards are particularly bad because each contact adds a small
> resistance that shifts the ground reference for everything upstream.
>
> The DAC inside a CPU like the dsPIC can be noisier than a standalone DAC
> chip. However, most CPU chips have separate analog ground pins dedicated to
> the DAC so that you can connect analog separately. If that's an option on
> your chip, then be sure to take advantage of it. The internal DAC should
> also have separate digital supply and reference pins.
>
> Also, when measuring noise, it's incredibly important where you take the
> ground reference. Because "ground" is not the same voltage at all points
> along the node, you'll get different readings at different points. The best
> way to evaluate the dsPIC DAC noise is to ground your scope probe at the
> dedicated analog DAC ground pin on the dsPIC. I've seen false noise
> readings when I left the ground connected to a convenient point in the
> circuit, only to see them reduce or even go away when moving the ground
> reference closer to the source.
>
> Finally, when you make your first PCB, you might want to design for an
> optional external DAC. If the dsPIC internal DAC is still too noisy with
> separate ground paths, you'll want to switch to an external (e.g. SPI) DAC.
>
> Brian Willoughby
> Sound Consulting
>
>
> On May 8, 2016, at 3:20 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> > Well, I'm seeing the noise in the audio output, but I've traced it back
> as far as the dsPIC's DAC output. It could well be produced internally by
> the dsPIC when the current spikes when the SPI kicks in.
> >
> > I've done as you suggested to find the noise (scope on AC, low level),
> and it amounts to about 20mV p-p on the DAC output. Can't believe how
> annoying it is for a signal at that level, but it's right in the part of
> the spectrum where you really notice, and where the filters that come after
> have *no* effect.
> >
> > I'll have a dig through the datasheet and errata to see if there's
> anything I'm missing, but this (or the 128K version, anyway) is one of my
> favourite chips, so I'm pretty up on that stuff.
> >
> > The resistors to limit SPI current is an interesting idea. I'll see how
> I go on, and maybe give it a try.
> >
> >> -----Original Message-----
> >> From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf
> Of Tom Wiltshire
> >> Sent: Sunday, May 08, 2016 11:36 AM
> >>
> >> I'm currently building a digital delay. The prototype is on stripboard
> (this
> >> may be a significant fact). I'm using a dsPIC 33FJ64GP802, which has a
> >> 12-bit ADC and 16-bit DAC on-chip. I had this working very nicely using
> the
> >> on-chip RAM for a short delay. This weekend, I've added an external SRAM
> >> (the 23L1024 128KB SRAM that Richie B suggested here a while back) and
> got
> >> it running with that. This produces a much longer delay, which is great.
> >>
> >> What's not so great is that it also causes a nasty whining sound in the
> >> output. I've checked on the o'scope and the noise coincides perfectly
> with
> >> the periodic block read/writes to the SRAM chip - so it's coming from
> the
> >> fast signals of the SPI connection (8MBits in my circuit). I'm using a
> 32KHz
> >> sample rate and read/writing blocks of 16 samples (the SRAM memory page
> is
> >> 32 bytes, so it's convenient) which gives me a 2KHz block rate, and a
> whine
> >> at the same frequency.
> >>
> >> I've got separate analog and digital supplies, and as far as possible
> I've
> >> got the analog and digital parts of the circuit separated, electrically
> and
> >> in space. The in/out op-amps and filters are all up one end of my
> board, and
> >> the uP and SRAM are down at the other, with a line of track breaks
> dividing
> >> the two in the middle.
> >>
> >> Given that I can't add a ground plane to a stripboard, is there anything
> >> else I can do that will help reduce the whining? Any other suggestions
> for
> >> reducing digital noise is mixed-signal circuits like this? How does the
> >> noise get from the SPI connection to the output? What techniques could
> I use
> >> to determine that?
> >>
> >> Thanks,
> >> Tom
>
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