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Hot!DSPIC ground lines

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JPortici
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Re: DSPIC ground lines 2019/08/07 07:53:33 (permalink)
5 (1)
mpgmike
Vss is common ground for the PIC.  AVss is Analog ground, in case you wish to have a different VREF- for Analog purposes.  I would recommend you use a ground plane to reduce/eliminate noise anyways, and I would tie the Vss pins to that ground plant (or ground ring).  AVss would be application specific.




No. AVSS is not VREF- !!!!!!
AVSS is the ground for the analog portion of the chip (ADC, comparator, ..) and it MUST be tied to VSS at some point.
You can provide separate grounding paths but they must going at one point.
 
VREF- instead is an analog input (which value MUST be within AVDD and AVSS, also probably with other constraints) which can be used as the negative voltage reference, in case you want to use an arbitrary voltage reference.
 
AVSS will always be available as a negative voltage reference, VREF- instead may not be available, depending on the chip.
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Jan Audio
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Re: DSPIC ground lines 2019/08/07 09:18:53 (permalink)
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JANickerson
Are you sampling the audio from a microphone with ADC ?

 
No, generating waveforms from sine table.
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/07 09:40:40 (permalink)
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And outputting via ?
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/07 10:25:07 (permalink)
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Which pins does the audio come out of ?
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Jan Audio
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Re: DSPIC ground lines 2019/08/08 05:34:10 (permalink)
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The audio DAC on both channels : pins 23, 24, 25 & 26.
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/08 07:11:13 (permalink)
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What does the scope trace of the DAC output look like ?
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Jan Audio
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Re: DSPIC ground lines 2019/08/08 08:42:01 (permalink)
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My scope is more noisey.
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/08 08:56:16 (permalink)
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In one of the posts mentioned in one of your other posts I see modcan mention  using this chip and it is not noisy ( or am I mis reading something ) 
https://www.microchip.com/forums/FindPost/482151
edit: link problems
 
post edited by Jim Nickerson - 2019/08/08 09:01:28
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Jan Audio
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Re: DSPIC ground lines 2019/08/08 09:11:20 (permalink)
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If you take 1 high harmonic signal over the 16 bit range it is not very noisey.
Depends on what you play on it.
 
Take a low frequency sinewave and you hear noise,
if you make a polysynth, that also has velocity, things get very soft, there will be lots of noise.
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/08 09:45:11 (permalink)
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Hard to quantify without a scope let alone guess where the noise comes from.
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du00000001
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Re: DSPIC ground lines 2019/08/08 09:53:34 (permalink)
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@ all
You won't be much happier with the scope: the already better ones are in the 7-8 ENOB range. Provided you have a really expensive one, the ADCs may have 12 Bits - leaving some 10 ENOB. The ear is way better in discerning "noise".
 
A spectrum analyzer (maybe best: the "good old analog" one) might do better in assessing noise figures/harmonic distortion.

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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Jim Nickerson
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Re: DSPIC ground lines 2019/08/08 09:57:13 (permalink)
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du00000001
The ear is way better in discerning "noise".

quite true, but the ear is not so good at displaying the Audio DAC output to corelate it with something going on in the Pic.
 
#32
rontaylor
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Re: DSPIC ground lines 2019/08/09 00:55:30 (permalink)
5 (1)
I've used this device in audio projects and found the DAC to be rather noisy. To get the best performance out of it you should;
  1. Use separate audio and digital ground planes, linking them at one point via a choke, typically 100uH.
  2. Use separate 3V3 linear regulators for the analogue and digital supplies.
  3. Decouple the supplies for RF and audio, i.e. combinations of 10nF, 0.1uF and 10uF.
  4. Use the +ve and -ve DAC outputs in conjunction with an Op Amp to produce the single ended audio output, (See back pages of Section 33 Audio DAC data sheet for a typical example).
  5. When observing the audio output, use the analogue ground as the scope reference - otherwise you will just be viewing audio plus the noise difference between the analogue and digital grounds.
Regards
 
Ron Taylor G4GXO
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Jan Audio
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Re: DSPIC ground lines 2019/08/09 07:04:45 (permalink)
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Hi Ron,
1 : what is the 100uH for ?, cant i just connect the two regulators ground at the power supply point ?
 
2 : LDO is also good ?, i use the LV33 that gets the power from a 5V switching regulator ( R-7805 ).
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rontaylor
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Re: DSPIC ground lines 2019/08/09 11:39:48 (permalink)
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The choke decouples the analogue ground from the digital ground. I use LDO regulators such as the MCP1703. Switching regulators are best avoided for these types of applications. They tend to generate a lot of noise. You can reduce this with careful decoupling but it is difficult - I recently tried this with one of the new three terminal +5V switching regulators. these are a drop in replacement for the 7805 series. It ran nice and cool but the noise on the audio output was noticeably greater than when using linear regulators.
 
Regards
 
Ron Taylor G4GXO
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du00000001
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Re: DSPIC ground lines 2019/08/09 14:28:42 (permalink)
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While some noise is inherent to the switching regulators, you can try a pi or T filter (or some fancy series connection of such) to get this noise down. Depends on whether you need the way better efficiency of these regulators.

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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rontaylor
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Re: DSPIC ground lines 2019/08/09 15:30:29 (permalink)
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Agreed. I needed to add a lot more decoupling and filtering on the supplies and regulator outputs to get the switching noise down to a reasonable level. It's a case of complexity or efficiency, in my case I wasn't worried about the extra burden of a linear regulator and so went for reduced component count. For applications where for example battery capacity is a critical factor then the switching regulators and extra filtering would be the choice.
 
Regards
 
Ron Taylor G4GXO
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Jan Audio
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Re: DSPIC ground lines 2019/08/10 06:31:28 (permalink)
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The 7805 got very hot with only 1 DSPIC, thats why i use switching now.
Maybe the 3v LDO will also take out some noise from the 5v switching regulator,
the 5v is further not used for audio, just for 8bit MCU and optocoupler.
post edited by Jan Audio - 2019/08/10 06:32:49
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