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Hot!DSPIC ground lines

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Jan Audio
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2019/08/04 06:12:00 (permalink)
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DSPIC ground lines

Hi, i plan to connect 3 DSPIC33FJ128GP802 running at 40MHz
they have 3 ground connections, VSS another VSS and AVSS
 
You have to connect them each to a seperate ground line/wire.
I like to have clean audio, it is the most important to me :
do i need all those 3 VSS a seperate wire ?
 
I use a 39 row experiment board, having 9 ground pins leaves little space for the rest.
Could i connect the AVSS for all chips together without problems on the audio quality ?
 
Is it very bad to tie all 9 VSS connections to 1 ground line ?,
the manual says it need to be first in chain, the 2 seperate VSS are not for nothing i assume.
 
Maybe my old project was very noisey because i tied all VSS together ?
 
Can someone explain ?
Maybe on lower clockspeeds it can handle 1 ground line without problems ?, just guessing.
 
thanks in advance
#1

37 Replies Related Threads

    mpgmike
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    Re: DSPIC ground lines 2019/08/04 08:36:54 (permalink)
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    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.

    I don't need the world to know my name, but I want to live a life so all my great-grandchildren proudly remember me.
    #2
    Jan Audio
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    Re: DSPIC ground lines 2019/08/04 08:57:15 (permalink)
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    I,m making this card with pin headers,
    suppose this card has 10 seperate ground pins, i all connect with a seperate wire to the power supply where the big elko is.
     
    @ Mike : The way you would do this is have 1 ground cable to the PSU ?, for sensitive things it is no good.
     
    The AVSS is for the DAC, maybe i can connect those 3 together ( making RA0 as input to have 1 strip to ground, dont need all the pins )
    #3
    du00000001
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    Re: DSPIC ground lines 2019/08/04 15:22:00 (permalink)
    4 (1)
    Make the ground connection "as good as possible": tie all pins together and use 1 wire more than for the VDD connections. (The basic rule behind that: have a larger copper cross-section for ground than for the positive supply. Works for me since 3 decades ago.)
    Although using a breadboard is always a silly idea when it comes to higher DAC or ADC resolutions (say > 10 Bits) - especially at audio where every ground bounce becomes audible.

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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    NorthGuy
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    Re: DSPIC ground lines 2019/08/04 15:38:09 (permalink)
    4 (2)
    When current flows through ground wires, the ground voltage rises due to wire resistance and inductance. It doesn't matter much for digital. However when you're doing analog, you do not want the voltage noise on the digital ground to affect your analog ground. How important is that? Estimate the level of voltage noise and figure out if it is acceptable for your analog circuits.
     
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    Jan Audio
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    Re: DSPIC ground lines 2019/08/05 06:56:32 (permalink)
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    Ok, so it is safe to tie all VSS together and use 1 cable to PSU ground,
    that is 6 times VSS to 1 cable for 3 chips.
     
    Then another cable for the audio VSS and opamps i also use.
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    Jan Audio
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    Re: DSPIC ground lines 2019/08/05 07:14:22 (permalink)
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    If i also build in a 3,3v regulator, does it require a third ground line ?, or can i add it to digital or analog VSS ?
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    du00000001
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    Re: DSPIC ground lines 2019/08/05 07:56:42 (permalink)
    4 (1)
    @ JanAudio
    You're not reading the answers, are you? Or just not understanding?
    Your "multi-wire approach" is prone to reasonable ground noise.
     
    Unless you really know what you're doing (maybe some time in the far future) it is best to tie all VSS (analog, digital, whatever) tightly together with really short connections of considerable cross-section. Then you can have a single wire (reasonable cross-section as well) to the PSU.

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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    Jan Audio
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    Re: DSPIC ground lines 2019/08/05 08:10:17 (permalink)
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    Funny, every project i made with tied VSS so far has noise.
    What should i know before i can use more ground wires ?
    #9
    NKurzman
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    Re: DSPIC ground lines 2019/08/05 09:08:06 (permalink)
    4 (1)
    Are you using a 4 layer board with a full ground plane? Are you using proper decoupling in caps? A quiet regulator regulator? And understand the limits of electronic noise running inside a chip with multi megahertz signals in it?
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    Jan Audio
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    Re: DSPIC ground lines 2019/08/05 09:11:56 (permalink)
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    I,m using 39 row stripboard :

     
    Caps are good : vishay brand for 100n, TDK for 10U.
    And a LV33 LDO regulator.
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    du00000001
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    Re: DSPIC ground lines 2019/08/05 09:17:48 (permalink)
    4.5 (2)
    Have your 
    JanAudio
    ... every project...

    all been on breadboards? No wonder.
    Even worse if you tied the VSS together with somewhat lengthy wires as you seem to describe above. (And I'm talking of "lengthy" = "just some inches", not feet !)
     
    Add some decoupling caps "somewhat far" from the chip(s) (again inches) and you cannot expect anything reasonable.
    (I'm currently having some issues with ground bounce on a PCB that's just about 2 x 3 inches and doesn't exactly employ "wires": the only significant VSS trace is about 4" long and 0.2" wide, the remainder is mostly a GND plane. OK - I'm sending 1+ Amps down the line...)

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
    #12
    du00000001
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    Re: DSPIC ground lines 2019/08/05 09:21:46 (permalink)
    4 (1)
    ... stripboard for microcontroller/audio ? Honestly ?
     
    And what does your wiring look like ?

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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    NorthGuy
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    Re: DSPIC ground lines 2019/08/05 14:07:38 (permalink)
    5 (1)
    Jan Audio
    If i also build in a 3,3v regulator, does it require a third ground line ?, or can i add it to digital or analog VSS ?



    You typically connect regulator somewhat close to the thing it's going to feed, then you connect the VSS of your target to the VSS of the regulator. This improves regulation.
    #14
    Jan Audio
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    Re: DSPIC ground lines 2019/08/07 06:21:25 (permalink)
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    NorthGuy
     
    You typically connect regulator somewhat close to the thing it's going to feed, then you connect the VSS of your target to the VSS of the regulator. This improves regulation.



    Do i then still need a seperate groundline for the audio as you say in earlyer post ?,
    even if its powered from the on-card regulator.
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    NorthGuy
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    Re: DSPIC ground lines 2019/08/07 06:46:33 (permalink)
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    Jan Audio
    NorthGuy
     
    You typically connect regulator somewhat close to the thing it's going to feed, then you connect the VSS of your target to the VSS of the regulator. This improves regulation.



    Do i then still need a seperate groundline for the audio as you say in earlyer post ?,
    even if its powered from the on-card regulator.




    No. The regulator will regulate the voltage relative to its own ground. If it regulates the voltage for your analog then you should connect the analog ground to the regulator's ground.
    #16
    Jan Audio
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    Re: DSPIC ground lines 2019/08/07 07:16:16 (permalink)
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    Ok thanks you, will try.
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    Jim Nickerson
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    Re: DSPIC ground lines 2019/08/07 07:24:35 (permalink)
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    How much noise are you seeing ?
    #18
    Jan Audio
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    Re: DSPIC ground lines 2019/08/07 07:33:47 (permalink)
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    I,m not done soldering yet.
    The noise was hearable in previous projects.
     
    This time i will add VCA for when it should be silent.
    For while the audio is playing i might need filtering.
    #19
    Jim Nickerson
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    Re: DSPIC ground lines 2019/08/07 07:37:50 (permalink)
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    Are you sampling the audio from a microphone with ADC ?
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