Digital and analog ground

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jivan
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2007/07/19 04:43:24 (permalink)
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Digital and analog ground

Hello
I m using ADC of PIC16f883. Input signal to ADC is from Opamp
 
I have two separate power supply (two transformers) for PIC and for opamp  I don’t want to tie up +/-15v`s gnd (analog) of opamp to 5v`s gnd of PIC
What can I do? PIC16f883 has two gnd pins, is one of them analog gnd for adc?
 
#1

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    Olin Lathrop
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    RE: Digital and analog ground 2007/07/19 05:01:05 (permalink)
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    PIC16f883 has two gnd pins, is one of them analog gnd for adc?

    Yes.  However both Vss and AVss must be tied essentially to the same voltage.  They might go to different ground connection points that ultimately are connected together.  If you are trying to make accurate analog measurements you might not want whatever offset voltage is caused by the power return currents thru Vss to effect the measurements.  In that case you would connect Vss to the local ground normally, then connect AVss back to the board-wide master 0 reference point for better accuracy.  AVss shouldn't be more than a few mV off from Vss.
    #2
    jivan
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    RE: Digital and analog ground 2007/07/19 21:57:34 (permalink)
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    hello
    thank you for reply
    I m a begenner
    Can you plz tell me in easy words I dont get what you are saying is. plz
    #3
    Stefan Uhlemayr
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    RE: Digital and analog ground 2007/07/20 05:47:45 (permalink)
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    Both ground-pin´s (analog AVss and digital Vss) must be tied to the same voltage. You can´t drive one pic with two different voltage-sources (one for analog and one for digital), if they don´t have a common ground.
    The reason for the two pin´s you can read in Olin´s reply.
    #4
    jivan
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    RE: Digital and analog ground 2007/07/20 21:44:03 (permalink)
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    Dear Sir
    Thank you for reply
    Again, I want to ask one query that, I just have seen about +Ve and –Ve external reference voltage for ADC.
    ADCON1 register of 16f883 has two bits VCFG0 and VCFG1, by setting these two bits, I can connect +/- voltage references to +Vref and –Vref pins.
    So now if I use 5v as a +vref, which is derived from +15V (using zener)
    and analog gnd i.e. gnd of +15v as a –Vref
     
    Then will it help to get the galvanic isolation?
     If not then plz suggest me, how I can isolate PIC ADC from analog circuitry ?    
    #5
    ric
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    RE: Digital and analog ground 2007/07/20 22:37:15 (permalink)
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    ORIGINAL: jivan
    I have two separate power supply (two transformers) for PIC and for opamp  I don’t want to tie up +/-15v`s gnd (analog) of opamp to 5v`s gnd of PIC 

    Please explain this requirement in more detail.
    You can NOT have your ADC ground floating in relation to your digital ground. If you need galvanic isolation, then you cannot use the ADC in the PIC. You would have to use an external ADC, with isolation on the digital connections btween the ADC and PIC.

    I also post at: PicForum
    Links to useful PIC information: http://picforum.ric323.co...opic.php?f=59&t=15
    NEW USERS: Posting images, links and code - workaround for restrictions.
    To get a useful answer, always state which PIC you are using!
    #6
    jivan
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    RE: Digital and analog ground 2007/07/21 02:15:46 (permalink)
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    Dear Sir,
    Thank u for reply
    more details are
    I am using ADC of PIC16f883. I/P to my ADC is from OP-07 OpAmp which is powered by +/-15V using one +/-15V power supply. For PIC I am using another 5V power supply.
    Both power supplies have separate transformers.
     
    For the proper ADC operation, I have to make common gnd of 5V and 15V supply.
    I do not want to do that. Because it is a DC motor control application in very noisy environment.     
    #7
    microlab
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    RE: Digital and analog ground 2007/07/21 05:09:58 (permalink)
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    Try ISO124 from Burr Brown (now TI). Ensure that the PICmicro's analog input never exceeds supply voltage. You may need an additional dual 15V power supply.
     
    Best Regards,
    Jayant
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    Olin Lathrop
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    RE: Digital and analog ground 2007/07/21 05:21:34 (permalink)
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    I/P to my ADC is from OP-07 OpAmp which is powered by +/-15V using one +/-15V power supply. For PIC I am using another 5V power supply.  Both power supplies have separate transformers. For the proper ADC operation, I have to make common gnd of 5V and 15V supply.  I do not want to do that. Because it is a DC motor control application in very noisy environment.

    Show your circuit.  I don't accept "Because it is a DC motor control application in very noisy environment" as a good reason (whatever it's supposed to mean) by itself for why you can't connect the two grounds.  What do these two power supplies control?  How much voltage can be between their grounds?  Where does this voltage come from?
     
    But if your problem is that you need to get a analog reading between separately floating isolated supplies, my first knee jerk reaction is to convert it to digital, then send the digital signal accross a isolated interface, like a optocoupler.  Get the cheapest PIC you can find with sufficient A/D resolution for the isolated side, then send the converted reading serially thru a opto to the PIC running from the other isolated supply.
    #9
    jivan
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    RE: Digital and analog ground 2007/07/23 20:48:32 (permalink)
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    We have five dc motors 50V 3A rating. A Drive rack has designed for this. Rack contains 5 drive cards.  +/-15V regulated supply is common for all cards. +/-15V supply is used for switching of mosfets in H-bridge. Where –15V is tied up with –HT  (+HT, -HT is motor supply). Thank u for reply      
    #10
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