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babidesai123
Visitor II
November 26, 2003
Question

noise in ADc chanel

  • November 26, 2003
  • 6 replies
  • 887 views
Posted on November 26, 2003 at 16:59

noise in ADc chanel

    This topic has been closed for replies.

    6 replies

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    babidesai123Author
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    hi,

    we are trying to sense temperature using upsd 323x. But what i have

    observed is in noisy enviornment. the reading are differed by more than 5-6 LSB.

    i have even tried with connecting same sensor to all 4 chanels., But all the four chanel reads the different values.

    Can anyone plz tell me the input impedence of ADC chanel. so that we can put suitable filter across the pin.

    Purvi
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    neil2
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    Hi,

    What is the physical layout? are you using a development system or do you have a PCB? The layout can be critical to analogue performance.

    If you have any common ground paths in your analog and digital circuits, this can result in common impedance coupling between the analog and digital circuits, and Microcontroller circuits are noisy!

    on the 3234, the pin next to Vref should be used as analog ground and the other ground pins for digital ground. It doesn't say this in the data sheet, but I was told this by an ST engineer.

    B
    babidesai123Author
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    thanks for the reply.

    We are trying to use our own PCB. But unfortunately we haven't provide the next to vref pin as analog ground.

    Will try to do the same.

    Plz let me know if u know the ADC impedence.. It should be specified

    in datasheet as Radc . But it's not...

    Thanks

    purvi
    N
    neil2
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    Just to clarify, the pin on the uPSD3234 that should be analog ground is pin 45 on the 52 pin device and pin 69 on the 80 pin device.

    pin 71 on the 80 pin device is NC (not connected) so should be left open.

    I can't find the input impedance in the data sheet either.
    J
    jinguanghua
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    Do you have a RC filter circuit on the board?

    J
    joseph2399
    Visitor II
    May 17, 2011
    Posted on May 17, 2011 at 11:56

    When the port1.4 ~ port1.7 are used for ADC inputs, the ADC input impedance is almost infinity.

    ADC input is connected to the internal capacitor that is used for sample-and-hold. There is no DC current path in the capacitor when an input signal is sampled. Therefore, the datasheet does not specify the input impedance for ADC.
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