• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 371 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

LM1972 Query

LM1972: Digital and analogue grounds

Ian Benton
Prodigy 210 points
Part Number: LM1972

The analogue part of my project is bipolar, so Vdd is +5V, Vss is -5V, and analogue ground is 0V.

Can digital ground be Vss?  or must it be 0V?

  • 0
    TI__Guru* 84915 points

    Hi, Ian,

    Yes, the digital ground can be VSS. However, I would recommend to join both digital and analog planes with 0-ohm resistors or single PCB traces. This is a good practice to reduce digital noise on the analog portion.

    I hope this helps you. Please let me know if you have additional questions or comments.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    Prodigy 210 points

    If the digital ground is Vss and the analogue ground is 0V, then joining them with zero ohm links isn't going to be a bit hard on the power supply!

  • 0
    Genius 3945 points

    Is your digital control logic referenced to -VSS or to GND?

    The real question is why would you want to do this.

  • 0 in reply to Ian Benton
    TI__Guru* 84915 points

    Hi, Ian,

    Sorry for the confusion, I made a mistake on my previous post. Yes, the logic level would be 0V. Then, this digital ground plane can be connected to the analogue ground plane through a 0 ohms resistor or single PCB traces.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    Genius 3945 points

    Luis Fernando Rodriguez S. said:

    Then, this digital ground plane can be connected to the analogue ground plane through a 0 ohms resistor or single PCB traces.

    Why is this outmoded and frankly bad advice -- two ground planes connected with a jumper or small trace -- still being given?

    Pour one ground plane. Then "partition" the routing -- that is, route digital traces away from analog. Always mind where the return current flows.

    See, for example, Henry Ott's papers and books about this.

  • 0 in reply to Andy Peters
    Prodigy 210 points

    Logic is between 0V and VSs.

    Because the logic output has to drive a 12V indicator panel, and the LM1972 can't manage +/-12V supplies, so I'm using +/-6V for the audio.

    The logic can operate at any level, so if I put it between 0V and -6V it can directly drive a ULN2003 which switches the indicator panel.

    If the indicator panel is on 0V/12V then I need an extra supply. If the logic is between 0V and +6V then I have to logic-shift the output to the ULN2003.

    That's why.