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PCM1794A: Regarding connections between analog ground, digital ground and amplifier ground.

? ??
Prodigy 20 points
Part Number: PCM1794A
Other Parts Discussed in Thread: PCM1794

Tool/software:

Hi team,

What are the disadvantages of inserting a low-value resistor into the connection between the analog ground of the DAC and the digital ground?
Also, what are the disadvantages of inserting a low-value resistor into the connection between the analog ground of the DAC and the ground of the IV amplifier?

Sincerely,

Satoshi

  • 0
    TI__Mastermind 30641 points

    Hello Satoshi, The 2 disadvantage that I can think of is the creation of ground loops and also injecting the noise from digital side to analog side or from one section or module to another. It won't effect the operation,  but might effect some of the parameters related to noise for example.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 20 points

    Hello Arash,

    Thank you for your answer.
    I would like to know more details. What is the mechanism behind the injected noise and what kind of noise increases?

    Regards,

    Satoshi

  • 0 in reply to ? ??
    TI__Mastermind 30641 points

    You are welcome. 

    On the digital side of any device, you have switching noise and that can be injected to the analog side . This is not specific to PCM1794 and  it applies to any device with digital and analog modules together on the same die. You can find more information  about noise injection in  text books as well as online search for various articles and papers.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 20 points

    Hi,Arash

    I understand that noise will be injected, but the manual says to connect the analog ground and digital ground. My question is, if I don't connect them, will there be any operational problems? Also, if I connect them via a low-value resistor or ferrite bead, will there be any degradation in characteristics compared to connecting directly?
    Similarly, if I connect the analog ground and the IV amplifier ground via a low-value resistor or ferrite bead, will there be any degradation in characteristics compared to connecting directly?
    Thank you in advance.

    Regards,

    Satoshi

  • 0 in reply to ? ??
    TI__Mastermind 30641 points

    Inside a given IC we bring the gourd of digital sections as well as analog section  to separate pins to isolate them as much as possible, however we  have to short theses GNDs ( as well as other GNDs of different chips)  at  some point on the board  or using 0 ohm resistors or ferrite bead to have  just one ground. 

    The above applies to shorting the GND of the IV amplifier and DAC . You want to have one GND at the end, so short them at the board level.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 20 points

    When I measured the actual chip, it seems that the digital ground and analog ground of the DAC are connected with a resistor of about 15 ohms inside the chip. I have also confirmed that it works without problems even if the digital ground and analog ground are not shorted on the board. What I want to confirm is whether there is a negative effect on the analog output characteristics when a resistance value exists between the digital ground and analog ground like this.

    I have also confirmed that there are no operational problems when a low-value resistor (for example, 22 ohms) is inserted between the analog ground of the DAC and the ground of the IV amplifier. I would appreciate it if you could also teach me whether there is a negative effect on the analog output characteristics in such a situation.

    I have also confirmed that inserting a ferrite bead between the analog ground of the DAC and the ground of the IV amplifier has an audible negative effect, and that inserting a resistor has an audible benefit.

    Regards,

    Satoshi

  • 0 in reply to ? ??
    TI__Mastermind 30641 points

    Hello Satoshi,

    It is very strange if there exists a dedicated digital pin for DGND and similarly dedicated pin(s)  for AGND and yet they are shorted internally. Specially that they  have separated ANALOG GND for left and right  channels as well as a separate GND for general analog block. I do not have access to design database of this old part to verify what you said,  but I will be very surprised if internally they are actually connected. These different GND pins must be shorted  to GND plane  on the board outside the IC.

    In our own EVM, we are connecting all these grounds as well as the V-to-I ground  to the same ground plane.

    There will be no operational problem if they are shorted or connected via a low value resistor or ferrite bead , however for different boards and their layouts you may or may not experience some noise . If that is the case,  it can effect THDN, SNR or/and DR. 

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 20 points

    Thank you for your kind reply.

    Regards,

    Satoshi

  • 0 in reply to ? ??
    TI__Mastermind 30641 points

    You're welcome.

    Arash