ADAU1701 /1401 Recommended Crystal Layout

Dear Analog,

We have designed a system using the ADAU1701 and have experienced inconsistent communication to the unit over I2C and SPI. We have narrowed it down to the crystal oscillator being at fault.

It appears to be an identical issue to a previous post.

When we remove the 100R damping resistor the communications works.

Can Anolog please comment on our findings based on our layout below.

Is there an obvious flaw in our crystal circuit layout?

Is there a recommended PCB layout for the crystal, load capacitors and damping resistor?

Should the damping resistor value be changed?

We are following the recommendation on page 18 of the manual,

Fundamental Crystal, 18pF. (Abracon ABLS-12.288MHZ-B4-T). 22pF load capacitor and 100R damping resistor.  

Our layout is shown below. It is a 4 layer stack with a ground plane directly under the DSP and crystal. There is no copper pour on the top layer

  • 0
    •  Analog Employees 
    on Jan 22, 2018 8:29 PM

    Hello euanbrown,

    I don't see an issue with the layout of the crystal circuit and its associated components. I do see an issue with the PVDD power and ground pins and the Loop Filter (LF) pin. This could be causing the PLL to take too long to lock. The power pin bypass caps can also cause issues with too much noise on the ground which can cause issues as well. By reducing the damping resistor you are getting over this noise. That is my guess based on the information you supplied.

    1) Loop Filter:

    This needs to be returned to the PVDD pin without going through vias and through a power plane. By going through vias and to the plane to return back to the PVDD pin  you are adding inductance to the circuit and then also adding in the noise on the plane.  

    2) All the power bypass caps in your entire design, including other parts, need to be close to the pins and not use vias to return to the ground pin. This will help make the caps more effective. This will help keep any noise generated inside the part from actually getting out onto the power plane. It will also help any noise that is on the power or ground plane from getting into the part. So it works both ways. By doing all your bypass caps like this you keep the noise down on the planes.

    The ADAU1452 datasheet has a good applications section describing how this should be done. I will paste in part of it here.

    What could be happening is that the PLL is taking too long to lock and without the PLL locked you cannot communicate with the part.

    At this point I don't think the resistor value needs to be changed. Try the other layout changes first.


    Dave T

  • Thank you Dave for your fast response.

    I have modified the layout to fit all the critical decoupling and crystal components as per your suggestion.(rest of circuit still to be completed)

    Can you please confirm the layout below looks suitable

  • 0
    •  Analog Employees 
    on Jan 24, 2018 1:00 AM

    Hello euanbrown,

    Yes, that looks good. We really needed to have a ground pin next to the IOVDD pin, pin 18, but we did not. I was not part of the design team so there may have been a reason for it.

    Anyhow, I think this should help. Let me know.

    Dave T

  • Thank you Dave,

    We will manufacture the new design and I will report back our findings before closing this discussion.

  • Hi David

    You have commented on the partial layout provided by Euan.

    Would you mind to review the layout below, now with all connections routed.