ADS7951: PCB layout

May,

To answer your question, we generally recommend connecting both digital and analog ground to a common plane.  When I look at the ADS79xx data sheet, I see the recommendations for layout on page 52 and 53 (fig 69 & 70).  Are we looking at the same document?  In any case, if you look at figure 69, you can see that BGND is connected to the ground plane.  In old literature you may see a split plane approach where analog and digital GND are separated and connected at one point.  This approach is typically not recommended in modern literature, and measured results show that one continuous GND plane is best.  I hope this helps.

We have a detailed video series on the subject.  The series can be found on TI.com and on Youtube.  Below are the links for TI.com.  The series focus is mixed signal systems similar to what would be required for the ADS7951.

Introduction to PCB design for Good EMC

PCB Trace as a Wave Guide

PCB Stack-Up Impact on Performance

PCB Trace Impedance Matching

Crosstalk on PCB Layouts

Data Integrity Issues

Decoupling Capacitors PCB Layouts

EMC Compliance Testing Methods and Standards

Art

May,

1. It seems measurements were taken on single a2d , how these are expected to change for multiple a2d based circuits where the currents would much higher? Answer:  Each similar device added will increase the digital currents proportionately.  However, the vast majority of the current will flow directly beneath the digital trace.  This means that with good "floor planning" (i.e. keeping sensitive analog away from the digital), the digital return current will have little impact on analog performance.  That is, the current will be contained beneath each digital trace.  See Crosstalk on PCB Layouts for a discussion of current distribution beneath a PCB trace.
2. Were a separate digital ground created at all ? or only one analog ground used to which digital and analog circuits connected ?  Answer: In general, we recommend one solid GND plane where both digital and analog grounds connect to.
3. The question does not answer how to connect analog and digital grounds if the circuit layout is already with two separate planes.  Answer: Using two different plans is generally not recommended.  This is especially true if digital signal cross over the split between the two planes.  Driving a digital across a break in a plane can cause RF emissions, substantial crosstalk, and interference with the sensitive analog.  Slide 6, 7, and 8 in Crosstalk on PCB Layouts gives  measured results for digital signals crossing over a split in the ground plane.  If you require a split GND plane, than the best way to do the layout is to create a bridge for any signals that move from the analog to digital sections.
4. The layout recommended on page 43 of datasheet does not meet any of the layout presented in your literature, why?  Answer: The data sheet is showing the top component layer.  The analog ground copper fill area shown is on the top layer.  The data sheet says:  A copper fill area underneath the device ties the AGND, BDGND, AINM, and REFM pins together. This copper fill area must also be connected to the analog ground plane of the PCB using at least four vias.  The data sheet does not specify if the "ground plane" is solid or split.  It should state solid.  If you choose to use a split plane, the most important thing you should do is to make sure that the SPI signal do not cross over the split.
5. Regarding your figure.  I agree with the good vs bad layout.  In the case of the good layout, the digital signals cross a ground bridge.  I do not agree with the idea that using the split will improve your performance when compared to a solid plane.  This is mainly true as the return currents will be confined under the signal trace.  I understand that you are skeptical on this subject, and in the past the idea of using a split plane for mixed signal systems was common.  I have used split planes in the past.  However, at this point most EMC experts and most mixed signal designers advocate the use of a solid plane for mixed signal systems.  Split planes are still useful for systems such as motor control and switched supplies that have very high di/dt.  If you use a split, but follow the bridge method in the picture you should be ok.
6. One last note regarding layout principles.  There is a lot of contradictory information on the web.  Especially older generation document may suboptimal recommendations.  I put together the material in the video series by reading experts such as: Henry Ott, Rick Hartley, and Eric Bogatin.  Here is a link to good videos on split vs solid: https://www.youtube.com/watch?v=vALt6Sd9vlY 

I hope this is helpful to you.  Let me know if you have questions.

Art

Hi Art,

Thank you the information, customer have follow up questions. 

Were a separate digital ground created at all ? or only one analog ground used to which digital and analog circuits connected ?  Answer: In general, we recommend one solid GND plane where both digital and analog grounds connect to.

In this case should it better to

1. Connect pin 27 to AGND
2. Connect pin 27 to DGND
3. Or create pin 27 to both through zero ohm resistors and keep an option to see what works better in practice?

1. Lastly , why is your datasheet does not make it explicit that there are two digital and analog grounds and are connected to common ground plane?

Thank you.

Regards,
May

Happy to help.  Suggestion noted.

Art