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TIDA-01022: ADC

Part Number: TIDA-01022
Other Parts Discussed in Thread: ADS5296A
Hello All,
 I have employed two ADCs in my design like those in TIDA-01022  and I have questions regarding AGND and DGND in the layout.  I greatly appreciate if you would help me with questions related to  TIDA-01022 
As Fig.1 shows, In this design, All fully differential amplifiers (FDA) are placed over AGND and the ADCS and the Clock are placed over DGND. As Fig. 1 shows one continuous ground is incorporated for the ADC which belongs to the DGND. However, the inputs to the ADC sees AGND where the  FDA are placed. Here I have some  questions 
1- AGND and DGND are connected close to the inputs of ADC using ferrite bead (FB), however, they are not placed and are DNP. Does it mean that there is no need to connectAGND and DGND. In other word if one uses this approach to separated DGND and AGND is using ferrrite beads mandatory?    
2- Why the GND for the ADC was not designed such that it belongs to AGND rather than DGND? (see Fig.2)
3-Why AGNF and DGND are not connected under ADC like the one shown in Fig.3. (In the evaluation board given for ADS5296A  this layout is suggested) (layout like Fig.4 as another example)

Regards
Samaneh
  • Samaneh:  Please see below.  --RJH

    1- AGND and DGND are connected close to the inputs of ADC using ferrite bead (FB), however, they are not placed and are DNP. Does it mean that there is no need to connect AGND and DGND. In other word if one uses this approach to separated DGND and AGND is using ferrrite beads mandatory?    
    RJH>> This is the age-old question on whether to split grounds or not and whether to connect via ferrite beads or not.  This design opted to split the ground and not place the ferrite beads.  Note there is still a connection between grounds at the power supply.  I do not think that there is a definitive right answer here.  You may opt to copy the ground split scheme but I would suggest to add the contingency for the ferrite beads to connect the grounds together near the ADC just in case.
    2- Why the GND for the ADC was not designed such that it belongs to AGND rather than DGND? (see Fig.2)
    RJH>> The ADC has a lot of digital so connecting it primarily to DGND makes sense.  In this design the analog inputs reference AGND and the rest of the ADC references DGND. 
    3-Why AGNF and DGND are not connected under ADC like the one shown in Fig.3. (In the evaluation board given for ADS5296A  this layout is suggested) (layout like Fig.4 as another example)
    RJH>> This is just another design choice to keep AGND and DGND completely separate.  The ferrite beads effectively give the option to tie the grounds together at a localized area close to the device.  I do not have any data one way or the other, but I think the approach of using the ferrite beads is the lowest risk.