AD7177-2 PCB Layout Recommendation

Hello everyone,

I am currently working on my first mixed-signal pcb project. I will be using the ADXL354 as my sensor and the AD7177-2 as my ADC. I plan to use separate ground planes for my digital and analog circuits.

Based on this article (https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html?fbclid=IwAR2e9D5R9xhRADLmzpivo81vmrB6PfGEZJhbZe2gHSwE8ZXt2GEOCWr-De0) the ADCs and DACs (and other mixed-signal ICs) with low digital currents should generally be treated as analog components and also grounded and decoupled to the analog ground plane.

1. How will I know if my ADC of choice have low digital current?

2. What should be the current range of an ADC for it to be considered low digital current?

3. Does the statement above applies to the AD7177-2? If yes, what pins of the AD7177 should be connected to or decoupled to my digital ground plane?

4. In the schematics provided by ADI on their datasheets, what do these grounds symbols below refer to? (Chassis ground, analog ground, digital ground)?

Also for the ADXL354 sensor,

1. I have separate 3.3V supplies for my analog and digital circuits and separate ground planes for analog and digital circuits as well. If I connect my VDDIO to my dedicated 3.3V digital supply, should I decouple it to my digital ground plane and also connect the VSSIO to my digital ground plane? Similarly for the VSUPPLY if i connect it to my dedicated 3.3V analog supply, should I decouple it to my analog ground plane and also connect VSS to my analog ground plane? Would this setup improve noise immunity?

2. Or can i just connect VSSIO and VSS together and connect it my analog ground plane, connect VDDIO and VSUPPLY together and power it using my 3.3V analog supply and decouple these pins to my analog ground plane as well? Which setup is recommended?



revised questions
[edited by: nognog024 at 9:27 PM (GMT -4) on 19 Apr 2021]
Parents
  • +1
    •  Analog Employees 
    on Apr 20, 2021 3:01 AM

    Hi, 

    1. I would consider AD7177-2 as low power meaning low current device as the total current consumption is only few mA. 

    2. Honestly, I believe this depends on how the market and manufacturers consider/categorize their low power devices. Looking at devices with Low power or low current classification I think the range is from uA up to few mA range. Again, this is not a standard range as I don't think there is really a rule of thumb here, but you can check and collect those information across different devices. But in general for AD717x we consider this part as low power and the digital current is also low (less than 1mA). 

    3. Since you are planning to use a separate DGND and AGND then you can just decouple your digital supply pins to DGND. However,  although AD7172 has separate supply pins, the AGND and DGND pins are tied together internally. Thus, it must not tie to separate ground planes unless connected together near the part or tied both ground planes to AGND. But ideally, the part itself can have a single solid GND plane considering good floor planning like avoid running digital lines under the devices, analog and digital sections are separated and confined to certain areas of the board etc. 

    4. The figures below are named as follows. DGND, AVSS, AGND. The AD7172 can operate with a split supply wherein AVSS is connected to a negative supply, thus a separate plane is required. But if you are operating to Single supply then you can just connect AVSS to AGND. 

    Overall, I would recommend to use/follow the Evaluation Board Schematic and Layout as it was already tested with a guaranteed performance. 

    As for the ADXL345 sensor, I would recommend to ask this query to Q&A - MEMS Inertial Sensors - EngineerZone (analog.com) Someone there can better assist you. 

    Thanks,

    Jellenie

  • Thank you for your swift response and your detailed explanation!

  • 0
    •  Analog Employees 
    on Apr 20, 2021 4:46 AM in reply to nognog024

    No problem. Just let me know if you have further questions and we'll try to assist you. But again, I would recommend to follow the EVB schematic and layout same with the ADXL part. 

    Thanks,

    Jellenie

  • Hi Ma'am, I have an additional question. Say for example I am using the ADXL354 output temperature as one input to my AD7177-2. Is there a limitation to the RC filter right before the AIN pin for this? How should I choose the values for the RC?

Reply Children
  • +1
    •  Analog Employees 
    on Apr 20, 2021 5:52 AM in reply to nognog024

    Hi, 

    Actually since your input is just DC then it doesn't need much of an RC filter. However, an anti-alias filter is required. However, because a sigma delta converter oversamples the analog input, the design of the anti-alias filter is greatly simplified. A simple first order/single pole RC filter should be good enough. Please take note that when the converter is operated in unbuffered mode, the inputs look directly into the sampling capacitor of the modulator. The modulator is continually charging and discharging the sampling capacitor. If the time constant of the anti-aliasing filter is too large, the modulator may be unable o fully charge the sampling capacitor and gain errors will result. To prevent the R-C combination from introducing errors, the R and C values used must be limited. If you have high RC value I would recommend to enable the input buffers. 

    But again, the easiest way is just to follow the values uses in our EVB since your input is just DC. 

    Thanks,

    Jellenie

  • One more thing, there are discrepancies between the values on the schematic diagram and the bill of materials. should I follow the bill of materials?

  • 0
    •  Analog Employees 
    on Apr 20, 2021 6:08 AM in reply to nognog024

    Hi, 

    Really? Apologies. We'll double check this, but I think the schematic is more correct. It should be 10r and not 10k. 

    Thanks,

    Jellenie

  • Yes, I have noticed that the values were different from the bill of materials as I was initially referring to the bill of materials for the final values. It a good thing that you were able to clarify this one. Thank you so much!