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Some Questions - Precision DAQ System based on ADS8568SPM

Light
Prodigy 70 points
Other Parts Discussed in Thread: ADS8568, ADS8556

Dear All,

I started to design precision DAQ system based on ADS8568SPM. I also used REF5025IDGK as a precise voltage reference. I have two ADCs. in same PCBs and a microcontroller manages ADCs and collects digital data. ADCs are used in daisy chain mode and SDO_A is only output for 16 channels. All channels are used simultaneously. Same Convst, SCLK, SDI and Frame sync. signal will be applied to both ADCs.

I have some questions as follows.

1- ADCs are used in daisy-chain mode and first ADC DCEN pin (# 23) is tied DGND second ADC DCEN pin is tied DVDD.  The datasheet says if DCEN = 0, DCIN_X pins should be connected DGND. However there is not any explanation when DCEN = 1 and DCIN_B,C and D are not used.  Second ADC DCIN_A input is tied first ADC SDO_A output. Other inputs won’t be used. I think DCIN_B,C and D should be connected to DGND. Am I wright?

2- AGND and DGND are separated in my design. I will make ground plane for both GND. Normally I will connect AGND and DGND only one point where as near as possible to power supply. Power supply will be on another PCB or backplane, but I try to make quiet ground for AGND. On the datasheet Page 44 the recommended layout is not clear for me. I think thermal pad (heatsink pad) is connected to AGND and also DGND over vias. That means AGND and DGND connected each other where near ADC. What is the best practice for connecting AGND and DGND?

3- I attached the schematics in pdf format. The ADC and Vref part are nearly same in Evaluation Board.  I made small changes. Please check it if there is any inconvenience.

Best Regards,

1715.adcs.pdf

  • 0
    TI__Guru**** 181351 points

    Hi Light,

    For part 1 above, you can't use SDO_A only in daisy chain mode.  The expectation from the device is that SDO_A/B/C/D from the first device in the chain will be fed into DCIN_A/B/C/D of the next, as indicated in Figure 40 on page 34 of the data sheet.

    For part 2, yes, we recommend tying AGND and DGND of the ADC together at the thermal pad.  It is OK to have a solid ground plane, just be careful of how you route your analog and digital signals.  We'd be happy to review your layout for you and make suggestions if you like.

  • 0 in reply to Tom Hendrick
    Prodigy 70 points

    Dear Tom,

    Thank you for your quick response.

    Please check the link below.

    http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/97296.aspx

    The subject is about ADS8556 but ADS8568's digital part is almost same with ADS8556. When I read your reply to Mr. Chris I supposed that I can use just SDO_A in daisy chain mode. Other ways daisy chain mode is meanless for my application. I can use just SDO_A for 8 channels for normal operation and for two ADCs I need just two SPI inputs. If I use daisy chain mode (based on your last comment) I need 4 SPI inputs and it is not exist in the microcontroller.

    I will send the layout when it is ready. I will route solid AGND and DGND seperately and they will be tied only one point as near as possible to power supply. I am going to use ADS8568SPM which does not have thermal pad under the chip. So I cannot tie AGND and DGND over thermal pad.

    Best Regards,

  • 0 in reply to Light
    TI__Guru**** 181351 points

    OK - let me check into this a bit more, I'll get back to you shortly.

  • 0 in reply to Tom Hendrick
    Prodigy 70 points

    Dear Tom,

    Ok- I am waiting your reply. If we can use only SDO_A for 16 channels in daisy-chain mode please read my first message and please answer my questions. Also please check voltage referance part in the schematic. Is it ok or not for accurate DAQ system?

    Best Regards,

  • 0 in reply to Light
    Prodigy 70 points

    Dear Tom

    I am waiting your answer. If you could not send I ask TI local field engineer. He will reach someone who knows. 

    Best Regards,

  • 0 in reply to Light
    TI__Guru**** 181351 points

    My apologies for the delay Light,

    I was wrong on my initial reply to you - you can daisy chain multiple devices through SDO_A only and yes, you should connect the unused DCIN pins to ground (I'll edit that entry). The reference circuit looks good. 

  • 0 in reply to Tom Hendrick
    Prodigy 70 points

    Thank you very much.

    I can continue the design.

    Best Regards,