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ADS8586S: ADC Circuit Design validation

Vineeth N
Expert 1180 points
Part Number: ADS8586S
Other Parts Discussed in Thread: CC3220SF, ADS8588S

Hi Team,

I am trying to develop an ADC circuit using ADS8586S, attached will be the schematic I developed. Could you please review it? While developing the schematic I came across certain doubts, they are

1. Can I use same supply for both AVDD and DVDD (say +5V), Since both of their ranges specified in data sheet are within -0.3V to +7V?

2. Is there is any problem for using common ground for all connections? I heard something about separate grounding should be required for analog and digital pins ?

3. How can calculate and develop the required power supply (AVDD and DVDD) for my application? is there is any documents there to help me with the design?

4. Planning to connect this circuit with CC3220SF Wi-Fi via SPI. That's possible right? 

I am a beginner in this field so please excuse me for the mistakes in the schematics. I referred ADS8588S schematic as my reference to this design.

I am thinking of using 250:1, 65A(rms) CT for my current measurements. Internal reference with +/-5V analog input is my aim for the design.  

ARC Detection.pdfDatasheet - CT - WCM604-1.pdf

Regards,

Vineeth 

  • 0
    TI__Mastermind 40165 points

    Hi Vineeth,

    Thank you for choosing ADS8586S in your design. My comments about your Schematic:

    1.  Ceramic type capacitors can be used for all capacitors.

    2. VDD power supply to pin 1 (AVDD_1) was missed.

    3. FRSTDATA is an output pin, leave it floating if you do not plan to use it.

    4. R2/R14/R8/R16/R6/R18 are not required, the nodes can be directly shorted to the GNDA.

    5. 20ohm pull-down resistor on each channel can get a smaller conversion code when the CTx+ input is floating (I'm not sure if this is an intended design purpose). However, this smaller value resistance may affect  your CT regarding a CT's output impedance. You can check it by yourself since this is a system level concern.

    For your questions:

    1. You can not use 5V supply for DVDD because your CC3220 controller does not supports a 5V logic and it supports 3.3V logic. Hence, you have to use a 3.3V power supply for the DVDD on ADS8586S. Make sure you have a clean power supply for AVDD and usually a LDO is recommended. 

    2. A solid ground plane is strongly recommended by the datasheet.

    3. As I mentioned in 1, clean power supplies are recommended especially for AVDD. LDO devices are recommended. 

    4. The SPI on CC3220 is a standard interface which is supported by the ADC. However, you have to double check your controller for the SPI timing requirement from the ADC, see the Fig 5 and timing parameters in the ADC datasheet.

    Thanks&regards,

    Dale

  • 0 in reply to Dale Li
    Expert 1180 points

    Hi Dale,

    Thank you for your response and your suggestions are really helpful. I would need a little more clarifications on some points here:

    Dale Li said:
    VDD power supply to pin 1 (AVDD_1) was missed.

    I will correct this. As you said if I have to keep two different voltage levels then it would be like 3.3V for all DVDD and 5V for all AVDD connections right. Please correct me if I am wrong.

    Dale Li said:
    20ohm pull-down resistor on each channel can get a smaller conversion code when the CTx+ input is floating (I'm not sure if this is an intended design purpose).

    The purpose of the 20 ohm resistor is to serve as a burden resistor to the CT. Which is mentioned in its datasheet. I really did not get your point in this. Is it required to have a pull down resistor here?

    All the capacitor values I used in the schematic was referred from the ADS8588 EVM module schematic. Will that be ok to use?

    As I am using internal reference for my circuit, are my connections for the same are correct?

    Regards,

    Vineeth 

  • 0 in reply to Vineeth N
    TI__Mastermind 40165 points

    Hi Vineeth,

    Your understanding about the power supplies is correct.

    If a CT transformer has included a resistor on its output, your additional resistor on the output will affect its related secondary output voltage. However, it looks like this is not the case for your CT.

    The datasheet has included all recommended capacitor values, so it's good to check the datasheet. The EVM schematic can be a reference. The RC filter on the ADC's input depends on your real requirements.

    Your connection for selecting internal reference is correct, however you should double check.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Expert 1180 points

    Hi Dale,

    How can I choose the current rating for the LDO? 

    How can I calculate the total power requirement of my board so that I can design input power supply.

    I am thinking of using some built-in power supply module for converting AC to DC. Do TI have any kind of Built-in modules that serve this purpose. With this power supply I am planning to generate 5V output which I will feed to both LDO circuits. Is this is correct flow. Can I give 5V input to an LDO and expect to give a stable 5V output, should the input be higher than the output for an LDO?

    Regards,

    Vineeth 

  • 0 in reply to Vineeth N
    TI__Mastermind 40165 points

    Hi Vineeth,

    You have to calculate all power consumption of all devices on your circuit board for each power supply, then select a proper LDO with a margin. For the ADC, check the datasheet to find the current consumption for AVDD which is 19.8mA maximum with internal reference.

    LDO requires a dropout voltage between input and output, you can check the information in any LDO's datasheet. For more questions about LDO, you can go to the power management E2E forum.

    Regards,

    Dale

  • +1 in reply to Dale Li
    Expert 1180 points

    Hi Dale,

    Thank you for your response. I will look into it and if I have further queries regarding the LDO I will ask them in the other forum.

    Regards,

    Vineeth