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ADS8686S: questions

Ivy Jin
Expert 8035 points

Part Number: ADS8686S

Hi team,

My customer is evaluating ADS8686S in their project.

They have below questions. Could you please help give some comments. Appreciate for your help!

1. Why the REFCAP capacitor need to use a ceramic capacitor, Whether there can use a tantalum capacitance?

2. Why need to avoid placing vias between the REFCAP and decouple capacitor? What is the impact?

3. For REFIO, there suggests add a 10uF capacitor, but in the previous page, the datasheet suggestions add a 100nF?

4.  What is the benefit of generating 4.096V via internal reference 2.5V through the buf? Why not use 2.5V directly?

Thank you!

Regards,

Ivy

  • +1
    TI__Guru 50555 points

    Hi Ivy,

    Please see my answers below:

    1. When selecting a decoupling capacitor for the voltage reference, it's important to minimize the load transient and maintain the stability, so the equivalent series resistance (ESR) and equivalent series inductance (ESL) of capacitor must be considered. Ceramic capacitors usually have the lowest ESR and ESL.

    2. Placing vias between the decoupling capacitor and the device's pins, as shown in the left figure below, will increase the inductance between the decoupling and the device and reduce the effectiveness of the decoupling capacitor. Also, make sure the capacitor has a short, low impedance ground return path. Below is the guideline for decoupling capacitor:

    • DO NOT have vias between bypass caps and active device – Visualize the high frequency current flow !!!
    • Ensure Bypass caps are on same layer as active component for best results.
    • Route vias into the bypass caps and then into the active component.
    • The more vias the better.
    • The wider the traces the better.
    • The closer the better

            

    3. This is a text error in the section Layout on the page 107. 100nF capacitor is recommended for the REFIO and shown in all other circuits and description in the data sheet.

    4. This is a design consideration for internal circuit because this device has to support bipolar high voltage input range.

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 8035 points

    Hi Dale,

    Very clear explanation. Thanks a lot!

    Regards,

    Ivy

  • 0 in reply to Dale Li
    Prodigy 10 points

    Hi Dale,

        I have a serious problem with the AD7616 which is pin to pin with ADS8686S. AD7616 used internal reference, REFCAP output 4.096V normally. But sometimes, it's output a oscillation waveform from 3.3V to 4.8V with 772Khz. I also test AVDD 4.95V ,  DVDD 3.32V , REGCAP 1.87V ,REGCAPD 1.89V,    REFINOUT  2.5V.  Do you know how did this come about?  If I switch to ADS8686S, will this problem not occur? This has been bothering me for months. I need your help. Thanks.

  • 0 in reply to Fangming Yu
    TI__Guru 50555 points

    Hi Fangming,

    Thank you for your query on TI E2E forum. Definitely I can help you solve the issue if you can switch to TI ADS8686S. I can guarantee you will not see this kind of issue on the ADS8686S. I will have a local FAE to contact you since I need more information from you. We will communicate with you offline. Thanks!

    Best regards,

    Dale