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Original question:

ADS8688: TIDU365B–Revised September 2014

ADS8688: question about TI design (TIDU365B)

Ben Kuo
Expert 7550 points
Part Number: ADS8688
Other Parts Discussed in Thread: TCA6408A,

Dear expert,

We found a TI design - TIDU365B, which is similar with our application.

But here are some question about schematic and BoM.

1. What's the function about TLP3123?

2.Why we need TLP3123? Can we use MOFET to replace TLP3123(F)? Both +5V_ISO and AIN2 refer to SGND, looks like not for isolated function.

Ben

  • 0
    TI__Mastermind 45155 points

    Hi Ben,

    This reference design can measure voltage or current signal. When the input is current signal(4-20mA or 0-20mA), the host controller commands TCA6408A to turn on or off the 200ohm resistor, switching TLP3123 by TCA6408A can connect 200ohm resistor to ground and convert 4-20mA current signal to 0-4V voltage signal which can be accepted directly by ADS8688. If you do not measure current signal, you will not need the TLP3123.

    +5V_ISO is provided for DVDD of ADS8688, the SGND is the ground for ADS8688 as well, and this ground is different from the ground on host controller side. This TIDU365 is an isolated reference design, please refer to below block diagram. Please check the schematic  in Figure 37.

    Best regards

    Dale

  • 0 in reply to Dale Li
    TI__Expert 7550 points

    Hi Dale,

    Thanks for your feedback.

    May I learn from you how to choose the optocoupler?

    Or we have to design with TLP3123 because it have something special feature?

    -Ben

  • 0 in reply to Ben Kuo
    TI__Mastermind 45155 points
    Hi Ben,
    Let me contact my colleague from system team since they created this reference design. Hopefully they can answer your question directly here.

    Best regards
    Dale
  • 0 in reply to Ben Kuo
    TI__Intellectual 1035 points
    Hi Ben,
    You will need a analog switch that has low ON resistance compared to the resistor that you connect (here it is 200E) depending on the system accuracy requirement. The ON resistance of the switch will add to the system error. Also select the switch with low ON resistance variation with temperature. The accuracy of the system can be improved if you are doing a calibration for the current measurement.

    regards
    Sankar