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Original question:

ADS8688: question about TI design (TIDU365B)

ADS8688: ADS8688: question about TI design (TIDU365B)

Zeeshan Ahmad Ansari
Prodigy 130 points
Part Number: ADS8688
Other Parts Discussed in Thread: TIDA-00310, TIDA-00118, TIDA-00119, TIDA-01333, TIPD164

Dear expert,

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

But here is a question related to current input.

As in this design we can measure current having input range from :-

1. 0 - 20 mA

2. 4 - 20 mA

My question is that can we use this for current input range for +-20mA ??

If no then how can we have this input range for this design.

Do suggest some technical documents for this.

Regards,

Zeeshan

  • 0
    TI__Guru 63855 points

    Hello,

    Yes, the ADS8688 can accept bipolar input voltages so if you use a 250Ohm shunt resistor with the +/-20mA input then you'll create a +/-5V input for the ADS8688 which can be accepted in the +/-5.12V bipolar input range mode of the ADS8688.

    We don't see +/-20mA that much outside of Siemens modules.  What are you interfacing with that produces +/-20mA?

  • 0
    TI__Guru**** 188639 points

    Hello Zeeshan

    ADS8688 can be used to measure positive and negative inputs with multiple ranges.

    You can use resistors such that at max current, the output across the divider is within ADC range.

    Bipolar DC inputs are typically used in grid transducer applications.

    You can look for reference designs on the TI designs and TIDA-00310 can be a good reference.

    Thanks

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Prodigy 130 points

    Dear Experts,

    ADS8688 can be used to measure positive and negative inputs with multiple ranges.

    But in your design as in document No:- TIDU365B

    There is a opto coupler TLP3123(F) which is in series with 200 ohm shunt resistor.

    So if we want to have +-20mA input range does it needs do be changed?

    If yes pls provide some low cost alternative for this.

    Is my question correct?

    Regards,

    Zeeshan

  • 0
    TI__Guru**** 188639 points

    Hello Zeeshan

    ADS8688 can be used to measure positive and negative inputs with multiple ranges.

    You can use resistors such that at max current, the output across the divider is within ADC range.

    Bipolar DC inputs are typically used in grid transducer applications.

    You can look for reference designs on the TI designs and TIDA-00310 can be a good reference.

    Thanks

    Sreenivasa 

  • 0 in reply to Collin Wells
    Prodigy 130 points

    Dear Experts,

    ADS8688 can be used to measure positive and negative inputs with multiple ranges.

    But in your design as in document No:- TIDU365B

    There is a opto coupler TLP3123(F) which is in series with 200 ohm shunt resistor.

    So if we want to have +-20mA input range does it needs do be changed?

    If yes pls provide some low cost alternative for this.

    Is my question correct?

    Note:- +-20mA input is provided by some transducers from grid side.

    This is our requirement.

    Regards,

    Zeeshan

  • 0 in reply to Zeeshan Ahmad Ansari
    TI__Guru**** 188639 points

    Hello Zeeshan

    The opto is used to make the input universal meaning it can measure 10V input or 20mA current.

    If you need to optimize the cost and if you are connecting to dedicated 20mA output sensor, just remove the opto and it will work.

    Thanks

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Prodigy 130 points

    Hello Sreenivasa,

    We need both voltage and current inputs, it should be software programmable.

    And i think for current input the opto will be resposible to connect 200 ohm resistor.

    But for current input +-20mA the opto can only work in one direction.

    We need to have current input as +-20mA as one of the inputs.

    Regards,

    Zeeshan

  • 0 in reply to Zeeshan Ahmad Ansari
    TI__Guru**** 188639 points

    Hello Zeeshan

    Tow questions that come up includes

    Is isolation required and 

    Number of input channels and isolation topology ( group or channel isolated)

    You will have to design the switch for your requirement and there can be multiple approaches.

    If isolation is required, TI recomended approach is to use  a digital isolator + MOSFETs to do the switching. 

    If you are looking at channel isolated design, adding isolated power will increase system cost.

    Please explore alternative and simple ways that you can use to optimize the cost. You can also reach out to your local TI sales.

    Thanks

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Prodigy 130 points

    Dear Sreenivasa,

    We require only group isolation with individual channel configurable as current and voltage independently.

    The external ADC will be isolated from the controller by isolated transceiver ICs.

    So for this " TI recomended approach is to use  a digital isolator + MOSFETs to do the switching"

    Can u suggest any reference note for this, so that we can read +-20mA when channel is configured as a current input.

    Regards,

    Zeeshan

  • 0 in reply to Zeeshan Ahmad Ansari
    TI__Guru**** 188639 points

    Hello Zeeshan

    Need for group isolation will simplify the design.

    The switch and the interface can be controlled using digital isolators.

    Please refer the below and note that we may not have solutions exactly matching your requirement.

    TIDA-00118

    TIDA-00119

    TIDA-01333

    TIPD164

    To get better reach please close this thread and start a new thread as required with tile ADS8688 based DC analog input module with +-20mA input 

    Thanks

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Prodigy 130 points

    Dear Sreenivasa,

    Thank you for your support.

    I will look into the documents you suggested.

    REGARDS,

    Zeeshan