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RM48L952 AD

dison wang
Expert 1620 points
Other Parts Discussed in Thread: RM48L952

Hi There,

I got a strange case on my circuit.vv

I use the RM48l952 ADC for a sample 4-20mA input sampling. The reference voltage is 3V. The input will be 0.54V, 1.08V, 1.62v,2.16v and 2.7V (when input is 4mA,8mA,12mA,16mA and 20mA).

The outpot of  the analog circuit is correct when the load is open. The driver is a Emitter follower which can output 8mA current before the ADC's pin of MCU.

When I connect the signal to MCU's pin, the signal became to half level.

Is there someone can tell me the reason. Is there some special requirements for  the MCU ADC input?

Thanks,

Chuan..

  • TI__Mastermind 49120 points

    Hi Chuan,

    The ADC input equivalent circuit is described in the datasheet. For the RM48L952, you can find the model on page 128 of SPNS177d.

    Can you include a picture of your circuit?

    Regards, Sunil

  • in reply to Sunil Oak
    Expert 1620 points
    Hi Sunil,

    Thanks for your reply.
    I have read the page. My circuit is very simple which is a Emitter follower's output connect to ADC pin of MCU directly with out the filter.
    All of analog output signal got half level when them connect to pin of MCU.

    Would you help me to find the reason?

    Thanks,

    Chuan
  • in reply to dison wang
    TI__Mastermind 49120 points

    Chuan,

    As you can see from the ADC input equivalent circuit, the ADC input pin is not designed to provide any path to ground for the input. This is why I ask for your schematic, as there should be another path to ground on your board. Can you check if there are any other resistors to ground on this ADC input channel?

  • in reply to Sunil Oak
    Expert 1620 points

    Hi Sunil Oak,

    Thanks for your reply,

    There is not any resister to ground on this ADC input. The enclosure is one of ADC input and MCU ADC interface.7658.pdf

    I read the page, it seem make any input half  to sample, because the Rmux is same Rsamp which both are 250ohm. If  I input ADREFHI to any ADC pin, I think the ADC will get a half value of the ADREFHI.  Is that correct?

    If it is correct, how i can get a full range sample value? 

    Thanks,

    Chuan6644.sch

  • in reply to Sunil Oak
    Expert 1620 points

    Hi Sunil Oak,

    I found the problem which my MCU was gone. I will try a new one today.

    Thanks,

    Chuan