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AD5421 + AD5700-1 wrong loop current and lack of modulation

Hello. 

Customer have a problem . 

When  RTS of AD5700 is set low (enable the modulator and disable the demodulator), IC AD5421 sets wrong values in range from 3,2 mA to 8 mA. (example set 4,0 mA, measured in the  loop 4,06 mA). if sets more than 8 mA - no issues.

Also there is no modulated signal in the loop,  but there are singals on pin 14 «HART_OUT» of AD5700 (oscillogram1.jpg) and pin 21 «Сin» ofAD5421 (oscillogram2.jpg) .
When RTS is high (enable the demodulator and disable the modulator) , curent sets correctly in full range from 3.2 to 24 mA

Any ideas ? Could you check the scematics ?
seems too big input capacitors of ADP2360 and  not obvious connection of zeners and diodes near the loop (ia block )

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Parents Reply
  • Hi,

    The hardware setup looks good to me. What SPI commands do you write on the AD5421? are you using direct readback on the fault register or just the auto fault read on the SDO line?

    I would also like to test it out without the big capacitors on the LDO input (this is seen at the REGIN pin too. 

    Best regards,

    Ian 

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