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


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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