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ADM3485E floating RS485 receiver inputs

Question asked by MaximGalkin on Feb 19, 2013
Latest reply on Mar 7, 2013 by ConalW

Hello.

I used ADM3485E transciever in my project. Power supply is 3,3 V, DI = High (3,3V), RO = High, RE = 0 and DE = 0. Both RS485 inputs (A and B) are float and unconnected from other components and nets. Input resistor 120 Ohm are absent. I measure voltage on input A and input B by means of multimeter. It's about 1,88 V. Differential voltage between A and B is zero, although both pins have not connected together. I read AN-960 application note. I see that ADM3485E have not true fail-safe receiver. ADM3485E have open fail-safe functionality and internal bias resistors. What kind of values of these resistors? Why voltages on A and B inputs are equal? I suggested to see A and B common voltage is equal to power supply mid range  and differential voltage is more than +200 mV. Why I did not see that?

Sorry for my bad english.

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