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ADM3485 RO output issue

Question asked by adi_susan Employee on Jun 27, 2014
Latest reply on Jun 27, 2014 by RAnslow

I'm using ADM3485 and found one problem, could you please help to look into it?

 

The schematic is shown as below:

1.png

The waveforms from the oscilloscope are shown as below (DI, DE/RE, RO, B and A):

2.png

The problem is 1v voltage drop for RO signal marked by red ellipse. That is, when DE=RE=1, ADM3485 transmitt data, RO will have 1V drop.

When I removed the pull up resistor at RO, the waveforms are shown as below, the voltage drop is much larger:

3.png

When I change the pull up resistor at RO to 1k, there is no voltage drop at RO:

4.png

My question is: when DE=RE=1, what’ the output state of RO pin? How to explain the phenomenon observed?

 

Thanks,

Susan

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