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ADM3078E pcb layout question regarding trace impedance

Question asked by failsafe on Mar 22, 2012
Latest reply on Mar 26, 2012 by ColmR


        I am currently working on a design that includes a RS485 network.  Currently there is a PC with PCI based RS485 transceiver card and I am designing a circuit that will be reporting data to that PC.  The RS485 node that I am designing will never receive data from the PC only transmit data to the PC.  This is why I selected the ADM3078E; It is half-duplex and quite fast.  Currently there are no other nodes on the RS485 bus. 


Also, the PCI based RS485 card comes with 100 ohm terminating resistance.  I am using some really high quality superflex cable specifically made for RS485/Can Bus communication.  The cable has a characteristic impedance at 100 ohms.  Lastly, my data rate is 1Mbit/s.


I am trying to make sure that I design my RS485 node correctly and I am a bit confused on the impedance requirement of the pcb traces connecting the A&B outputs of the ADM3078E to the screw terminal block.  The pcbs traces going from the screw terminal block (which is where the RS485 twisted pair wire attaches) are quite short.  The distance from the output of the ADM3078E to the screw terminals is approximately 40 millimeters.


Here are my questions:


How much do I need to worry about the impedance of the short pcb traces connecting the ADM3078E to a set of screw terminals?

Where between the screw terminals and the ADM3078E should I place my terminating resistor?

Is the input impedance of the ADM3078E's terminals approximately 100 ohms?  If not how do I minimize reflections due to mismatches?


I have attached a crude drawing to further explain my particular setup and questions.