ADM3078E pcb layout question regarding trace impedance


        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.

  • 0
    •  Analog Employees 
    on Mar 26, 2012 8:39 PM

    Hi failsafe,

    the general rule for the stubs is to keep them as short as possible: shorter than the 1/10 wavelength Rule-of-thumb.  Anything greater than 1/10th wavelength is considered electrically significant from an imedance mis-match point of view.  It is more dependant on the rise and fall time of the device rather than the datarate (though they are related).  This is why matching the datarate of the part to the datarate of transmission is optimal.

    Keep stub length < 0.1 * rise time * velocity through PCB trace.

    5nS/m is a typical value used for PCB trace delay per meter (2e8ms-1), derived from microstrip theory. (it does vary depanding on the x-sectional dimensions of the Cu and dielectric permitivity)

    0.1*15nS*2e8 = 7.5m

    So 40mm is << 7.5m, stub length is OK.

    The impedance of the receiver input is 96k Ohms min. so termination takes care of reflections.

    Hope this helps.