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DS91M040 - Minimum Termination

David Fischer
Intellectual 1705 points
Other Parts Discussed in Thread: DS91M040, SN65MLVD047A

Hello,

I have a customer using the DS91M040 for SPI interface extension.  They have the 4 channels (SCLK, MISO, MOSI, and CS) distributed on 16" multi-drop backplane with 16 end-points.  They are all using the same DS91M040.  I know we recommend using 130 ohm traces with double 80 ohm terminations for uTCA and ATCA chasis.

However, my customer cannot use traces witih that much resistance due to PCB stack-up.  Would it be alright for him to use 100 ohm traces with double 60 ohm terminations for this device?

  • 0
    TI__Expert 5275 points

    Hi David,

     

    I have done some reasearch, and this is what I found:

    Zo: 130ohm -> 100ohm is OK, but it will lower the noise margin. (>80 ohms minimum requirement)

    Zt: Lower value termination resistors mean lower DC load for a signal driver and ultimate lower signal amplitude. Even though the M-LVDS drivers feature control circuitry that keeps the output amplitude constant, the output amplitude is only constant for a load of 40ohms or highter.  See Figure 18 in the DS91M040 data sheet.

    Per Figure 18: 60ohm termination will result in ~485mV amplitude instead of the standard 565mV amplitude at 80ohm termination. So 60 ohm termination would NOT be recommended for this device.

    Additional References:

    http://www.ti.com/lit/an/snla113a/snla113a.pdf

    http://www.ti.com/lit/an/snla082a/snla082a.pdf

    If the customer is using this part only in driver mode (not receiving) they could consider the SN65MLVD047A - Multipoint-LVDS Quad Differential Line Driver (not transceiver) which allow for operation with loads as low as 30ohms.

    I will be visiting the customer in person today and will share this information. 

     

    Regards,

    Adam Hoover