• State TI Thinks Resolved
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 27 views
  • Users 0 members are here
Support feedback
Options
Options
Related

AM26C32: Output drive on logic side

Ian Benton
Prodigy 210 points
Part Number: AM26C32


Tool/software:

This also applies to other RS485 receiver ICs, 65C1168, for instance.

I note that the output drive on the logic side (not the line driver side) is asymmetric. Datasheets quote Vhi=3.8V, Vlo=0.2V for Vcc=5V. Approximating to a resistor that is 200Ω on the high side and 33Ω on the low side. Is there a reason for this? Are there more symmetric devices?

The reason I ask, is that I am turning a PDM signal back to analogue, and a simple 2-pole filter on the output of the receiver seemed the easiest way, but it needs a symmetric drive.

  • 0
    TI__Guru 74781 points
    Ian Benton said:
    I note that the output drive on the logic side (not the line driver side) is asymmetric. Datasheets quote Vhi=3.8V, Vlo=0.2V for Vcc=5V. Approximating to a resistor that is 200Ω on the high side and 33Ω on the low side. Is there a reason for this? Are there more symmetric devices?

    This is generally true for most TTL CMOS drivers that I've seen. From an industry point of view, it's cheaper to make an NMOS than a PMOS (the doping process requires more surface area to create a PMOS of the same Ron as an NMOS because from a physics stand point the 'holes' are much larger than 'negative charge').

     You could try to place another device infront of the output like a push pull level translator with rise/fall time accelerators if you want to try to make them more symmetrical. 

    -Bobby