1

I'm designing a schematic with opto-coupled digital inputs (BNC). For better noise/ground loops immunity/reflections should I add 50 ohm resistors on both input lines, signal(+) and GND, along a filtering capacitor, and make GND1 in the attached figure earth grounded or floated?

Input digital signal is <10 kHz. The whole device is LED driver, switching large currents >15 A based on the digital input.

enter image description here

winny
  • 13,064
  • 6
  • 46
  • 63
Mr No
  • 71
  • 2
  • 1
    I would prefer to use no R77. BNC inputs are asymmetric anyway. – Uwe May 13 '22 at 01:40
  • 1
    Mr No - Hi, Since this seems to be the improved version of [your previous question](https://electronics.stackexchange.com/questions/619462/instrumentation-box-digital-input-signal-conditioning) I have closed that one as a duplicate of this one. In future, please remember to edit a question to improve it, rather than reposting a slightly updated version. Thanks. – SamGibson May 13 '22 at 03:04
  • 1
    "For better ... reflections " Unlikely. The time constant on your RC is about 10 usec. Unless you're using about 10,000 feet of BNC cable, reflections simply won't be an issue. – WhatRoughBeast May 13 '22 at 10:21

1 Answers1

0

The series resistor isn't needed on an input. If you want to terminate an input to 50 ohms, R52 should be 50 ohms and the other resistors removed. Note however that if the source is also 50 ohm output impedance you will receive half the voltage at the input, so your circuit needs to be able to handle that.

Alternatively if you control the source then you could simply limit it's rise time. Since your edges rate is so low you could tolerate long rise time and therefore need no termination at all. I would consider this if possible (e.g. you are designing the source driving that input).

user1850479
  • 14,842
  • 1
  • 21
  • 43