Quote:
Originally Posted by thingstodo
Our instrument supervisor came up through the ranks, so he's the most experienced tech I know. He has explained grounds to me many times .. Obviously I'm a slow learner. I just don't seem to be able to keep it straight.
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I just talked to Rob and I need to get this written down before I mess up the details:
- recommended cable is #18 twisted pairs, individual shield on the pairs, and an overall shield. We just moved to a new inventory system ... I'm having trouble finding the 600V rated cable. This cable has the right specs, except for the 600V insulation. I think we use this stuff in panels only? Under $3 per foot.
Belden - 9552 0601000 - Cable - Multiconductor - Allied Electronics
- tieing both shields together, on the same end, is OK if you are not in the Mhz or Ghz range. 500 Kbits is just barely into the Mhz range.
- we don't use a braided shield any more. It was used in the 60's and 70's for noisy environments. It no longer appears to be needed (not sure about that part)
- when you have an isolated supply, you can *make* an isolated ground with a large copper buss bar that is isolated from all else by insulators. This seems a bit extreme to me. I didn't ask how to figure out what 'large' is.
- using negative on the power supply as a ground has some issues, particularly for switching power supplies. Adding a low Equivalent Series Resistance (ESR) capacitor across the power supply leads helps if the power supply does a soft start. Otherwise you need a filter so that the capacitor does not short out the power supply on startup. The capacitor looks like a short circuit to high frequency noise.
- grounds come to a common point. NO DAISY CHAINS. Use finely stranded wire to give it low resistance to high frequency. And make the ground cables at least as large as any of the conductors that are tied to it.
That's all my notes and all I remember from the conversation
