Driving a Discrete Input Having a 50ohm Impedance

Don · 2005-11-08T14:58:08+00:00

Any one know of a good reference design for a driver for an input tounit with a 50ohm impedance and that's looking for 8 to 11VDC as a"high" and "-1 to 2VDC for a logic 0? Needs to have a fast (50nsec)rise time.I have been pondering just driving a transistor cable of handling about500mA with the collector ties to the mid-junction of two 50ohmresistors hooked in series. The high side resistor connected to 20VDCand the other resistor connected to ground.Any references or suggestions will be greatly appreciated!Thanks!Don

Nov 08, 2005 25 Replies

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Fred Bloggs 20 years ago

Good catch, that is a good safety net. View in a fixed-width font such as Courier.

. +5V . | . +----+--[10K]--|>|---+----------------------+---| . | | 1N6263 | | | | |/. | | | . | | | | | | . | | | . | ----74AC241 | | | | |\ | | | . | | | | | | +--| o-+ | | . | | |\ | | [560] | |/ | | . | | +--| >-+ | | | | | | . | | | |/ | | | | ---------- | . | | | . | | | | | | . | | | . | | | | | | . | | | |\ | | | | | | . IN>--------+--| >-+------+---------+------------+---|>|-----' . | | | |/. | | 1N5817 . | | | . | |

Can you find one that has 20pF input capacitance, handles 200mA at low threshold, and costs 27 cents? I assume it would be a P-channel.

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Fred Bloggs 20 years ago

You can reject it but I won't. You either do 10V @200mA or 5V@400mA. I don't call a common logic 5V supply, an octal logic driver, standard decoupling caps, and some Schottky's excessive. If you have no logic in the system then maybe something else makes sense, but if it's already there, the additional loading is 200mA E-6 which looks like leakage current to me.

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Tony Williams 20 years ago

A novel circuit that is interesting to look at though. I'd never thought about a single 5V pump of the rail and a simultaneous 5V swing of the output.

Will the output reliably swing from 0 to 10v in about 50nS? A possible killer might be load capacitance and the effective transient current load on the 74AC241.

What might be other ambushes?

Tony Williams.

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Tony Williams 20 years ago

No.

Tony Williams.

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Winfield Hill 20 years ago

What about ON Semi's NTA4151P that was just suggested? Only 0.49 ohms with -1.8V drive, and 16 cents each. Oops, 156pF of input capacitance, well OK, then a similar but smaller MOSFET. The NTA4151 can handle 760mA, well over the 200mA requirement.

Thanks, - Win

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Fred Bloggs 20 years ago

The OP said coax- if its RG-58 then the transition times should be

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