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Driving a Discrete Input Having a 50ohm Impedance

Look at the TC4421 and TC4422 gate driver chips, and try to limit the 'high' time to a microsecond or less, so that the duty cycle is very small, and therefore you will not need to waste a lot of power.

As long as the load is 50 Ohms, which you say it is, then you don't need a resistor at the driving end, only at the load end. Those who say otherwise are mistaken. You can test this all quite nicely if you have a fast scope and a few tens of metres of 50 Ohm coax cable, this length being desirable so that you can clearly discern the reflections or absence thereof in the waveforms. You need to have a low capacitance scope probe or you need to use the scope in 50 Ohm mode in place of the load termination for testing. Be careful not to put too much DC for too long a time into a 50 Ohms scope input as this will fry it.

Chris

Reply to
Chris Jones
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LOL- the choice is not available, so my vote is for a super-duper bootstrapped driver constructed from a buck's worth of readily available logic drivers. A 1us PW should be more than adequate: View in a fixed-width font such as Courier.

. . . . CMOS BUFFER 1PPS CABLE DRIVER . . . +5V . | -> ->

. +---+----+-[10]--|>|-+------+--------+ . |-> | | sd | | || . | | | === [1K] v| . === | | 0.47u | | . 0.47u|| | | | |\\IC2 . | v| | ^| +-------| >------> >--+ ~10V . | | | || | |/ -> | 200mA . | | | | | | | ---- . | |\\IC1 | -> | | | | | | .IN>-+--|--| >---|-----------+---------------+ | | 1U | . | | |/ | | | | | . | | | | | | - - . | | | | -or'03 OD gate- | | . | | | |\\ |-+ | . +--|---|---| o--------------||NMOSFET | [50] . | | |/ |-+ v | . | | | | | . | | | | | . | | | | | . +---+----+------------------+ | . | | . ---

Reply to
Fred Bloggs

Maybe a little protection diode: View in a fixed-width font such as Courier.

. . . . CMOS BUFFER 1PPS CABLE DRIVER . . . +5V . | -> ->

. +---+----+-[10]--|>|-+------+--------+ . |-> | | sd | | || . | | | === [1K] v| . === | | 0.47u | | . 0.47u|| | | | |\\IC2 . | v| | ^| +-------| >------> >--+ ~10V . | | | || | |/ -> | 200mA . | | | | | | | ---- . | |\\IC1 | -> | | | | | | .IN>-+--|--| >---|-----------+---------| C= ----- 1u=200nF . 1V .

Reply to
Fred Bloggs

A bead or small inductor in series with the blocking SD should make for fastest turn-off transition- and IC2 can lose a gate to buffer the 1K pull-up into the other IC2 gates: View in a fixed-width font such as Courier.

. . . . CMOS BUFFER 1PPS CABLE DRIVER . . . . +5V . | 10uH -> ->

. +---+----+-~~~~--|>|-+---------+-----+ . |-> | | sd | | || . | | | === [1K] v| . === | | 0.47u | | . 0.47u|| | | | |\\IC2 . | v| | ^| +----| >------> >--+ ~10V . | | | || | |/ -> | 200mA . | | | | | | | ---- . | |\\IC1 | -> | | | | | | .IN>-+--|--| >---|-----------+--| C= ----- 1u=200nF . 1V .

Reply to
Fred Bloggs

This one might be too fast, but it's a cheap way of driving that 1PPS trigger pulse train from something like a PIC. I am assuming that up to

1us pulsewidth is more than adequate- the giveaway is they do not care much about trailing edge: View in a fixed-width font such as Courier.

. . . . . +5V . | . +----+--[10K]--|>|---+----------------------+---||---|---------+------------+--| o-+----+->

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

Reply to
Fred Bloggs

This is the kind of circuit we think about from time to time, and quickly reject. For example, a flying opamp with supplies pinned to the rail of another opamp, to extend ordinary cheap +/-15V capable opamps to +/-30V operation, rather than using a HV opamp to begin with. In the case of some steady-state AC signal applications, the flying opamp's entire power supply can also be obtained from the first opamp's AC output. Arrggghhh!

--
 Thanks,
    - Win
Reply to
Winfield Hill

Perhaps add a Schottky to stop the gates of the 74AC240 from going above it's supply rail when the input goes down. Maybe even change the 74AC240 for a mosfet.

Tell you the truth Fred I'm sitting here, buttocks well- -clenched, waiting for the next edition of this bloody hairy circuit.

--
Tony Williams.
Reply to
Tony Williams

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.

Reply to
Fred Bloggs

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.

Reply to
Fred Bloggs

Hi Robert,

Cool, do you have any sources?

Marte

Reply to
Winfield Hill

To protect your Saudi "Friends", no doubt!

The millitary serves the needs of the Nation, Your Nation - by choice - needs Saudi Oil. So, there was a price attached to driving them SUV's. Why is this a surprise ... ??

Pah - There were more people to that party: Syria, Libya, Iran, PLO and a few warlords and militia.

Sorry to lack your standard, but I am not the one having a problem dealing with reality.

Reply to
Tony Williams

hi, if any one got experience with using KODAK , KAC-9630 CMOS image sensor i2c interface, please help me. the problem i got is that the sensor does no acknowlege the data transfer. im using inetrnal pullups for scl and sda with in the FPGA.

Thank you. CMOS

Reply to
Tony Williams

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

Reply to
Fred Bloggs

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