50 ohm output ideas

Dave, I suggest a video amplifier configured for again of 2. Insert a 50 Ohm back termination resistor in series with the output. The gain of 2 compensates for the 2:1 voltage divider formed by the 50 Ohm back termination resistor and the 50 Ohm load. Regards, Jon

I designed a circuit that produces some low frequency (.1 Hz to 100 KHz) square wave outputs in the TTL range (Voh =>

2.4, Vol

Hello Dave,

That seems to be a popular method. However, never parallel unless it's all on the same chip.

Another option is the usual push-pull follower. At 100kHz almost any old npn/pnp combo should do. Considering that CMOS chips drive rail-to-rail that would still leave enough swing, provided you are supplying the driver ahead of it with 4V or more. 3.3V won't cut it with this setup.

Regards, Joerg

Did you mean TC4427? Sounds like you have a pretty good parts bin! Those parts do moderately well when powered from 5 volts: 15-ohms Ron typical, 50-ohms max, according to the datasheet. It's amazing how high the ON resistance of these serious 1.5A MOSFET drivers can get at low supply voltages. 50 ohms is pretty wimpy.

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 Thanks,
    - Win

There is also the preferrable although more expensive TC4427A and related parts (TC4426A) that offer better matching of rise/fall times and delays.

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I would use a CMOS hex inverter, all 6 in parallel.

Sheesh, is all that stuff really needed, and is it all pulling its full weight? At the other end of the spectrum, Dave could consider the mind-blowing shortcut solution offered in The Art of Electronics (page 612), which works amazingly well:

. +Vcc --/\\/\\--, 10 ohms, 2W . | . logic output |/ 2n4401 . ---|>o-------| . not critical |v output destination . | jack logic - 74LVCxxx, etc, . '----o -------- o--+---|>o------- . cable | 51 ohms term . '--/\\/\\-- gnd

The destination sees a logic HI of Vcc - 0.7V and the 50-ohm termination acts as a prompt pulldown for the 2n4401 transistor. Any pulse traveling down a 50-ohm coax is properly terminated at the end, so there's little reflection energy, allowing one to get away with an absence of source termination. The jack on the destination should be labeled "50 ohms" or "50-ohm term."

Alternately, one can employ an open-circuit at the destination. This reflects back to the source, at which point the cable input also appears as an open circuit. But to insure the destination gets the intended source amplitude (and doesn't spike to more!), the source must present at least 50-ohms of impedance (for 50-ohm coax). For example, with a roughly 25-ohm CMOS gate Rout, one could add at least 35 ohms, to be on the safe side, or one could go for it and add 25 ohms. This simple scheme works surprisingly well, and only requires one resistor. If you use a scope to look at the signal at the source, it starts out reduced, and only pops up to Vcc after the reflection arrives back. But if you look with your scope at the end, you'll just see an ordinary full-size pulse, as intended. Just one resistor... Cool, huh?

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 Thanks,
    - Win

Thinking out loud: how about something like an MC1488 or MAX232, followed by a Schottky diode (to keep the 'low' voltage reasonably low) to throw away the negative swings you don't care about?

Matt Roberds

If adding another IC, there are several 50ohm drivers readily available. For logic signals like this, I had a similar need a while ago, and a look in my 'parts bin', found the MC4427 MOSFET driver, which worked very well indeed, and had the advantage of being really small.

Best Wishes

Yes TC4427. However my data sheet only gives the Ron Max at 25R, not 50R at 5v. I was driving 10v though, so this aspect was less of a problem for me.

Best Wishes

A high speed non-inverting line driver operating off the 3.3V with TTL compatible output and short-circuit tolerant would be something like this: View in a fixed-width font such as Courier.

. . . 3.3V . | 1u . +---+--------------------+----+--||--+-----+ . | | | | | | . | [100] | | | -+- . | | | | | /// . | e 2x e | | . | \\| 2N4403 |/ | | . | |--+----------+--| - | . | /| | | |\\ ^ sd | . | c | - c | | . | | | ^sd | | | . | | | | | | +--------- . | +----+ +----+----+-----------------Vout . | | |\\ | | +--------- . | [2.2K] +-| >o-+ | | | . | | | |/ | | | | . |\\ | | |\\ | sd | - | . Vin>--| >o-+----+-| >o-+----|

As time goes on, ICs keep killing discrete circuits. If you parallel all three sections of an NL37WZ16 and run it from 6 volts, it will put

5 volts into 50 ohms in about 600 ps, for 13 cents.

John

I eventually used an AD817 Op Amp in a similar requirement for a 50 ohm output buffer for a VCO. As below, the AD817 needs a 50 ohm output resistor, and set to a gain of 2. Works good on single supply voltage as per the spec sheet example - but needs therefore a DC blocking cap at the output. JEFF

A very nice factoid, thanks John! That implies three parallel NL37WZ16 stages have Zout = 20 ohms, and therefore needs about 30 ohms in series to transparently drive a coax?

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 Thanks,
    - Win

The original question was low speed (only 100KHz max). Things get different if you need to go faster.

Best Wishes

Something close to that, maybe 10+40 or something in-between. I'll let you know if the opportunity arises, and you can tell us ditto.

John

The trouble with these things is the propagation delay, like,

40ns is forever. Last time I had to do this for just a few channels I rolled my own, using two transistors and a few resistors. Sort of like this:

No good if you need many, though. A simple emitter follower works in fact very well, especially if you drive it with high speed CMOS.

Jeroen Belleman

Here is an idea:

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Sorry, right, I meant to say 10-ohms Ron, 40-ohms series out.

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 Thanks,
    - Win