SN74AUC16240: 74AUC suitable for use as Logic Analyzer (LA) front end?

Ted Mawson 3.0

Part Number: SN74AUC16240
Other Parts Discussed in Thread: SN74AXC8T245

I am working on a LA cable/front end design for an interface to an oscilloscope. The scope takes LVDS inputs on 16 channels at up to 200MHz and I want to create a front end. The circuit attached is my basic idea (only 1 channel shown, 4 x Device B would be needed) and I have a few questions:

1. Is the SN74AUC16240 one of the devices that can be used as a voltage level translator using Vpullup of 3.3V

2. What value would you recommend for Rpullup?

3. The 3 places in the table where Vpullup is shown at 3.3V would suggest that, with Vcc at 1.8V, the AUC device would work for 3.3V and 1.8V logic input levels - changing the AUC to Vcc of 1.2V would then allow 1.2V logic levels to work. Have I got this right?

4. Device A is really my device under test but what protection would make sense going into the AUC device? Would a 100 Ohm resistor in series suffice?

If you think I should take a different approach or use a different logic family, please say so.

Thanks in advance for any help / advice you can give.

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 259910 points

1. This kind of voltage translation works only with open-drain outputs. The SN74AUC16240 does not have open-drain outputs.

2. The pull-up resistor and the parasitic capacitances in the circuit form a low-pass filter. To reach high speeds, you must use a very low resistor value, which results in very high currents. Better use an actual translator with push/pull outputs like the SN74AXC8T245.

4. Any I/Os that go to the outside world need proper ESD protection. (Again, consider how the 100 Ω combine with those additional capcitances.)

0 Ted Mawson 3.0 over 4 years ago in reply to Clemens Ladisch

Expert 1795 points

Thanks for the suggestion of the SN74AXC8T245, it looks like it would well in my design. I'll make your reply up as the answer. Any suggestions for the front end protection? I'm trying to keep it simple and lowish cost, if I don't get much above 100 MHz, that's probably fine.

0 Clemens Ladisch over 4 years ago in reply to Ted Mawson 3.0

Guru 259910 points

See, for example, figure 5 of System-Level Protection for High-Voltage Analog Multiplexers.

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SN74AUC16240: 74AUC suitable for use as Logic Analyzer (LA) front end?