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I plan on routing two supply lines (one 5V and one 3.3V) into an AND gate powered by the 3.3V supply. The 5V supply is dirty and the 3.3V supply is clean. I'm concerned about the noise from the 5V supply polluting the 3.3V supply and/or the output of the gate. Are some AND gates (series, mfg, etc.) better than others at isolating the inputs?

Thanks,
Bryan

I like VHC logic from Onsemi or Fairchild.

What speed do you need?
A resistor divider taking a "1" down to about 3V maybe enough.

The faster, the better I guess. This is basically a protective feature of the chip I'm routing the output to (as a resistive pullup to a differential digital line)... the chip mfg doesn't want the line pulled high when the dirty 5V supply is not active.



Enough for what? Can you elaborate?

Thanks,
Bryan

I couldn't find any single Fairchild parts, so what about this one from OS: http://rocky.digikey.com/WebLib/On-Semi ... HC1G08.pdf

Thanks,
Bryan

NEIN!!!! Unless you want EMI.

As fast as necessary, but not faster!

Jocko

Given that the state of the gate will be switched very infrequently (most frequent may be a few seconds, but more like minutes), is this really an issue?

Thanks,
Bryan

So, what is the benefit of using an AND gate over using a relay? An AND gate would take up a lot less PCB space - but if you're doing this after the fact, is there any benefit to using the AND gate? (Sorry, I hope this isn't considered thread jacking)

No...........because I was wondering if it is only going to be on a few seconds every now and then, why speed is an issue.

Even if it is every now and then.......getting out of the habit of picking as fast as needed, but not faster is not a good habit.

Jocko

The way I see it voltage divider from 5V is the best solution. Not all logic families have 5V tolerant inputs when supplied with 3.3V. If you do not take care about that you might get the whole 3.3V section supplied from 5V through AND gate protective input diode. Voltage divider will also form light low pass filter with AND gate input capacitance. (Very slight in modern logic families, I confess.)

Any suggestions on resistor values? For 1mA going from 5V to ground, I get 1700 and 3300 Ohms. Any problems?

I guess from the filtering standpoint, it would depend upon which frequencies were of interest. The 5V is a computer USB bus, and I'm not familiar with the frequencies contained therein...

Thanks,
Bryan

You would be fine even with 100uA. IIRC USB clock frequencies are 12MHz for USB1.1 and 480MHz for USB2. You can use additional capacitor in parallel with 33k resistor if you need additional filtering. Choosing AND gate with schmit trigger input would be a good idea also, considering slow input waveform.

Does a VHC and gate exist with Schmitt Trigger inputs?
I couldn't find one...

Thanks,
Bryan

Like Jocko said: go with the slowest gate you can find, no need for VHC:

http://focus.ti.com/lit/ds/symlink/sn74ahc1g08.pdf

is one possible solution.

Ditto about need 4 speed!

Cheers Michael