Component naming confusion

I'm trying to make a new version of the LadyAda SD logger Data-Logger Shield for Arduino but based on a Mega, rather than an arduino. (I'm moving it to a Mega shield becuase I also need to add various other periperals).

Anyway, the LadaAda schematic uses a 74AHC125N level converter. The supplier I normaly use, doesn't stock this exact component, but it does stock lots of similar components with extra letters in front, different letters in the middle and different ones at the end to.

I've looked at the datasheets for these components, and they all seem to be more-or-less the same. So, my questions are...
What do the different letters mean? and how can I work out if one device will work as a replacement for another? Most of the differences I can spot seem to be really subtle stuff about response times, inter-lead capacitance (whatever that is) and some minor differences in operating temperature. Does any of this matter? should I really just be going for whatever is cheapest?

(BTW, I've actually got the exact component from another supplier - this is just an question for the next time I can't find an exact match)



Letters at the front are just the supplier's brand letters, Letters at the end are package, or voltage ratings version. Usually any of them will do the same job, as long as its the type of package you want. (you obviously wont want a micro SMD device arrive when you want to solder it into a standard PCB)
When I do a search for a device I always drop the letters and see what turns up,
example, CD4017B CD4017C, both pretty much the same, I search for 4017
CD is manufacturer.
4017 decade counter
Hope that helps:)

Characters at the end usually mean package type, speed, temperature range or voltage range. At the beginning, manufacturer.

.. and the characters in the middle (e.g. 74XYZ123) on standard logic families typically tell you something about the underlying technology of the device.


Letters being different usually indicate that there is some difference.

In this case what you have to look at is that the input signal is going to be at 5V but the supply is going to be a 3V3. So the inputs have to be 5V tolerant. Most parts with the same number only allow the input signals to be 0.5V above the chip's supply.

The key bit is the HC - this means its high-speed CMOS (which supports 2V to 6V supply range - other families don't have this tolerant supply voltage range). The circuit is using it at 3V3 so other families like 74HCT or 74LS won't do.

[EDIT: my bad, I misread - its an 74AHC, which accept higher-than-Vdd input voltages, so the circuit's right, but only a 74AHC will do....]

Thanks guys, is there a key anywhere? i.e. what letters mean what?


Not that I know of you just have to pick it up. Wickipedia might have a page on logic families.

Go to, they have a logic family handbook you can download with those kinds if details, pinouts, voltage & currents, etc.