Today, I tried to use a ULN2003a darlington array as a buffer for my CPLD's outputs. While it works well, I have a concern. The max. voltage where the Max V is guaranteed to read a low is 0.8V. The darlington array's low level output was 0.7V. I was expecting closer to 0V, but that was not the case.
Because I'm operating near the logic level threshold, I'm not sure if using this darlington array is a good idea. Instead, I'm looking at other dedicated logic level buffers. My current requirements aren't that high - I think about 50 mA to 100 mA per pin would suffice.
I did find a 16-bit tri-state buffer, but I the Enable inputs control 4 outputs at a time. I need a way to control every output. I essentially require a open collector/drain at every output.
With that said, I did the 74LVC1G125. The advantage of this IC is that its max. output voltage for a low is 0.8V. The min. output voltage for a high is 2V. The voltage at which the Max V is guaranteed to read a high is 1.7V. I think this make it a good match.
Now, I haven't really used such a IC before and even though it has two inputs, am I correct in guessing that I could use this just by using the Output Enable (OE)? I could have a pull up resistor at the output, and the A input can be grounded. When the OE is low, the IC tri-states and the resistor pulls up the voltage. When OE is high, the IC outputs A, which was a low. Am I correct in my understanding regarding this or am I missing something?
The downside is that it's just one per package. The other issue is the limiting current of 50mA. SInce this is a absolute rating, I better get too close to this. I would definitely like to have some more legroom for this.
Anyone have any ICs in mind for logic buffering?
Finally, is it wise to have buffers on both ends of a long wire? The length is approximately ~10m. The wire feeds a signal from the output of the CPLD to the input of another CPLD.
The loads are just long (~10m) wires. So, capacitive + resistive load? On the other end, the wires connect to another CPLD. Because the other end has digital inputs, I don't think the load is going to be significant. The frequency of operation is less than 1kHz.