How can I determine the maximum frequency at which a digital IC can operate?
I often find that I'm looking at a datasheet, wondering if a certain IC can receive and drive a signal at my desired frequency. I'll get a spec that a customer needs a signal of a certain voltage level at a certain frequency, and it can be frustrating sourcing parts when it's not clear if they can do the job. Unfortunately, I never have time to breadboard, so I need to make decisions based on the datasheet and be conservative in my designs. I'd like to know if there's a way to tell the max frequency from the datasheet.
Some datasheets do include a max frequency or bps. Some include a graph of power supply current versus frequency, so I can infer that the graphed frequencies are possible. These are not a problem.
Many datasheets only include propagation delay, which I'm not completely convinced is what I need. I could take the largest propagation delay that I would expect (based on temp, supply voltage and load capacitance), and figure that the Fmax = 1/(2*tpd), but I don't think that's necessarily correct. I think the max frequency could be either lesser or greater than that. Even though a signal edge takes a certain amount of time to propagate through an IC, that doesn't mean the IC has to wait that long for the next signal to start propagating through. Likewise, if a signal edge has finished propagating through an IC, that doesn't necessarily mean that the input is ready for the next one. But propagation delay currently seems to be my best hint, so that's what I use (with a good margin for error).
Also, I'm not always concerned with the amplitude of the signal produced. I realize that at higher frequencies, the signal will attenuate. What I want to know is, if there's a healthy signal on the input, up to what frequency will there be any signal on the output? I think this removes the effect of load capacitance, but not temperature or supply voltage.
If an example helps, you can use the 74LCX86. I've driven it at 100MHz, but I don't know how much faster I could trust it to go.
Thanks. This isn't a current problem. It just comes up repeatedly, so I'd like to know how to handle it next time.
I often find that I'm looking at a datasheet, wondering if a certain IC can receive and drive a signal at my desired frequency. I'll get a spec that a customer needs a signal of a certain voltage level at a certain frequency, and it can be frustrating sourcing parts when it's not clear if they can do the job. Unfortunately, I never have time to breadboard, so I need to make decisions based on the datasheet and be conservative in my designs. I'd like to know if there's a way to tell the max frequency from the datasheet.
Some datasheets do include a max frequency or bps. Some include a graph of power supply current versus frequency, so I can infer that the graphed frequencies are possible. These are not a problem.
Many datasheets only include propagation delay, which I'm not completely convinced is what I need. I could take the largest propagation delay that I would expect (based on temp, supply voltage and load capacitance), and figure that the Fmax = 1/(2*tpd), but I don't think that's necessarily correct. I think the max frequency could be either lesser or greater than that. Even though a signal edge takes a certain amount of time to propagate through an IC, that doesn't mean the IC has to wait that long for the next signal to start propagating through. Likewise, if a signal edge has finished propagating through an IC, that doesn't necessarily mean that the input is ready for the next one. But propagation delay currently seems to be my best hint, so that's what I use (with a good margin for error).
Also, I'm not always concerned with the amplitude of the signal produced. I realize that at higher frequencies, the signal will attenuate. What I want to know is, if there's a healthy signal on the input, up to what frequency will there be any signal on the output? I think this removes the effect of load capacitance, but not temperature or supply voltage.
If an example helps, you can use the 74LCX86. I've driven it at 100MHz, but I don't know how much faster I could trust it to go.
Thanks. This isn't a current problem. It just comes up repeatedly, so I'd like to know how to handle it next time.