Keeping it completely analog, is there a way to high-z an open collector output? An open collector always allows a path to or from a specific voltage potential and this is complicating a rather simple task for me. The only way I can think of to do this is to interject an analog switch IC into the mix, but is there a way to do this with passives? I could use a transistor but then I'd also have to add logic to control the transistor and at that point a switch IC would be just as easy. Thanks much!
Open collector to high-z
analoghigh-impedanceopen-collector
Related Solutions
A couple of thoughts:
- To make an opto's output transistor switch on hard, quickly, you need to put a lot of current into the LED briefly, but I don't know that you'd have to keep it on that hard. I would think that driving the LED briefly with a higher current and then using a lower current to hold it might be helpful; if the time between cutting back on the holding current and switching it off altogether was sufficient, that reduced holding current could help the transistor switch off more quickly.
- It might be helpful to add some circuitry on the output side of your opto. You may have to switch 15mA, but that doesn't mean the opto has to do so. If you will only have to start asserting the output when it's high (meaning you won't have to do things like stretch pulses which originate with other devices) and won't have to hold it too long, you could perhaps use an NFET, a diode, a small cap, and two optos. One opto would connect the gate of the NFET to the drain via diode. The other would connect it to the source. The drain would have a small cap to the source to maintain the gate voltage while the NFET is on. To start asserting the output, one would pulse the opto connecting the gate to the drain. To stop asserting it, one would pulse the opto connecting the gate to the source. When the device is idle, one should drive the latter opto just enough to overcome any possible leakage through the former. There are many possible variations on this circuit; having a "supply" capacitor may be better than having a capacitor on the gate, but putting the capacitor on the gate would probably yield more consistent behavior (since the capacitor would always start discharged when one started to assert the output, and since releasing the output would not require charging the cap).
Depending upon how much voltage drop you can tolerate, other approaches may be workable as well. For example, you might be able to use an opto together with a BJT to form a Darlington arrangement, but use a second opto to accelerate the turn-off behavior. Such an approach would have a much higher voltage drop than the MOSFET-based approach, but would eliminate any need for a capacitor.
Typical open collector led setup would be:
simulate this circuit – Schematic created using CircuitLab
You are essentially creating two paths two ground, which are both variable and not perfect. There is a C-E voltage drop as well. If you measure the output pin voltage compared to ground, when the output pin should be Low (open-collector active), what do you get? The datasheets list that max would be 0.8v. U2-U5, are they buffers? What voltage level activates the buffers? And what part number?
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Best Answer
Open collector outputs are always hi-Z when they are high. The only way to source current from an OC output when they are high is via an external pullup.