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| Temperature indicator circuit with LEDs |
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| dazz:
Cheers Hero. I really need to learn to read datasheets more carefully. "Output Load Current... max 50uA" doesn't get much clearer than that :palm: --- Quote from: Hero999 on August 31, 2018, 08:06:51 am ---A unity gain buffer, made with the cheap and cheerful LM358 will be better. --- End quote --- Wouldn't that opamp need to be full rail to rail output? |
| Zero999:
--- Quote from: dazz on August 31, 2018, 12:09:00 pm ---Cheers Hero. I really need to learn to read datasheets more carefully. "Output Load Current... max 50uA" doesn't get much clearer than that :palm: --- Quote from: Hero999 on August 31, 2018, 08:06:51 am ---A unity gain buffer, made with the cheap and cheerful LM358 will be better. --- End quote --- Wouldn't that opamp need to be full rail to rail output? --- End quote --- Why do you think you'd need full rail to rail operation? What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine. |
| dazz:
--- Quote from: Hero999 on August 31, 2018, 12:44:51 pm ---Why do you think you'd need full rail to rail operation? What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine. --- End quote --- The sensor output range is 100mV to 2V, but I'm just using the interval between 800mV - 2V really. I'm confused now. The reason for my concern is that I simulated the buffer with a TL082 (that's the only opamp I have right now) and the output doesn't go all the way to zero. The TL082 datasheet states that the voltage swing for a +/-15V supply is -13.5 to +13.5V which seems consistent with the simulation showing the buffer kicks in when the input voltage gets above 1.5V If I'm reading the datasheet correctly, the LM358's swing is pretty much the same: "Input Common-Mode V+−1.5 V Voltage Range |
| Zero999:
--- Quote from: dazz on August 31, 2018, 01:05:14 pm --- --- Quote from: Hero999 on August 31, 2018, 12:44:51 pm ---Why do you think you'd need full rail to rail operation? What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine. --- End quote --- The sensor output range is 100mV to 2V, but I'm just using the interval between 800mV - 2V really. I'm confused now. The reason for my concern is that I simulated the buffer with a TL082 (that's the only opamp I have right now) and the output doesn't go all the way to zero. The TL082 datasheet states that the voltage swing for a +/-15V supply is -13.5 to +13.5V which seems consistent with the simulation showing the buffer kicks in when the input voltage gets above 1.5V If I'm reading the datasheet correctly, the LM358's swing is pretty much the same: "Input Common-Mode V+−1.5 V Voltage Range --- End quote --- Yes, the TL082 is unsuitable for this because its inputs don't function close enough to the negative rail. The LM358 should be fine for what you're doing. The part of the data sheet you've quoted is the maximum i.e. the highest end of the common mode range, which is V+-1.5V. The lower end is 0V and is specified with a single 5V supply. In other words, the LM358 will work with its inputs between the voltage on pin 4 and pin 8 minus 1.5V. In fact the LM358 is slightly better than the data sheet suggests because it will work when its inputs are slightly below 0V, down to about -0.3V but it's not guaranteed to, so be cautious about using it in designs where it inputs can go too negative. In short you don't need a rail-to-rail op-amp, just a single supply op-amp, which the LM358 is. |
| dazz:
Oh, I see. thanks again, Hero :-+ |
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