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| Generating precise outputs from a comparator |
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| guymo:
Hello, I am trying to design an analog circuit to perform frequency addition for sawtooth waves. Given two sawtooths running 0 - 5V, if we sum the voltages, detect when the sum crosses 5V, and subtract 5V in that case, we get a new sawtooth wave whose frequency is the sum of the input wave frequencies. The obvious way to do this is to use a comparator and somehow clip or set the output voltages to be 0V and 5V. Obviously a certain amount of precision is desirable and ultimately I'd like to be able to chain a bunch of these stages which calls for more precision and a low parts count if possible. So: what's the best way to get precise voltage outputs from a comparator? Ideas I have had: - use an LM311 with a 5V reference for the pull-up; I'm not sure if the 0V output will be precise and there will be loading issues for the pull-up. - use a rail-to-rail op amp powered from 0V and 5V. This seems quite a good approach although there is some danger that the input waves go beyond the power supply rails of this component and perhaps give it trouble - run the comparator output through a CMOS IC (inverter for instance) powered from 0 and 5V - clip the outputs of a regular comparator with some sort of shunt reference -- but what? I've played with a TL431 and didn't get very good results - use a precision clipping circuit. Parts count is going up at this point... Any advice or ideas welcomed! I'm posting in Beginners because I basically don't know what I am doing... |
| D-Jack:
So the output should be: (wave1 + wave2) or (wave1+wave2)-5v? And can you use dual supply op-amp or must it be single supply? |
| guymo:
--- Quote from: D-Jack on July 04, 2018, 11:16:24 pm ---So the output should be: (wave1 + wave2) or (wave1+wave2)-5v? And can you use dual supply op-amp or must it be single supply? --- End quote --- Yes, overall output will be wave1+wave2 if that does not exceed 5V, and wave1+wave2-5V otherwise. The system as a whole will be running on good old fashioned +/-15V dual supplies. |
| Giaime:
As far as output accuracy goes, I have obtained best results with CMOS ICs, especially when run in parallel. Their low RDS(on) for the output stages help to reduce loading effects: I use them for precision PWM DACs. Of course they cannot be more accurate than the power supply rail they're being powered with. |
| Zero999:
What issues did you have with the TL431? |
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