Hi,I am using a MSP430G2553. I wanted to use the comparator as an inverting Schmitt-Trigger.Configuration is as follows:- P1.0 configured as + channel reference- P1.4 configured as - channel for input signal- P1.7 configured as comparator ouputReference voltages are:- V high ~ 78 mV- V low ~ 35 mV
Strangely comparator only changes output from high to low for the double reference voltage.I.e have to input a rectangular signal at approx. 150 mV instead of 78 mV as expected.Tried this for different reference voltage, always the same behaviour.
Checked for inter-pin short cuts, but could not find any. Signal source and controller are on same ground.What am I missing? Cant find any explanation for this behaviour why threshold voltage is doubled, instead of applied reference voltage.
Any tips appreciated.
Could an impedance mismacht of comparator input and my source cause this behaviour?
FYISeems I need to use an high impedance source, then the problem is gone and comparator works as expected.Problem solved.
How did you configure the OpAmp? Maybe that's the problem?
Your rectangular signal, maybe it is symmetrical, so you're actually applying +-75mV instead of 0V/150mV? Most function generators produce a symmetrical signal on the adjustable output. Only the TTL output (with fixed level) is 0V/3.5V.
So what is your ground level on signal generator, MSP and measuring device (scope)?
Edit: using a high-impedance source shouldn't be necessary. However, in this case the negative half of the signal is cut more cleanly and won't appear in the measured input voltage anymore. It adds to my suspicion that your input signal is symmetrically.The input impedance of the signal shouldn't matter as long as the up to 50nA input leakage curent do not make a difference.
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Hi,thanks for your advice. Comparator configuration is:
/* CACTL1 Register */ CACTL1 = CAREF_0 + CAON + CAIES + CAIE; /* CACTL2 Register */ CACTL2 = P2CA3 + P2CA0 + CAF; /* CAPD, Register */ CAPD = CAPD4 + CAPD0;
The waveform generator has only dc ouput, so the signal is asymmetrical from 0 V to set value. I also checked this with an oscilloscope.Reference voltage also checked with multimeter and according to calculations.I did not check current output of source but I think it can generate more than 45 µA for leakage current.But since it is working now with my intended source I am satisfied, but still unhappy not knowing why it had problems with rectangular pulse source.Thanks for pointing out possible problems,Josef
Well, if the two (generator and MSP) use the same GND and the signal is not AC-coupled (with a series capacitor, which would raise a virtual GND on the average signal value).then I realyl do not understand why twice the voltage is required.
Josef Irlingerstill unhappy not knowing why it had problems
Dear Jens-Michael,you were right in the end.The offset of the signal generator was off, and hat to be reset to 0V.Then also the comparator worked as expected with the rectangular signal.Thanks again!Josef
Josef Irlingeryou were right in the end.
Josef IrlingerThe offset of the signal generator was off, and hat to be reset to 0V.Then also the comparator worked as expected with the rectangular signal.
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