Schmitt Trigger for logic level conversion voltages up to +-20V

Question

I want to be able to safely convert any voltages above a threshold voltage of +-1V with a maximum voltage level of +-20V to the logic level (3.3V). I ended up using an op-amp to construct a Schmitt trigger with the resistors and the power supply for the high threshold voltage of ~1V and the lower threshold voltage of ~0.5V. The circuit is shown below

Upon building this circuit using a LF353N op-amp and testing it out, I found that the output of the circuit just hits a DC level of about 3V when the input passes a voltage of 6V peak-peak. I also found that the waveform it produced was has curves on the top looking a bit like this:

The frequency of the waves I'm dealing with are around 10kHz - 100kHz and I was wondering if these effects are due to the fact that the slew rate of the LF353N is too slow?

Answer 1

This requires you understand how a comparator needs to work.

Your Spec: Single Vcc 5V, +/-x to 20V input, Vih=1V Vil=-1V Voh=5V, Vol=0V or CMOS worst case logic level specs.

• Thus Vth the center switch Vref must = 0V not 1V ( you have 20% of 5V.)
• 20% hysteresis is correct but at wrong Vref.
• Vin below Vss causes loss of gain which is necessary for positive feedback.
  • this is defined by Negative Common-Mode Input Voltage Limit
• choose the non-inverting inputs then Vcm will be more positive.

^{simulate this circuit – Schematic created using CircuitLab}