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

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?

• Why not use an comparator instead of opamp? Jul 22, 2017 at 12:46
• You're giving an op amp that cant pull down to the neg rail or up to the positive rail only 3V to work with, then apparently giving it a 1V sine wave centred on the negative rail, then specifying "above plus or minus 1 volt" as a spec. That's three reasons it isn't going to work to be going on with Jul 22, 2017 at 13:32
• Must use Rail to Rail COmparator with high slew rate, not an OA with saturation offset and limited slew rate that cannot accept inputs below Vee or wide Vdiff input. 3 faults. Even a differential amp with high gain would be better. Jul 22, 2017 at 18:28
• How critical are the thresholds and what are the noise levels? Jul 22, 2017 at 19:04

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