Electronic – Negative peak detector hits “minimum” value


I am using LTSpice simulation to average value of a function. To do that I detect positive and negative peaks and average them together. My function, however, starts at ~-3.3V. And the circuit I am using is unable to register any negative peak values lower than ~-1.7V. It results in this:
Simulation results

What changes should I make to my circuit for it to be able to detect lower values of negative peaks?
Vdd=5V; Vee=-5V;

Best Answer

Input common mode range is the problem: -

  • The signal input voltage range for an LT1057 starts from about 3.5 volts above the lowest supply voltage. If your lowest supply voltage is -5 volts, then all bets are off if the input is lower than -1.5 volts
  • The output voltage swing may get to within 2 volts of the negative rail.

These are typical figures only. I'd look for a different device or increase the supply voltage levels.