Op-amp driving mosfet changes hystheresis thresholds?

Question

I have a non inverting push-pull output op-amp with additional hysteresis (Maxim MAX9015) that should drive an N-Mosfet.

I calculated the resistor values for the op-amp(R5, R6, R7) for an older version of the circuit, when there was no mosfet yet and the "OUT" of the op-amp was going directly to an other IC.

Now I am not sure if the resistors that I have to put for the mosfet, R8 and R9, are going to modify my hysteresis thresholds. Do I need somehow to isolate the hysteresis part from the mosfet?

See part of the circuit below.

Thanks in advance for your help. Cheers

Answer 1

The only connection R8 makes to the MAX9015 is on the output. R8 is 100 kohm therefore it will have virtually no effect on hysteresis levels. Even if R8 terminated to ground it would only load the MAX9015 output extremely lightly.

If you look at graphs in the data sheet they show how the peak output voltage can be reduced by load current. At a load current of 1 mA, the output voltage (high) might be Vcc - 50 mV and the low output might be about the same. With a load of 100 kohm your output current might be as high as 50 uA so expect that the error induced in the output might only be 2 or 3 milli volts.

See page 6 of DS.