Using external comparator instead of the one in atmega

[shater]

Hello.
First, apologizes for my English.

Here's the deal:
I made very simple circuit which is using analog comparator in atmega8. There is Ultrasonic receiver plugged into comparator. When it measure some vibrations, it makes very tiny wave and comparator tells me when it passes zero point.

It is in attachment,

I saw circuits where there were many operation amplifiers between ultrasonic receiver and atmega but I have to do it this way and i know it works because it works

Anyway I can't use atmega's analog comparator any longer because of some reasons and i figured out that I will use external comparator. So I made the same exactly circuit with LM311 Comparator but it doesn't work :/ Of course i gave there pullup.
There is always 0 on comparator output.

I think that atmega's comparator might be more sensitive than LM311. Anyway tell me, what to do?