LM311N Comparator with Strobe

[C_Electronics]

Hi,

I built a simple circuit using the LM311N comparator. I am inputting a 6V pp amplitude, 3V offset triangle wave on the + pin and a 3V d.c. voltage on the - pin. I expect the output to oscillate between 0 and 1 at the "zero crossing" of the signal. The output is correct (as seen on the scope).

The problem is that the strobe pin is not working for the model. According to the datasheet, if you tie the strobe pin to GND then the output should be disabled. I tried adding in a transistor with a 1k resistor to ground and a TTL signal at the base as per the datasheet and this doesn't disable the output either.

Could the component model be the issue? Any help is appreciated.

Thanks.

[C_Electronics]

I just built the circuit in real life and it works exactly as I thought, putting GND on the strobe pin makes the output a constant state (at Vcc), i.e. it is disabled. Why is it not working in Multisim?

[niarena]

maybe it is a bug on the SPICE model, did you try another comparator?

 also, attach the .msXX file to run it

[C_Electronics]

Hi,

Sorry for the late reply, I was refreshing my spice knowledge to try and fix the problem. Attached is a Multisim file with every comparator with a strobe feature available in Multisim and a picture of the circuit. All of those comparator outputs should be a high state which means the LED will turn on (connected through the 1K resistor to Vcc). If you manually switch the voltage at the + terminal to be less than at the - terminal then the output should be ground, and the LED shuts off; this indeed occurs. 

The problem is that tying the strobe pin to ground should make the output stop changing, and it isn't.

Any help is appreciated.

Thanks.

[niarena]

It does switch on/off if you change the + signal.

I think it keeps functioning because of the way the model was created, according to the datasheet, the strobe pin should not be grounded, it is warned several times on the datasheet, and on the component models, that node is not connected. (Check image).

As grounding unused pins is a practice on electronics, I guess it was the way of the designer to make sure the device was not disabled for a ground wired to that pin.