Why is LM393 output reduced by half when I connect this?

[blapcb]

In the attached circuit, I have a comparator (LM393) that takes a sine wave (from pint "A") and produces a 5v square wave (at point "B"). Then it is fed to a limiter circuit to condition it for input to a TTL 3.3v micro.

When only the comparator circuit is present (i.e. other part not connected), I get a nice 5v square wave. But when I connect the second part of the circuit, the amplitude of the square wave at the output of the comparator itself is reduced to about half (and with some distortion).

Why is this happening? I have previously used the same second part of the circuit for conditioning all types of inputs to bring them down to 3.3v and worked fine but not here.

Thanks

 

[keith1200rs]

The LM393 has an open collector output which you have pulled up with 10k but then you limiter circuit loads that with 10k to ground.

Keith

 

[blapcb]

The LM393 has an open collector output which you have pulled up with 10k but then you limiter circuit loads that with 10k to ground.

Keith

I was given to understand this this is required when working with a LM393 (i.e. the pull-up at the output).

The limited circuit is actually part of my device and I can't really change it. It is meant to condition signals (of square wave type) and logical levels from 10v to 40v for use with TTL 3.3v micro.

The comparator circuit, on the other hand, is my attempt to use a sound card as a variable frequency generator for testing (feeding the sine wave in to it and getting a 5v square wave).

So, my "test circuit" is not compatible with my end device? How should I change it so that I can use it in the way I intended? (eliminate the 10K and the output and just connect it directly to the limiter?)

Thanks, appreciate your advice

 

[keith1200rs]

Yes, you need a pull-up. Just make it a lot lower so it isn't influenced by the other circuit as much. Something like 1k or 470 ohms.

Keith

 

[pauloynski]

Not only R7 loads the output of the LM393 but also the series combination of R8 and C2 (which distorts the signal by delaying it). If decreasing the value of R1 (as posted above) does not solve your problem you can add a voltage follower to the LM393 output. Just take a NPN transistor (eg: BC548/BC547) and connect the collector to +5V, the base to the output of the LM393 and the emitter to the input of the limiter.
Regards