LM339 is open-collector output-- what pullup resistor did you use?
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Last night, I put together a simple addition to my square wave generator based on chip XR2206.
The addition was a comparator chip LM339. I needed it to produce two separate, logically opposite signals for two inputs of gate drivers.
I used two pairs of inputs:
Input 3+: 1/2 Vcc (I used a voltage divider based on 3k resistors) Input 3-: output from XR2206
Input 4+: output from XR2206 Input 4-: 1/2 Vcc
The result is square wave output from outputs 3 and 4. What I did not like was that they were not very square looking. It seemed as though there is some capacitance somewhere, like a capacitor that needs to fill before the voltage would rise. It was kind of exponential looking like shape, like 1-e^-x would look.
Could it be that I was not using proper technique for displaying the wave on the scope? I use some length of coax, and then a coax to test leads adaptor, and alligator test clips.
i
I will check tonight, that could be it, I used no pullup resistor. I will use something from my pile of boards.
i
Did you use a suitable pull-up resistor from the LM339 output to VCC? The LM339 is an open collector device.
Regards, Joerg
Nope, and I think that it is the issue. Thanks to you and Spehro.
i
You are welcome. I try to avoid posting the same answer as others but Spehro's didn't show on my server before I posted. Seems he beat me by
12 minutes ;-)Regards, Joerg
I will try your suggestion tonight, if I have enough time.
i
As others have said, one reason is the pullup.
That is not the correct way to display fast rising signals. A x10 scope probe is needed, so the attenuation (x10) is constant with frequency. With the setup you use, that is not so as your cable and scope form a low pass filter, attenuating the higher frequencies.
Most scopes have a square wave available on the front that you use for adjusting the probe (adjust until the display is square).
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