Hallo Prahlad,

Steve, Erik, me and many others want to know, whether you have added pull-ups to the outputs of ULN2803, like this way:



We want to know this for some simple reason: ULN2803 is not designed to work as a line driver, but more to switch inductive loads. If you want to use it as line driver, you need an pull-up somewhere. Otherwise you will not be able to produce logical high state!! Logical low state is accomplished by having the darlington turned-on. But what when the darlington is turned-off? Only a pull-up will then guarantee that the voltage goes high.

ULN2803 has so called 'open collector outputs'. As line driver these are not well suited, because pull-up resistor must be choosen very low, when fast rise times shall be realized. Think of the time constant formed by pull-up and capacitive load!

Beside this, I think your optocoupler is responsible for the malfunction. My datasheet says: rise time 300µsec, fall time 300µsec!! So, for a full range swing you need 300µsec plus 300µsec, makes 600µsec, which corresponds to a maximum frequency of 1.7kHz!!!
So, your observation at 3kHz is no surprise with this data.

Why not using 6N137? Is a very fast one...

By the way, 180Ohm resistor at opto LED is much to low. 20mA current through the opto LED is not good design practice. Choose an opto which needs only 5mA (for instance 6n137).

Good luck,
Kai