I'm using a device which generates different output pulses based on different input but I dont know how I can convert all these output pulses to digital signal so that my another device (which takes in digital signal) can understand them.
If your digital input device is just one wire, you can use pulse width methods. Convert the analog parameter to a pulse width by some proportional factor that the digital receiver can use to calculate the analog parameter from measuring the pulse width.
What do you mean by different output signals?
OP AMP can be used for low frequecny data(?) but as the frequcy goes higher other signal conditioning may be needed.
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