Reducing Rise/Fall Time in the digital using an Op-Amp Comparator

Thread Starter

Saro

Joined May 10, 2019
3
Hi Everyone,

I am new to circuits. I am trying to find out a way to reduce the rise/fall time in a digital line[for digital protocols] caused by cable capacitance using some external circuits. We tried to create a prototype using an Op-AMP comparator [using UA741CP], which acts as a switching circuit and eliminate the rise/fall time in the digital line. We saw a 6us rise/fall time in the output signal but based on the slew rate of the device 0.5V/us, we understand for 3.3V around 6 us rise time will be there [To avoid this rise/fall time we can order Op-AMP with better slew rate]. In addition to that, we saw a delay of around 9us second between the falling edge of the input signal to the falling edge of the output signal. We were not sure about the reason for the delay. Can anyone help us in understanding the reason for this delay or suggest a proper Op-AMP to avoid this delay. I will be also good to explain us if there is any other way to reduce the rise/fall time in the digital line. I have attached our connection and observation images.

Thanks in advance!
 

Attachments

crutschow

Joined Mar 14, 2008
23,814
What is the maximum frequency of the signal, and what is the cable capacitance?

Note that reducing the rise/fall times can generate ringing and overshoot in an unterminated line.
 

MrChips

Joined Oct 2, 2009
19,774
Cable capacitance is going to be your limiting factor. You cannot eliminate it.
What you need is a low impedance cable driver with series impedance that matches the cable impedance. Sub 50ns should be attainable.
 

Thread Starter

Saro

Joined May 10, 2019
3
Hi,
The capacitance of the cable we are using is 100pF. With some additional circuitry after it, the capacitance is around 250 pF. The signal we are sending will be around 70-100 MHz or at least 15 MHz.
 

crutschow

Joined Mar 14, 2008
23,814
Hi,
The capacitance of the cable we are using is 100pF. With some additional circuitry after it, the capacitance is around 250 pF. The signal we are sending will be around 70-100 MHz or at least 15 MHz.
That makes no sense. o_O
You are taking about a 6μs risetime in your first post, which is in the kHz region, not the MHz.
 
Top