Switching RC delay network in and out of circuit

[JBourke]

Hi all,

I have an existing RC network in a digital circuit to delay a certain signal by ~70ns. The resistor value is 10R and the capacitor's is 680pF.
The delay does its job, although I know there are better ways to delay digital signals but anyway that is not an option right now.

I would like to be able to switch this delay network in and out of circuit with a mechanical switch but I have found that if I add any additional wiring the circuit no longer works.  I understand the wiring will add capacitance and resistance but I didn't expect this to have that much influence; beginner here obviously, I must be violating some fundamental principles :-)

Would changing the combination of R an C help in this?

Your help and expertise is much appreciated,

John

[Ian.M]

No.  The problem is stray capacitance,  and you are already at minimum R, less than the output impedance of most logic families, so cant increase C to swamp the stray capacitance.  Wire inductance may also be causing problems.

There are a couple of possible approaches.  Use your mechanical switch to apply a DC control voltage to control:

• a SPDT analog switch, choosing the delayed or non-delayed signal.
• small signal N-MOSFET, with its drain providing a switched ground to the capacitor of the RC.  Add a 1Meg pullup to logic Vcc to bias the drain so the body diode doesn't conduct on falling edges of the signal when the MOSFET is off.

[Doctorandus_P]

Your whole approach is defective by design. Using RC delays in combination with logic gates is a dubious and amateurish approach. With an IC like a "retriggerable monostable mutivibrator" such as for example the 74xx423 you will get much more reliable results.

I also do nut understand why you want to use a mechanical switch. Long wires add inductance and capacitance and this does not combine well with high speed signals. A better option would be to use your switch to adjust a logical gate that reroutes your signal.

But if you want to proceed with your RC approach.
For a "quick" solution I pretty much agree with Ian.M.
First increase the resistance, so it does not draw more current from the output pin then it can deliver, and adjust the capacitor to get your time constant.

Then, you can insert a small MOSfet between the capacitor and GND.

[JBourke]

Thank you both, I learned something and have useful solution suggestions to boot.