SN74HC14: RC circuit design help for buffer/inverter

ArTzy said:

How should I design RC circuit?

What is the purpose of your RC circuit? I can't really help you design it if I don't know what it's supposed to be doing.

ArTzy said:

Please find below the RC circuit arrangement connected to the input of SN74HC14. Some values are shown on the circuits. Could we have some equations, notes, etc.  to verify those values?

The RC circuit shown has two basic components together -- first is a voltage divider (R88 and R90), which produces a maximum voltage of Vin / 3 across R90. I'm afraid I don't know Vin since it is not shown in your schematic (I'm labelling Vin as the node voltage on the far left wire -- assuming this is the input).

C52 is being charged by R88, so the voltage across R90 will charge with a time constant of R88 * C52 = 1ms. The full charging equation for the capacitor C52 is thus:

V_C52 = Vin/3 * (1 - exp(-t/1ms))

Generally you won't need this level of accuracy though. You can get a reasonable estimate for the delay of an RC + Schmitt-trigger buffer with 2.2*R*C, which is the time it takes for an RC circuit to charge from 10% to 90%. Although this is not precise, it does put you "in the ballpark" and should work for most applications using this type of delay.

Long story short -- the estimated rising edge delay of this circuit is 2.2 ms, which is 2.2 * R88 * C52

Because of R90, the falling edge delay will be different. Basically R88 and R90 are in parallel if the input is 0V, so you get a discharge rate related to R88 * R90 / (R88 + R90) rather than just R88.

I built a quick simulation to show this:

Here you can see that the direct RC (red) charges and discharges in about the same time (5ms), while the RC with a voltage divider (green) charges to maximum in about 5ms, but discharges faster -- in about 3.3ms.

R89 does also produce some filtering effect together with the input capacitance of the SN74HC14. Assuming a typical input capacitance of 3pF, the RC of 100k + 3pF is 300ns, which is really negligible compared to the other stage. It may provide a small amount of reduction for high frequency noise or sharp voltage spikes, but otherwise won't do much.