1. ## Electronics Help Needed

Bascially I need a 550k clock for an ADC, now it's not really possible to go this high with a 555, I've tried and they keep falling over, about 100k is really there limit.

Now I know it's more than possible to knock up a simple astable multivibrator using a couple of inverters and a cap & to resistors, which will easilly provide the clock frequency I want.

What I don't know is the formula to calculate the capacitor & resistor values, based on the desired frequence?

Is it a case of T = 1/RC or similar?

I'll try and post a scematic of what I'm on about.

Any help would be great, if not it looks like it'll be a case of trial and error.

Rob

2. ## Worked it out

Here's the circuit:

And here's the formula:

F = 1 / ( C * ( R1 + R2 ))

Worked it out via trial and error.

3. ## Re: Worked it out

Originally posted by Rob
Here's the circuit:

And here's the formula:

F = 1 / ( C * ( R1 + R2 ))

Worked it out via trial and error.
If you use a Schmitt trigger input you can do it with one R and one C and f = 1/RC

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