Hi, I need to make a clock source for my sample and hold circuit (9V single supply). I guess the good properties of this type of clock would be that it can have a very short asymmetric on time, and be quite sharp without seeping into the rest of the circuit somehow. Does anyone know which op amps should I be looking at, or is this just to general?

I was thinking 555, but I thought maybe there would be something better that I could get a really short on-time with.

Dan

 

Hello,

Any idea about the wanted duty cycle?
(how many time between pulses vs pulse time).

Bertus

 

The sample pulse width depends on the characteristics of the sample and hold, and on the characteristics of the signal. Post a schematic of the S&H, and tell us what your signal source is (frequency range, amplitude range, source impedance, etc.).

 

This circuit is the basic setup. 2 TL072 buffers with a 4016 switch. Just need the clock now. My first attempt was to use this LM358 I had lying around as an oscillator and messed with components until I had the duty cycle I wanted. The only problem was that when I had a really small duty cycle, I don't think it was strong enough to trigger a sample. So I wanted a chip that produced a really skinny signal, but very strong.

The goal is to have a duty cycle as small as possible. Since my variable resistor is noisy, I don't want to have to hear it when it's sampling. The shorter the sample the better, as long as it's long enough to register.

 

Think you may find that the hold capacitor value is too large. This app note may help - http://www.national.com/an/AN/AN-775.pdf

 

OK I will try a 555 with a smaller cap. But just so I know, how would one choose an oscillator based on it's ability to provide a short duty cycle? By short duty cycle I mean a highly asymmetric square wave. Or is that a quality that any oscillator can have simply by inserting the correct components?

 

The 555 is very noisy is the problem.

 

Actually, the end application should drive the S & H considerations. If this is going to feed an A to D converter, for instance, the cap only need to hold the signal from significantly drooping while the conversion goes on. The cap needs to be as small as possible so as to get an accurate sample of the dynamic signal.

What is the S & H to be used for?

 

The application is for audio. I'm sampling a noisy source to get discrete control voltage steps that sound interesting when applied to a voltage controlled filter. Drooping is not my main concern. The problem I am hearing now is that during the sampling phase, all of the noise passes straight through to the filter so the filter sounds bad. So on the high clock pulse I can hear noise or at least a noticeable break in the signal which is my problem. I need to make the S/H so it samples so fast that it cannot be heard, right now it's not doing that. But I guess I have the most primordial setup possible.

 

You might have some more luck with an IC solution - http://www.analog.com/en/circuits-from-the-lab/CN0058/vc.html

 

I can't see how you are going to create audio by sampling a pot that you manipulate by hand.