Back Next

Sample and Hold

sah1.gif levshift.gif

There is not much to say to this, a S&H is frequently used to create random voltages, or to sample LFOs resulting in repeating voltage patterns. When constructing this take care to deflux the board. The S&H can handle the "full" +/-10V scale. Operation is very simple, when the Gate of the FET is at -15 the FET is completely off. Thus no current can flow out of the capacitor (exept some leakage). When the Gate is turned towards +15V the FET is turned on and can charge the capacitor to the input voltage. The circuit on the right converts 5V triggers to the +/-15V triggers needed by the S&H

Jörgen Bergfors has pointed out that with a BF245A and the gate resistor R1 is altered to 47k, the circuit can reach -10V input. (Which isn't always the case with a BF245B.)

Because I get this question often: Will it also run on 12V? Yes it does.

* YASH * (Yet another Sample and Hold)

My newer S&Hs are build arround the LF398 which contains all that is needed to make a fine S&H. Additionally there is an oscillator that can be used for free running sampling, without an external trigger signal.

Yet another Sample and Hold

Polystyrene caps are now difficult to source, so a polypropylene (PP), polycarbonate (PC) or polyphenylene sulfide (PPS) cap can be used instead. The timing of the sample pulse is aimed at fast aquisition (i.e. short pulse) at the cost of less precise capture. Mainly to mimic the sound of low sample rates. There is a curve in the datasheet, 3us gives about 1%. If you are interested in more precision I suggest raising the value of the timing circuit (470p + 10k) by a factor of about 3. Either by using 1.5n for the cap, or 33k for the resistor instead of the values given.

Because I get this question often: Will it also run on 12V? Yes it does.