sample and hold op-amp

hello everyone,

i'm trying to build a sample and hold circuit. based on my research, i've found a simple circuit using a couple of op-amps. but it doesn't define the type of op-amp to use.

based on my understanding, when a pulse appears at the sample input, the first op-amp will turn on, therefore charging the Vi. I guess this the capacitor will only discharge it only after a period.

so, any suggestions of the type of op-amp to use? and what could possibly be the function of the S2 switch in the figure attached?

regards.

The type of op-amps depends on the speed of the sampling and the frequency of the input signal. In general, you would use a FET input op-amp if you wanted really long "hold" times (in the seconds).
You only need S2 in this circuit if SI is not a bidirectional switch. If SI doesn't conduct in both directions, there would be no way for C1 to discharge and you would have a "peak hold" circuit.

just to make sure, is this applicable if i want to let and LED light on for a certain period of time. which is caused by the hold mode?

beplusso said:

just to make sure, is this applicable if i want to let and LED light on for a certain period of time. which is caused by the hold mode?

Click to expand...

hi,
Its the type of circuit you need if you are sampling a 'analog' signal, if you require to keep a LED ON, for certain time,
using TTL/CMOS logic levels, you require a monostable.

A sample/hold OPA has to have a high input impedance and a very low input 'leakage'. I have used a CA3240 OPA as a sample/hold.

Its also important the Cap holding the charge has a very low leakage current, polycarbonate caps are OK

The S2 switch is to discharge the cap ready for the next sampling period, it can also be a FET or bipolar transistor.

The S2 FET/BPT would be pulsed ON, [to discharge the cap] after the 'held' voltage has been digitised
or for whatever purpose you are using it.