If you need to hold an analog signal for infinite period then this digital sample and hold circuit is the solution. This circuit uses a Exar XR-2240 programmable timer/counter as main component. The Exar XR-2240 is triggered when strobe input is applied. The Exar XR-2240 start count when the timer is triggered and output of bistable latch is set to high state. Staircase voltage is generate by this circuit at opamp output. This circuit need supply of 4-15V. Here is the schematic diagram of the circuit:
If the level of analog input that will be be sampled is reached by the stair-case voltage, the Exar XR-2240 stop count because comparator changes state and bistable latch is activated. Then Opamp output voltage level corresponds to sampled analog input. Until next strobe signal, the sample is held by this circuit. This circuit has Minimum recycle time of 6ms.