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LF398-N: What is advantage of using two of these sample-hold circuits in tandem?

Jae Jeon
Prodigy 10 points
Part Number: LF398-N

Hello,

On page 20 of the datasheet for this part shows an example of typical applications, titled "Fast Acquisition, Low Droop Sample and Hold". The datasheet I'm looking at is:

http://www.ti.com/lit/ds/symlink/lf198-n.pdf

In this example, there are two LF398s in tandem. Could you please explain how this achieves "fast acquisition" and "low droop" compared to doing it with a single LF398?

Thank you,

Jae Jeon

  • 0
    TI__Guru 54416 points
    Hello Jae,

    The acquisition sample time is proportional to the size of the capacitor, since there is limited amount of current to charge the cap.

    A smaller cap will charge-up faster. But a smaller cap will "droop" faster due to the leakages. It is also easier to get "good" caps in the lower values.

    In the tandem circuit, the first 398 stage uses a small cap so that it can acquire quickly. But it cannot hold the charge for too long.

    The second 398, using a much larger cap and longer sample time, samples the output of the first 398 and is able to hold the sample longer.

    Though the overall throughput is still slow and dominated by the second stage sample time, the actual sample window time can be made much narrower and more accurate.