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Sample and Hold, input voltage.

Discussion in 'General Electronics Discussion' started by XPAX, May 10, 2013.

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  1. XPAX

    XPAX

    1
    0
    May 10, 2013
    Hi,
    I am using the Sample and Hold LF198/LF298 in my project.
    But there is something that does not make sense.
    I am unsure what the diodes in the circuit do?
    Are they blocking current or are they maintaining a voltage potential off 1,4 V?

    Secondly, when I use -5V +5V as supplyvoltage i can only sample signals in the range -2 to 2V. Is this because of the "The following specifcations apply for −VS + 3.5V ≤ VIN ≤ +VS" on page 3?

    Should it not be like: maximum input voltage equal to +Vsupply?
    Like written on page 3:
    Absolute maximum ratings:
    Input Voltage : Equal to Supply Voltage

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

    Best,
    XPAX
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In general, those inverse parallel diodes prevent the differential voltage from exceeding a diode drop.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    They prevent the first op-amp's output from slamming against either rail.
    When the sample/hold switch is open, the overall feedback loop (from the first op-amp output, through the second op-amp and back to the inverting input of the first op-amp) is broken. Without those diodes, the first op-amp's output would no longer have any effect on its inverting input and the first op-amp would behave like a comparator - its output would swing hard against the positive or negative rail. When the switch was closed, there would be a delay before the first op-amp recovers, and this could cause a voltage overshoot. The diodes ensure that the first op-amp's output is never very far from the input voltage, so when the sample/hold switch closes, the first op-amp is still operating within its linear region and regulation will resume quickly as the feedback loop is re-established.

    Yes. The outputs of the op-amps inside the device cannot swing all the way to the rails. That note tells you that when the outputs get closer than 3.5V to either of the supply rails, the device's performance degrades and it may become unusable. You've found that it still works to within 3V of the rails; they only guarantee that it works properly to within 3.5V of the rails. If you need a wider range than +/- 2V (or +/- 1.5V guaranteed by the manufacturer), you need to increase the positive and negative rail voltages.

    If it was a perfect op-amp, yes. In the real world, no.

    Have a look next to the text "Absolute Maximum Ratings". It says "See note 1". Note 1 says: "Note 1: "Absolute Maximum Ratings" indicate limits beyond which damage to the device may occur. ..."

    In other words, absolute maximum ratings are ratings that must not be exceeded; if you exceed them, you'll damage the device. Absolute maximum ratings do not specify ranges over which the device will operate correctly. Hence the name "Absolute Maximum".
     
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