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LF398 input signal is disturbed when supply voltage is low

Stone Zeng
Prodigy 700 points


Hi,

my customer uses LF398 for sample and hold. The input signal is 50k sine wavefrom(+/- 5V peak to peak), the supply voltage is +/-12 at normal.  when positive supply voltage is 5V below, the input signal is disturbed (look there is a clamp diode from input to V+/V-), see attached picture, could you help explain why this happen and share the detail?

  • 0
    TI__Guru 61286 points
    Hello Stone,

    The distortion is caused by hitting the input common mode voltage limits. The LF398 is NOT a rail-to-rail device and is not really designed for low voltage operation.

    The input needs a certain amount of headroom to operate, 3.5V from each rail. When you lower the upper supply voltage, you bring that limit down closer to the signal range.

    This is specified in the conditions at the top of the tables.

    "−VS + 3.5V <= VIN <= +VS − 3.5V"

    Or in other words, the legal input range is 3.5V above the V- rail to 3.5V below V+.

    So for a ±15V supply, the legal input range is ±11.5V.

    So for a ±5V input, you need a minimum supply of ±8.5V.

    So the LF398 is operating as expected...you cannot operate it with a +5V supply for the intended input signal.

    Regards,