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LF398-N: Sample and hold circuit with the LF398S8 is not working

Fabian Milczewski
Prodigy 70 points
Part Number: LF398-N

Hello everyone,

I am using a LF398S8 Sample and Hold Amplifier and am trying to give it an analog signal in the range of 0-10V that the SaH should hold on logic low and sample on logic high.

I am giving it the required supply voltage of +/- 15V and am changing the logic input that I hooked up at pin 8 every 5 sec. Logic high is 5V.

Pin 7 is grounded and on pin 6 I am using a 10nF capacitor. As an anaolg input I am using 5V for testing. However once I start giving it my logic input. The output always stays at about -14V.

Can someone tell me what on my circuit went wrong or is the SaH that I am using not the correct one for this application?

I am looking forward to any help I can get. 

  • 0
    TI__Guru* 93105 points

    Hello Fabian,

    The LF398-N is sample and hold usually straightforward in application. Your circuit description sounds like a basic application, but if you could provide the circuit schematic showing the supplies, input signal, logic input signal and output load that would provide us more information to work with.

    Do note that the LF398S8 is not produced by TI, but by one of our competitors. You may want to check with their Applications team as there may be something they would advise when applying their device.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 70 points

    Hello Thomas, 

    I am using the same circuit as on the first page of your datasheet http://www.ti.com/lit/ds/symlink/lf198-n.pdf for my SaH.

    For the analog input I am generating sin 0-5V with the periode of 5 seconds through a waveform generator.

    For the Logic input I am generating a rectangle signal with logic low= 0V and logic high= 5V that changes every 2.5 seconds.

    V+ = 15V and V- = -15V through a power supply.

    First I turn on the supply Voltage and I receive noise at my output between +/-0.7V.

    (During that time, pin 3 receives a voltage of -14V.)

    After that I turn the analog signal on and nothing changes.

    However as soon as I turn on the logic signal, the output jumps down to -14V and doesn`t change anymore no matter if the logic signal or analog input change or  get turn off.

  • 0 in reply to Fabian Milczewski
    Prodigy 70 points

    I already tried the same application by turning the analog input signal on first and then the supplying voltage with the same result

  • 0 in reply to Fabian Milczewski
    TI__Guru* 93105 points

    Hello Fabian,

    It would be really helpful if your could provide a schematic image of your LF398-N circuit showing the supply connections, input sources, output load, etc. Even though you have described your setup in detail it is always easier for us to work from a schematic.

    You mentioned you are using an LF398S8 specifically.  Do you have another one you can test in your circuit? Do you have a TI LF398-N you can test in the circuit? Is it possible you might have a faulty LF398 device? 

    Regards, Thomas

    Precision Amplifiers Applications Engineering