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Buffers

Shafeeq Muhammed
Expert 1200 points
Other Parts Discussed in Thread: SN74GTL2014

Hi,

I have to use a buffer to isolate external sensor inputs to the processor. My sensor outputs are open drain outputs which have a low level of 1V. The output of the buffer, we will connect to processor GPIO which is 3.3V CMOS. Could you please suggest buffers which have Vil(low-level input voltage) more than 1V.

Regards,

Shafeeq

  • 0
    TI__Guru 62160 points

    Hi Shafeeq,

    The logic threshold for the 3.3V CMOS node is 0.8V. 1V is above the Vil and hence the buffer may not recognize it as a valid low signal.

    Is there a schematic you could provide and details on the processor and the sensor and its current sinking abilities(IOL)?

    As the input of buffer is high impedance(only input leakage current), a buffer should isolate the loading on the sensor output.

  • 0
    TI__Guru* 97046 points

    Hi Shafeeq,

    It sounds like you need a comparator rather than a buffer.

    We do have one part with a selectable threshold, the SN74GTL2014 has a VREF pin that allows for selecting the threshold of a set of comparators.  Vref must be between 0.5V and Vcc/2 -- so with a supply of 3.3V, then the threshold could be set to 1.5V.

    There are a lot of options out there for a comparator -- I'm only recommending the GTL2014 because my group supports it. You can probably find a better match in TI's comparators.

  • 0 in reply to ShreyasRao
    Expert 1200 points

    Vibration sensor_ST-GLD-5S11_EN.pdfHi,

    I have attached the sensor datasheet for your reference. We are using the IMX 6 Duallite processor from NXP semiconductor

  • 0 in reply to Shafeeq Muhammed
    TI__Guru* 97046 points

    Hi Shafeeq,

    That datasheet is exceptionally hard to decipher. I think this part:

    Output residual Voltage: DC 1 V or less (load current DC50 mA)

    is trying to indicate that the VOL @ 50mA load is 1V max.

    To me, this means that under normal operating conditions, you should see a _much_ lower voltage at the output -- probably ~0.2V.  I would recommend reaching out to the manufacturer to see if they can clear up that spec for you.

  • 0 in reply to Emrys Maier
    Expert 1200 points

    BUFFER.pptxHi,

    I have included more details in the attached PPT. Please help us to select the right Buffer our requirement.

    Regards,

    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    TI__Guru* 97046 points
    Hi Shafeeq,
    As I stated previously, you need to learn more about that sensor. TI doesn't make it, or I could find out for you. I seriously doubt you are sinking 50mA into that device just to drive a CMOS input... I would guess at more like 1mA or less. The VOL is very likely ~0.2V under this condition, but I can't confirm this.

    Otherwise, just use a comparator, as I have already suggested. There are open-drain options that you can use, and you can set the input threshold to any value you'd like.
  • 0 in reply to Emrys Maier
    Expert 1200 points

    Hi,

    If I use the comparator which you have suggested, as attached schematics, Can I connect the output of the comparator to a 3.3V CMOS processor input?

    Regards,

    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    TI__Guru* 97046 points
    Yes - the output of the device will be a 3.3V logic output.