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SN65LVDT41: Pull-down resistors on inputs - why?

Supira Medical
Intellectual 300 points
Part Number: SN65LVDT41
Other Parts Discussed in Thread: SN65LVDS31

Tool/software:

HI,

The layout section of the datasheet (as well as the schematics for the SN65LVDS31_33 EVM) show 49.9 ohm pull-down resistors on the single-ended inputs. However, the datasheet itself does not seem to make mention of these.

Are these required? The value seems very low and may load the SPI bus significantly.

Thanks.

  • 0
    TI__Guru 53575 points

    Hi Supira,

    The EVM is designed to accept a 50-ohm impedance output from a waveform generator. This would allow the signal source to be set to the desired input voltage to the pin to evaluate the device in a lab environment. In an actual application, the signal driver for this will likely not be a 50-ohm impedance source, so this resistor would not be needed. Instead you may choose to include high-ohmic biasing resistors to ensure that the signal is in a known state even when the signal driver is disabled. However, because each input also has a weak internal pull-down, this is not a requirement for device to work properly. 

    Let me know if you have any more questions. 

    Regards, 
    Eric Schott

  • 0 in reply to Eric Schott1
    Intellectual 300 points

    Thanks, Eric. I'll leave them off in my application.

    Much appreciated!