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79669Re: OPI-ONE cables again...

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  • rtstofer
    Dec 14, 2012
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      --- In Electronics_101@yahoogroups.com, Howard Hansen <hrhan@...> wrote:

      > Hello Richard,
      >
      > Don't give up on the I2C approach. From recent comments from Chuck it
      > looks like the existing system is switching the current source going to
      > the diodes and only one diode is energized at a time. If this is true
      > then one could connect the I2C temperature reader to the input of the 8
      > channel multiplexer. Hence only one I2C device is required. What is
      > the on resistance of a typical analog multiplexer these days?
      >
      >
      > The other Howard
      >


      Absolutely! That would be my first approach. I don't want to do any more analog stuff than I have to.

      One of the single chip solutions I reviewed could compensate for 1500 ohms of series resistance going to the diode. The little bit of resistance in the analog switch will be insignificant.

      I think I would spend a day or so reviewing the single chip options. It will be time well spent. There are some very interesting devices but, unfortunately, some sell in quantities of a gazillion (minimum). It comes down to selecting a device that will work and is readily available in qty 1.

      Keywords seem to be 'diode remote temperature sensing IC'

      A device like the TI TMP400 can compensate for up to 3k of series resistance:

      http://www.ti.com/lit/ds/symlink/tmp400.pdf

      See Figure 5

      As it turns out, this device is not a stocked item at Digikey but they have 250+ available in qty 1 at $2.87

      I worry about CDiff as it has a huge effect on the accuracy. There is a discussion where it is recommended to keep the capacitance below 1000 pf Figure 6 and Page 7. I guess I have no idea how much capacitance the cable presents.

      Richard
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