SPI through a cable in medical device

Hello,

  One suggestion might be to consider using a pair of the LTC4332 SPI extenders. 

   These parts robustly extends a SPI bus with up to 3 remote devices with a robust full-duplex bus in a nearly transparent manner.  The local side LTC4332 behaves as up to 3 target devices, receives the data for the selected target, sends it across the link to the remote side LTC4332 which behaves as the controller talking to the specified remote target device.   Data readback from the remote device does require a 2nd dummy read at the start of each transaction due to the extra buffering due to the link.  Depending on the physical distance between the nodes, SCK speeds up to 2MHz are supported.

   The physical interface link is a robust RS-485 transceiver with a 25V operating common mode voltage range, and well protected link pins with 60V fault tolerance,40kV HBM / 6kV IEC ESD protection and 4kV IEC EFT protection.  

Eric

With that sort of extremely high noise, I'd be inclined to use either differential signals or Optoisolators and light pipes.   The advantage to the light pipes is good isolation from ground or other hardware in a medical environment.    It also lets you use tight RF shielding, since only light has to go through the RF tight enclosure.    I think you would take a hit on data speed, but it would simplify the medical concerns about ground loops, Etc.

                         Jim Pennell

Our device is body-worn, and the cable should be flexible. Also, we are using a tiny battery (68mAh) that must work for 20 hours. LTC4332 drains our battery. SPI is currently running on 300 KHz so we do not have an issue reducing the data speed.