An oximeter is a sensor commonly used in medical applications where one needs to retrieve the oxygen saturation of a patient. The reference MAX86141 from Maxim Integrated is one of them :


The principle is as follows : the sensor drives an LED at a given frequency, and reads the optical output of a photodiode placed close to the LED. When the patient puts his finger on the LED/photodiode, it is possible to compute the oxygen saturation based on the variations of the reflected signal on the skin.

On the datasheet, Maxim gives strict recommandations regarding grounding these two signals in order to minimize noise.

Now let's take a look at this oximeter :


As you can see, the LED/photodiode system is placed close to the finger. The oxygen saturation is computed and displayed on the device.

I was wondering what signals could possibly transmit through the cable, given that EMC must be met in medical appliactions :

  • It cannot be the analog signals, otherwise it would be too noisy and the device would not meet the standards of EMC.
  • They cannot use buses like I2C or SPI which are typically proposed by sensors, because they cannot be integrated outside of the PCB.
  • There is maybe a microcontroller which transmits the data using a differential protocol like CAN, RS232, RS422, RS485 for instance. However I have opened a remote oximeter myself (which is equivalent to the one on the URL above), and there are only 3 wires in the cable ! It is not sufficient to transmit supply AND a differential protocol.

So... Do you have an idea ? Am I missing a point here ?


  • \$\begingroup\$ How do other medical devices transmit signals? \$\endgroup\$ Commented Nov 13, 2020 at 14:20
  • \$\begingroup\$ RS232 is not differential. Also, you assume these cheap devices go through EMC compliance testing, which might not be the case \$\endgroup\$
    – Wesley Lee
    Commented Nov 13, 2020 at 14:23
  • \$\begingroup\$ @WesleyLee Pretty sure it is mandatory to use a 3rd party notified body for testing medical devices, at least in EU. Other countries might use Ali Express certification ("I received it in the mail so it must be legal"). \$\endgroup\$
    – Lundin
    Commented Nov 13, 2020 at 14:32
  • \$\begingroup\$ @Lundin - I mention that because OP posted a device which has a connector with 5 connections and mentions disassembling an "equivalent" one with 3 wires, so it is impossible to tell whether the device with 3 wires actually is tested or properly designed or not - or is even sold in the EU/UK/US etc. \$\endgroup\$
    – Wesley Lee
    Commented Nov 13, 2020 at 14:38
  • \$\begingroup\$ Once you've got a digital signal, there's not much need for great care in transmitting it. It'll be (relatively) low speed in a shielded cable with a fairly high signal level (there's nothing stopping you from using a 5V serial protocol.) \$\endgroup\$
    – JRE
    Commented Nov 13, 2020 at 15:01

1 Answer 1


Assuming the device is EMC compliant and legal (which isn't necessarily the case).

It is possible to design a rugged, 3-signal interface using current loops. That is, some proprietary protocol which uses current levels to modulate the digital protocol, rather than voltage levels. You can even do this with 2 signals if you also supply the device through the current loop signal.

Lots of high integrity systems use/used this before the CAN bus era. You'd preferably use a tiny MCU which translates the MAX IC SPI output to the current loop protocol. I suppose it could even be done without a MCU and just some manner of current driver, if the SPI baudrate is kept slow enough.

  • \$\begingroup\$ That is interesting and could possibly be one explanation ! I will take a deeper look at this method, thanks. \$\endgroup\$ Commented Nov 13, 2020 at 15:18

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