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 ?