In the MAX30101 datasheet, pins 5,6,7 are NC. But in the MAX30101 Eval kit datasheet, these 3 pins are connected to an LED. What are the function of these 3 pins ?
They are there primarily for production test. However, parallel external LEDs can be used with caution. The drive current will split between the internal LED and the external LED. Matching the LEDs will not be viable. In addition, the external LEDs will need to have v-drop compliance for the system. If this device is used for SpO2, there will be performance issues associated with simultaneous emission of two different wavelengths. For SpO2, The ratio of Red/IR is used to estimate SpO2. The Green LED, typically used for heart rate measurements, will probably be a little more tolerant to an external green LED. This emission does not involve a post processing ratio calculation, but rather identification of Pk-Pk timing.