Why use PWM signals?

Question

Why, in a lot of applications (e.g for bioengineering where very small devices are developed for the recording of biosignals), after an amplifier there is a comparator (with two inputs, one for the analog signal and the other for a triangular waveform) to modulate an analog signal to a PWM signal?

Why should we convert the analog signal to a PWM signal? What is the purpose?

So far I have understood that we want a PWM signal because it is the first step to convert an analog signal to a digital signal through an ADC. Is that true?

Paper: "Using Pulse Width Modulation for Wireless Transmission of Neural Signals in Multichannel Neural Recording Systems", Ming Yin, Student Member, IEEE, and Maysam Ghovanloo, Member, IEEE

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3579626/

Answer 1

It appears they are trying to avoid noise. I found an open (legal) copy of their paper on the NCBI website

However, analog samples are susceptible to noise, and the transitions from one channel to another in short sampling periods can result in significant crosstalk among adjacent channels on the receiver side

This is for implantable devices with the signals going directly to RF. Adding an ADC as suggested in the comments would be counter productive since then a microcontroller would have to read said ADC and then modulate on the signal.

doing a direct AM or FM would likely be difficult since the values are so small and they are multiplexing 15 channels. by going to a PWM they get a digital representation that they can send using Phase Shift Keying (PSK) which is 'digital-like' in that the value are very distinct 1/0 (0 deg/ 180 deg) over the transmission medium. Since they need low power the more distinct the signal out the better.

In short they get the value of a digital representation without the added cost of digital components

Answer 2

Another reason to convert analog signal to digital PWM is that it can easily passed though an opto-coupler when isolation is needed between power domains.

You can pass an analog level through an opto-coupler but the response curve is NON linear and temperature dependent.

Answer 3

when you send a signal wireless you have 2 important reasons to use pwm. 1: it increases efficiency by a lot, a transistor or mosfet driven by a analog signal wil be partly on or of a lot of the time, this will make it generate a lot of heat, a PWM signal will set it to high or low, this reduces resistance and increases efficiency and signal strength. 2: it will send over exact data, a radio signal is just a very fast wave, sending analog data would modulate the amplitude, however this will not result in exact data since the receiver might be in different locations and so get different strengths, in things like radio this works due to some trickery and audio not needing that much accuracy, however here you want to send data fast, so a PWM signal is essentially just like serial communication, it sends a frequency modulated signal with different highs and lows, this makes auto tuning the receiver more easy and gives exact data transfer.