THVD1406: About connection of THVD1406

Hi Kiyoshi,

Yes - it is possible to use a device like the THVD1406 to communicate over a power line with a DC source, but there are 2 main  significant disadvantages with this implementation shown below:

A reference design for using standard RS-485 parts for Power line Communication can be seen here: https://www.ti.com/lit/ug/tidu993/tidu993.pdf?ts=1661268214137&ref_url=https%253A%252F%252Fwww.ti.com%252Freference-designs%252Findex.html 

It is a different part but it applies to  any standard RS-485 device. 

1. The signal cannot have a DC level for its lower frequency signals due to the series capacitor - essentially the capacitor needs to be low impedance for the entire data signal length; while still blocking the DC capacitor. For lower data-rate applications this means using Manchester Encoding or similar encoding structures -  8b/10b can also be used. 

2. The low speed of the THVD1406 is going to require very large inductors and capacitors. The capacitors need to be low impedance for the data signal to pass through while the inductors need to be "high" impedance to block the data from the power supply. Depending on how many nodes are on the RS-485 bus is going to influence how large those inductors are - but they will be rather large in most cases which can impact board size and cost. 

For the connection diagram I have a few comments:

1. All the RS-485 transceivers must be grounded - but I think it is connected to the power supplies ground on that side - so if that is the case its okay. There just needs to be a stable reference point

2. Is this showing all the nodes in the system? If yes - I'd suggest 1 termination between A and B (120 Ohms) on each transceiver (between the transceiver and the series capacitors). This will help with reflection mitigation.

3. what is the intended data-rate + inductor/capacitor sizing? 

4. I'd also suggest TVS diodes on each node for extra protection in case of transients.

For the THVD8000/8010 the advantages are as follows:

1. OOK modulation scheme removes the requirement for special encoding of a low speed data signal - which will improve system throughput and reduce the design complexity greatly. 

2. OOK modulation will be much higher frequency (125KHz - 5MHz (THVD8000) or 125KHz - 300KHz (THVD8010)) which will allow for lower values for inductors and capacitors saving board space and potentially cost savings as well. To keep duty cycle distortion to within +/-2% the max data-rate should be 1/10th of the modulation frequency. 

3. We have a lot of technical resources for Power line Communication with the THVD80x0 so there is less ambiguity with some of the more nuanced use cases so I'd say the support for these types of applications is generally more robust than typical RS-485 parts due to the fact that this part was specifically designed for these types of applications. 

Please let me know if you have any other questions!

Best,

Parker Dodson