PCB Review: 420mA transmitter/ Pic dev board - Page 1
Forum section: electronics / projects

author: Tripplecheckuser
author_rank: Newbie
timestamp: January 04, 2023, 06:09:30 pm
content: Hello All,I have been trying to understand 420mA transmitters for work. Simultaneously, I also want to get a better understanding of PIC controllers (also for work). I did some research and found this document by Texas Instruments on 420mA transmitters:https://www.ti.com/lit/ug/tidu299/tidu299.pdf?ts=1671415998013I took much of the design but changed a few things:1.I don't want to use a DAC, so to replace this I used a PWM output by a PIC16F887 microcontroller pin (the pic does other things too aside from providing the output)2.I added a RC low pass filter to make the PWM output more like an analog signal inspired by this document:https://ww1.microchip.com/downloads/en/Appnotes/90003250A.pdf3.The PIC microcontroller is powered separately from the 420mA circuit (because the PIC needs to do other stuff even if a 420mA receiver is not connected)I have successfully checked the base circuit in Falsad but I was not able to simulate the complete circuit. While analyzing said simulation, I noticed that the OP AMP power rail is connected to the 420mA receiver power supply (24vDC) but is grounded with the same ground as the PIC. Would this cause a ground loop? If I were to ground the op amp with the ground of the 420mA receiver power supply and keep it separate from the PIC ground, will the 420mA circuit still work as intended? I tried making a separate ground for the simulation but it didn't work (or I just messed it up).Another question I had was the effect of the RC filter on the 420mA circuit. It was a late consideration so I wanted to make sure it didn't add any unwanted effects.All this is new to me so I am trying to get as much guidance as possible before I order my boards. Any and all suggestions are welcome!

author: Terry Bites
author_rank: Super Contributor
timestamp: January 04, 2023, 06:54:45 pm
content: Current loop transmitters are not intended for rapidly changing signals. You can slug the PWM as much as you need, your RC filter won't create a problem. The current loop transmitter's "ground" should only be thought of as a local reference point. It floats independently of the receiver's mains earth "ground" propper. So connecting them together will cause an error current to flow. This may will also cause the local voltage regulation to fail or die misserably as well. Loop signalling is not intended to be earth referenced. That's its greatest advantage over other systems.Not all grounds are created equal. Beware!In your case the work around is simple. Isolate your PWM source's (mains earth refd or not) ground from your transmitter "ground" with an optocoupler or icoupler.eg TI ISO7710-Q1 or AD ADuM210N. A digital output optocoupler: eg VO3150A or similar.

author: Tripplecheckuser
author_rank: Newbie
timestamp: January 05, 2023, 01:42:35 pm
content: I did some brief research and the FOD8314T(https://www.digikey.ca/en/products/detail/rochester-electronics-llc/FOD8314T/13493358)seems to be good Isolator to use. Can you explain a bit more about slugging the PWM? Is this related to the RC circuit?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 11, 2023, 05:12:55 pm
content: By slugging I mean that using a longer RC time constant (within reason) to smooth out your PWM won't present a problem in a low bandwith signaling system like a 4-20mA loop.BTW, there are ways to smooth PWM without long time constant RCs. seewww.edn.com/cancel-pwm-dac-ripple-with-analog-subtraction/

author: RoGeorge
author_rank: Super Contributor
timestamp: January 11, 2023, 05:32:19 pm
content: Quote from: Tripplecheckuser on January 04, 2023, 06:09:30 pmNot 420mA. 420 is something else.It is called 4-20mA, meaning the current through the wires varies between 4mA (for minimum) and 20mA (for maximum).