I want to read three analog voltages (over NTC-Resistor temperature sensors) in a solar thermal controller with another MCU to process the values and store them in the Cloud.
I want to keep the two electronic circuits electrically isolated.
In the device from where I want to read I have a 5 V power supply which should have some mAmps spare.
Since the second device already has a Rfm69 radio receiver, I actually have the idea to use an Arduino clone with Rfm69 transmitter to send the values to the second device (Gateway).
But I wonder if there isn’t a better and cheaper solution using special electronic circuits (e.g. isolated SPI or I2C A/D converter). The distance between the devices is small, so that using wires is not a problem. Any advice is appreciated.
Kind regards
RoSchmi
what about RS485 (instead I2C , SPI) …
Thanks @valon_hoti_gmail_com. Would be possible as well but I would at least need a RS485 module on my second MCU (Gateway) and the problem of electrical isolation (no common ground) would only be solved if there were RS485 commerially available compatible A/D converters with included electrical isolation (e.g. opto coupler). Do you know such circuits?
@philippeb and @valon_hoti_gmail_com. Thanks, yes, both would do the job. It’s amazing what they have. For I2C I would additionally need a three channel A/D converter with I2C Client function. (Do you know one) For RS485 I could use a cheap Arduino and e.g. Modbus protocol. Remains the price. With an Arduino Clone with Rfm69 I have to pay about 20 Euro.
I did something similar to read a 4-20mA sensor via an MCP3428 with 100ohm sense resistor connected to the ADC as a differential input measuring the positive rail. The ADC was powered from an isolated DC-DC power supply and the I2C through an ADUM1251 for bus isolation. It worked in the same way a multi-meter would in measuring the sense resistor as there is no common ground reference.
They effectively have a lot of boards there. 4 ADC single-ended or two differential input channels 16 bits with I2C
@Dave_McLaughlin I knew that you would have a solution 
If I had a SitCore Board with microBUS Socket I think that I would prefer @Dave_McLaughlin solution with MCP3428 and @philippeb Mikroe i2c-isolator-click. This is expected to be more reliable than Arduino + Rfm69.
Without mikroBUS I will have more wires and soldering. Since my reliability demands are not high I think that for now I still stay with my first solution. Electrical isolation and e.g. I2C one board/chip would be a game changer perhaps. Or I once more think about if I really need electrical isolation. Thanks to all for your valuable advice.
@RoSchmi : if you go with MCP3428, you could then use the ADC 3 Click module with our driver : MBN-TinyCLR/Drivers/Adc3 Click at master · MBNSoftware/MBN-TinyCLR · GitHub
This may save you some time 
(although the MCP3428 is quite easy to use).
Thanks, I’ll think about it. One question to the i2c-isolator-click. Can it be operated with 3.3 V level on one side and 5 V level on the other side?
The AD devices allow different voltages. I use 3.3V on the CPU side and 5.0V on the MCP3428 side.
Just ordered adc-3-click (MCP3428) and i2c-isolator-click, actually with 20% discount.Looking forward to see how it works.
Just because I didnt see it in this thread, there are also high speed digital optocouplers that are used on communication buses in noisy environments. The I2C isolator click uses a silicon dioxide barrier in the chip, but you can achieve a similar thing with optocouplers.
Thanks, which are the special circiuts names/brands? Advantages? Price?
I assume your ADC3 click will have a separate supply to the i2c isolator and just share the same ground on the isolated side?
Yes, that’s how I wanted to do it. I want to use the 5V power supply and GND of the device from which I want to sniff for the ADC3 click and VCC2 and GND2 of the i2c isolator and want to use 3.3 V power and GND from the sniffing MCU (Feather M0) for i2C isolator VCC1 and GND1. GND1 and GND2 will not be connected,
Alternatively you may try isolated SPI with 3 channels A/D converter.
LTC6820 is a isolated SPI transceiver. With a twisted pair you can stretch the distance between 2 device to 100 meters / 328 ft.
There are many A/D converters support SPI interface.
The only catch is the LTC6820 limited the data rate to 1Mbps.
Thanks. Would probably work as well. With the other solution I’m happy to have the circuits on breakout boards (Need it only one pice)