Optical isolated I2C Bus - Page 1
Forum section: electronics / projects

author: rej
author_rank: Contributor
timestamp: January 17, 2023, 09:05:31 am
content: Hello,i often used the CPC5902GS as a Optically Isolated I2C Bus Repeater, but according to mouser and digikey it is discontinued. I could not find a replacement. I am afraid that they will also discontinue the THT version CPC5902G. Do you know a replacement?I know you can build this with optocouplers, but the solution with one IC was foolproof. I also want to avoid digital isolators.Greetings rej.

author: gamalot
author_rank: Super Contributor
timestamp: January 17, 2023, 09:16:46 am
content: ADI and TI produce many I2C isolator devices:https://www.ti.com/isolation/isolated-interfaces/i2c/products.htmlhttps://www.analog.com/en/parametricsearch/11033#/

author: rej
author_rank: Contributor
timestamp: January 17, 2023, 09:41:06 am
content: yes they have, but unless i've missed something, they're all digital isolators, which is what i'm actually trying to avoid. In an analog design i try to avoid all clock sources.

author: Gyro
author_rank: Super Contributor
timestamp: January 17, 2023, 09:58:18 am
content: I2C is a digital interface. It already has a clock..... Ok, I think I see what you're getting at.

author: rej
author_rank: Contributor
timestamp: January 17, 2023, 10:17:49 am
content: This digital isolators have some kind of isolation barrier and have to transmit over this. My understanding is, that this ICs have also a clock source. On the I2C bus there is no clock switching, when there is no communication, but I suspect, that the clock source of this digital isolators is always active. In the past I had problems with an analog design because one of this digital isolators made the GND noisy. So I switched to the mentioned optical isolators and the problem was gone. Thats the reason I try to avoid this digital isolators.

author: Gyro
author_rank: Super Contributor
timestamp: January 17, 2023, 10:41:30 am
content: Yes, dielectrically (capacitively) isolated devices do appear to have internal oscillators for coupling the signal. Looking at the datasheets, they will be extremely small (as will be the capacitances) and differential too, so it's pretty unlikely that they would be detectable externally. The datasheet, looking at one of the TI parts at random, also says that these coupling oscillators are only running in the active state, so wouldn't introduce any more noise if the bus is idle.I'm sure all of the isolators are going this way because they are far cheaper to manufacture.

author: rej
author_rank: Contributor
timestamp: January 17, 2023, 11:12:08 am
content: QuoteUnfortunately I have not found this. Can you tell me where it is?My understanding is that the oscillator is always running. The signal is modulated with on-off-keying and the I2C bus is idle high, so there would be transmission all the time.Probably a digital isolator is not a problem in an analog design with good layout and stackup. I was just avoiding it to keep it simple.

author: gamalot
author_rank: Super Contributor
timestamp: January 17, 2023, 12:12:28 pm
content: Quote from: rej on January 17, 2023, 11:12:08 amFor example, ISO164x datasheet page 31:"The ISO164x family of devices has an ON-OFF keying (OOK) modulation scheme"https://www.ti.com/lit/ds/symlink/iso1641.pdf