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I am making a device that collects AC and DC voltage data. AC and DC input should be isolated from each other and from the microcontroller. So, I used an I2C isolator, which I've used on many other projects without any problem. But I am facing a problem that I cannot detect one ADS1115 on "DC" section. The one on "AC" section works fine.

In the following picture, the left section is "AC", works fine. The right section is "DC", not working. The jumpers are for address selecting. I could see all 0x48, 0x49, 0x4a, ox4b addresses when changing the jumper position on the left "AC" section.

dual ADS1115 PCB

The following picture is schematics.

enter image description here

I also tried to probe I2C signals on each section. At the working "AC" section, I could see clean I2C waveforms on AC_SDA and AC_SCL lines. At the non-working "DC" section, there's not signal on DC_SDA and DC_SCL lines. They are just pulled-up to 3.3V (DC_3V3).

I tried to measure all the pin voltages on the U2 (ADS1115 of "DC" section), and 3.3V is correctly applied to the VDD pin.

It just looking like the I2C isolator U10 (ADUM1250 of "DC" section) is not working.

At first, I thought I may have buggy ADAU1250 assembled. But after testing another board, the problem behavior was exactly same. Thus, I presume that my design has some issue I cannot find myself.

What could lead such problem?

Picture of the U10

Thanks to @Justme's answer, I could find the problem. Here's the unconnected GND of isolator.

GND connection fault

After soldering a wire to the GND, the "DC" section worked fine.

fixed I2C isolator ground

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1 Answer 1

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There's no proper supply to isolator.

Your isolator GND supply is on HGND while it should be on DC_GND.

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  • \$\begingroup\$ Yes, correct. Thank you a lot! I added a picture of it on the above post. \$\endgroup\$ Commented Jul 12, 2023 at 5:31

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