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I designed my own PCB with an ADS1115 chip on it. I didnt connected the ADDR pin to GND and the ALERT pin to VDD, because I thought, that it wasnt going to be needed. When I wasnt able to detect it with the Raspi, I "hacked" the connection.

The ADDR pin was on the outside, and I connect it with a resistor to the GND side of the capacitor.

The ALERT pin, wich is on the inside, got connected via a very thin wire to a resistor, that is connected to a ferrite bead on the VDD line.

I measured 2.6V between GND and ALERT, VDD -> GND I got 3.3V.

What is wrong there?

P.S.: The resistior have an resistance of 10KΩ

Here is an picture of my schematic:

enter image description here

And here are two pictures of the PCB:

enter image description here

enter image description here

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  • \$\begingroup\$ You don't say which resistor values you used. How can we know if the values are even remotely suitable? Please post the schematics with component values. \$\endgroup\$
    – Justme
    Feb 15, 2022 at 18:42
  • \$\begingroup\$ Sorry. The resistors have an value of 10kΩ. \$\endgroup\$
    – Bugsia
    Feb 15, 2022 at 18:51
  • \$\begingroup\$ Please post a schematic! \$\endgroup\$ Feb 16, 2022 at 0:33

2 Answers 2

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It won't work because R1 and R2 are 10 kilo-ohm resistors in series with SDA and SCL pins.

Usually, if there even are series resistors on I2C bus, they should be in the order of 100 ohms max.

So short the 10k resistors on SDA and SCL and try again.

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  • \$\begingroup\$ So pull-up/down resisitors go between the chip and VCC/GND and not between the raspi and the chip? \$\endgroup\$
    – Bugsia
    Feb 17, 2022 at 14:03
  • \$\begingroup\$ Correct. Parallel. Not series. Raspberry Pi should already have pull-ups on I2C pins, so you should not need pull-ups at all on your board, but if you do put another set of pull-ups on your board, don't put too strong pull-ups \$\endgroup\$
    – Justme
    Feb 17, 2022 at 14:06
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I'm seeing some very interesting things in your component arrangement. A circuit diagram would help us to understand further without needing to look up the IC pinout in order to understand your design. I guess your supply is 3.3V

  • Why is there 10Kohms in series on the SLC/SDA lines? I'm not sure in which part of your system the pull-ups on the I2C bus reside, but that won't matter much at this point. The I2C wont be able to pull the lines to ground with such large series resistance

  • The ALERT pin is open drain output. If it is sitting at 2.6V instead of 0 or 3.3V, either its Hi-z but you are not supplying enough to pull it all to way to 3.3V and the pin is somewhat floating (is that wire really connected? that solder joint looks kinda rough). OR the open drain is pulling to ground but some other part is pulling it up to hard. That last one could be damaging the chip. Re-check these connections and the value of the pull-up

  • I would do away with the ferrite bead on the ground of the chip. It will add impedance to the I2C signal loop as well which you dont want. At 400Khz you might start seeing effects of that.

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