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How can a chip (HX711) work without a ground pin connected?

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Tonn:
Hi,

I am working with a HX711 board. They are used with load cells and weight sensors. You can find them in many Arduino projects and they are probably sold by the thousands.



This is the schematic. It's copied all over the internet.



By comparing the board, the schematic and the HX711 datasheet I found several issues, marked in the schematic.
I's #4 that has me confused.

The HX711 chip has three power pins (VSUP, DVDD and AVDD), but only one ground pin (AGND).
The AGND pin should be connected to the supply ground but it isn't.

Power is supplied to JC9.
VCC (JC9, pin 1) is connected according to the datasheet.
Ground (GND - JC9, pin 4) goes to RATE (input pin, configures the chip for 10 samples/sec) and X1 (crystal pin, configures the chip for using the internal oscillator). It does not go to AGND.

The datasheet confirms that the power ground should go to AGND. No surprise there.

I checked five different boards from two suppliers (at least two different batches) and indeed AGND is not connected to GND.

Yet, the boards work well.
What am I missing?

Thanks,
Tonn

theworldbuilder:
Are you sure they aren't connected on the PCB layout despite what the schematic says?
Like a connection under the IC or something along those lines?

In Vacuo Veritas:
You mean like how pin 14 in connected to GND via the symbol at legend 1? And the trace on the PCB as well?

I don't follow the question. Granted, that's a piss-poor schematic.

wraper:

--- Quote from: Tonn on June 13, 2018, 02:06:39 pm ---I checked five different boards from two suppliers (at least two different batches) and indeed AGND is not connected to GND.

--- End quote ---
Did you measure actual resistance between AGND and GND?

NiHaoMike:
The ESD diodes can provide a ground path, but I would expect charge injection to adversely affect the accuracy of a precision analog part. (Had that happen with a dsPIC in my senior design project since I only used series resistors to connect it to the 5V Arduino that handled the networking.)

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