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

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wraper:

--- Quote from: SiliconWizard on June 14, 2018, 05:06:21 pm ---
--- Quote from: edavid on June 14, 2018, 04:29:02 pm ---AGND is not necessarily a power connection.  It could be a reference voltage that is only connected to some MOSFET gates, so no current is drawn.

--- End quote ---

So there would be absolutely no ground connection on this chip?  :wtf:
As per the datasheet, it IS meant to be connected to ground.

--- End quote ---
There is GND connection to IC, digital GND. The issue is that resistive bridge is connected to AGND but not actual GND, therefore is powered through AGND pin.

SiliconWizard:

--- Quote from: wraper on June 14, 2018, 05:37:47 pm ---
--- Quote from: SiliconWizard on June 14, 2018, 05:06:21 pm ---
--- Quote from: edavid on June 14, 2018, 04:29:02 pm ---AGND is not necessarily a power connection.  It could be a reference voltage that is only connected to some MOSFET gates, so no current is drawn.

--- End quote ---

So there would be absolutely no ground connection on this chip?  :wtf:
As per the datasheet, it IS meant to be connected to ground.

--- End quote ---
There is GND connection to IC, digital GND. The issue is that resistive bridge is connected to AGND but not actual GND, therefore is powered through AGND pin.

--- End quote ---

What is the digital GND pin on this IC?

wraper:

--- Quote from: SiliconWizard on June 14, 2018, 05:45:11 pm ---What is the digital GND pin on this IC?

--- End quote ---
Oops, yep there is no digital GND pin, it's powered through protection diodes on X1 and Rate pins.

Tonn:
The board works well with both E- to ground or not. It seems the HX711 is not much affected by the diode voltage drop.
Accuracy or noise may be affected, but I don't have the means to verify that.

It seems I can call this one solved. Very educational.
@NiHaoMike: nice catch; you made me look for a diode, not just connection or resistance.

Everyone thank you very much for your help.
Tonn



Dave_PT:
Sorry to come dig up this topic.

But I have some problems with these chinese boards from a project point of view  :o.

Clearly they have design mistakes (GND, etc...). But can someone explain me the feedback resistances?
Why they use R1=20k and R2=8.2k?
To get a AVDD of 1.77V?  :o Why?


[EDIT]
Vbg=AVDD*R2/(R1+R2)
AVDD=Vbg*(R1+R2)/R2 -> the datasheet equation is wrong.



Another thing, has anyone used the oscilloscope to measure the AVDD? Can anyone explain me why the AVDD comes up as a "square wave" of about 50Hz?

Actually I can use the IC to take measurements ... but I can not figure out how the analog supply regulator works.


Thank you all

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