0
\$\begingroup\$

I'm working on an electronic weight scale using MSP430 and I need your help:

I've written the code using Energia and I've used the HX711 module for testing and everything works just fine. The final project requires everything being on a single PCB, which means I had to give up using the HX711 module (I have to only use the HX711 integrated circuit.) I've remade the schematic that was on the module (see the attachment below.)

hx711 schematic])[1]

I've done some measurements on the circuit and here's what I've discovered:

  • The Vsup and Dvdd pin are set to 3.3 V as expected.
  • Base has 2.6 V.
  • AVDD, VFB and AGND are all equal around 0.7 V.
  • Also the VBG is 1.8 V.

The schematic is identical to the one attached, but because AVDD and AGND are equal, the voltage that arrives at the load cell is equal to AVDD - AGND which is zero, so I think this might be the issue; I just need a second opinion from you guys (and ways that I can raise that differential voltage.)

\$\endgroup\$
  • \$\begingroup\$ It usually helps to explain what doesn't work. From your description, it is not clear what is wrong. For all I can tell, the chip may be doing nothing at all. Assuming Andy aka has found the problem, then it would appear that communication with the chip works, and that it is delivering measurements but that the measured values are wrong. As a programmer, I deal with error descriptions on an almost daily basis. "It doesn't work" is not all that useful when trying to find and fix a bug. \$\endgroup\$ – JRE Jun 1 '20 at 9:54
4
\$\begingroup\$

I think in your schematic that you haven't tied AGND into proper GND: -

enter image description here

The above is a sample circuit I found on google that clearly shows that AGND connects to a GND symbol and your circuit doesn't appear to. There may be other errors too but this is the most obvious.

Red box and red AGND are my additions to make it clearer to you.

Here's another schematic that confirms this error is quite common: -

enter image description here

Notice the AGND label on the node that goes to the pin named AGND - nodes that are labelled need to be connected.

Picture taken from this forum that also have folk getting confused about the same issue - node names imply connectivity and, it seems, a few folk have just copied the circuit without looking at the data sheet.

And, if you look at the Sunrom schematic directly above where you picture came from you can see all the 0 volt connections inside red boxes (my addition): -

enter image description here

\$\endgroup\$
  • 1
    \$\begingroup\$ Thank you Andy..I've tied the AGND to my global ground and now everything works as it should \$\endgroup\$ – George Bogdan Ion Jun 1 '20 at 14:46
  • \$\begingroup\$ @GeorgeBogdanIon very good. Glad to help. \$\endgroup\$ – Andy aka Jun 1 '20 at 15:45
  • \$\begingroup\$ Hi Andy. At the schematic I have and alsp the one you post it I saw there are 2 low pass filter between the load cell outputs and hx711.. Can you tell me why we need them please? \$\endgroup\$ – George Bogdan Ion Jun 19 '20 at 14:59
  • \$\begingroup\$ As a comment @GeorgeBogdanIon I can only say they are either used to reduce signal noise or to protect against some form of interference. \$\endgroup\$ – Andy aka Jun 19 '20 at 15:31
  • \$\begingroup\$ Hello again, I have one last question...the schematic I've found online has some capacitors...one at the VBG,one at the AVDD and one at the VSUP...the one at the VSUP I think its a decoupling capacitor...but what about the other ones? \$\endgroup\$ – George Bogdan Ion Jun 30 '20 at 16:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.