Hi all,
I've made a pcb with a Pro mini and four HX712 A/D 24 bits converters. It is intended for connecting 4 weight cells.
I previously used the HX711 with good results, but the HX712 should be better in terms of gain and thermal drift.
For the moment, i use a library for the HX711, as the HX712 has the same logic, at least for a gain of 128.
Unfortunately, i can't get anything stable and usable for these chips. Putting some weight on the cells doesn't change anything. Unbalancing the differential input with a resistor gives me some variation, but that would correspond to a very heavy weight. The cell can't give such a voltage.
It is like if the gain is very low...
I wired the HX712 the simplest way, except that i use a 5V for the analog part and 3.3V for the digital one.
Has anyone used these chips and got results ? Any suggestion ?
Disclaimer: I am not familiar with the chips so no experience yet.
I had a quick look at the libraries and the HX711 looks more mature (more explanations, more commits etc).
Furthermore I noticed the example for the HX712 is a .pde file which points to "old versions" of Arduino, say Pre 1.0 versions. So that code could be incompatible.
some questions:
Does the 712 example compile without warnings?
If not please post
When the 712 example runs, what are the (error) messages?
please post output so I can compare to the code.
I had no problem compiling and running the .pde.
Everything is working as expected, except that i can't get the beast to work with some weight on the scale.
The dialogue between the Arduino and the chip is just fine, as i can power it down and up. As well as change some parameters.
The HX711 library SHOULD work as both chips are using the same protocol, except that the HX712 has 2 more options to read battery input and select more gain.
But the few kgs i put on the scale do not produce any difference in numbers sent by the chip.
The same scale and weight with an HX711 works perfectly , is stable and accurate.
I will make a test tomorrow with a HX712 soldered on a test board and using the same power supply for the analog and digital parts. Will see if it has something to do with that...
It was a hardware problem. The AVDD supply was not the same as the REFP ! It works fine now at gain 128.
But, i still can't switch the chip to gain 256... Seems to be an internal problem.
Indeed was it (partly) a hardware problem. The voltage on AVDD was different from that applied on REFP pin. Bad documentation induced me in a wrong direction.
Once i put both on same voltage, it worked.
But i never could switch the chip to have a gain of 256, what was the first reason for me to use the HX712.
I wrote to Avia company to ask if the batch of chips i bought were genuine or not, and asked why i can't use them with a gain of 256.
Here their answer :
Hi,
This batch is coming from our factory. HX712 can' t be used at a gain of 256.
We are sorry that we don' t modify the datasheet in time. We will revise it.
Thank you.
Best regards,
Mandy
海芯科技（厦门）有限公司
AVIA SEMICONDUCTOR (XIAMEN) CO. , LTD.
ADD: 301E NO. 34 Guanri RD, Xiamen Software Park Ⅱ, Xiamen, China
Tel: 0592-2529530
Fax: 0592-5996329
A shame ! The HX712 is on the market for a few years now !
All in all, stay away from these chips if you're looking for more gain than the HX711. There is no reason to use them compared to the HX711. They do have a separate analog ground and a internal MOSFET switch, and a bit less noise.
But twice as much thermal drifts and no inside voltage regulator.