HX711 with higher load cell voltage

Hi !

I'm working on my first Arduino project which is on a good way.. My last concern is to connect a 150 kg loadcell to a HX711 board.

The board i bought seems to have been designed by a first year student.. long wires, decoupling caps far away from pins, my ground planes, etc...

To increase my chances of success, i want to design my own board and i'm thinking of supplying the loadcell with a 10V supply. I want to use an external 10V voltage regulator and use 2 separate grounds for the digital and analog parts.

Is this possible ? Has anyone done this before ?
What regulator can i use for this ? Low voltage drop, very clean output, very low quiescient current and ON/OFF capability.

Thanks for any help.

It seems that i have to do my own experience... :confused:

If you want to design your own board with a 24 bit ADC, check out this link. See also the datasheet for the Analog AD7780 for good design suggestions. But I'm not sure why you would want to build your own, except for fun, because it seems plenty of folks find the HX711 suitable for their work and you haven't said what is functionally wrong with yours.

Also, I'm no expert on this subject, but if you use 10v for your load cell, the common mode voltage at the input to the ADC will be 5 volts. That exceeds the common mode capabilities of the HX711, according to the datasheet.

Hello DaveEvans,

Thanks for the very valuable links you provided. These Analog devices datasheets and design notes are a mine of interesting informations !

So far, i haven't done any tests with my load cell and HX711 board as i still need to build a frame on which a can fasten the load cell !

But, looking at this board, i doubt that i can get the best of the HX711 itself ! My approach is to understand what i'm going to do in order to be able to correct the problems i'm going to encounter.

Can you clarify what you mean when you mention common mode voltage ? I see that the datasheet gives +1.2V for AGND and -1.3V for AVDD, but i don't know what these figures mean.

From the HX711 datasheet, i also see that the maximum AVDD is 5.5V.

I thought i could put 10V excitation on the load cell only to get the biggest voltage variation possible, but if the excitation voltage has to be connected to AVDD also, my hopes are ruined !

Thanks a million for taking the time to read and answer me.

Patrick,

The HX711 has selectable gain of 32, 64, or 128. Why mess with doubling the supply voltage to it, when that only doubles the output?

It seems to me that using it as intended increases your chances for success. Are you trying to use this for weights considerably lower than 150kg?

Hi Steeve,

The goal is to get the best signal/noise ratio and that can only be done by increasing the input signal at the amplifier and/or reducing the amplifier gain !

Noise (of any kind) reduces the DAC capabilities dramaticaly.

I will use the system to weigh beehives, they can start at around 25 kgs to up to 150 kgs (that would be a beekeeper's dream...)

I see that we're from the same ham community!
AE7HD de TK5EP, 73.

Are you having any problems with noise now? The key is to fix all those things you know to fix, as you mentioned bypass caps too far from the ICs, wires too long, etc.

I'd also put the HX711 right at the load cells.

I'm a sad case, got all three at one sitting, then had to move and have not gotten a station set up yet. Barely on the air at all.

The common mode limits in the datasheet mean that your common mode voltage has to be more than 1.2v above the analog ground and less than 1.3v below the analog supply; if you use a 5v AVCC analog supply (so that it is less than the 5.5v max), then that means the common mode voltage must be between 1.2v and 3.7v.

If you use a 10v supply to your load cell, then your common mode voltage will be 5 v (half of the load cell supply), which is outside the acceptable range and no good. However, as I said in my first post, I'm no expert on the subject.

Have you googled common mode voltage bridge? The first hit I get is quite helpful.

Also, if your load cell is intended to be used with 5 volts and you double the input to it, you will quadruple the self-heating (V squared over R), which could be a problem, or not.

Hi Dave and Steve,

Thanks for all these infos.

After reading all these papers and the HX711 datasheet, i think it is not possible to connect the load cell to a 10V source. The cell is not the problem, as it is rated for a 18 V excitation voltage.

But the HX711 can't handle this.. It is not well explained, but i suppose that the DAC reference is connected at AVDD which maximum admissible voltage is 5.5V.

The only thing a can do it supply this AVDD with the maximum allowed voltage, instead of the 4 V coming from the onboard regulator. Not a big difference... but always 20-25 % !

I will try to design a new board with better grounding, decoupling and onboard low noise regulators. 5.5V for the AVDD and 3.3V for the DVDD.

The board will be close to the cell in a shielded box.. Another thing i learned is that the 2m long cable which is provided with the cell is part of the temperature compensation and must NOT be cut !
I don't know how a heavy gauge wire can do that compensation, though...

Regards,

There is a shielded version of the HX711 available. Why not get one and give it a try before you spend a lot of time designing and building your own?