HX711-Load-Cell-Amplifier
HX711-Load-Cell-Amplifier copied to clipboard
sparkfun
Analog/Digital VDD
I originally posted this to tech support, but they suggested I post here too.
If you guys respin this breakout board could you separate analog and digital supplies. The hx711 has separate digital and analog supplies but your board ties them together so its a pain to use with 3.3v micro and 5v strain gauge. If you bring the digital supply to the header people can externally tie them together if they want 5v digital.
The digital pins have to be the same voltage as the DVDD pin so running on 5v wasn't ideal, level shifting it possible but not ideal, running the whole thing on 3.3v kind of works but with less voltage on the strain gauge its a lot more susceptible to errors.
Ultimately I cut the track to pin 16 (DVDD) to isolate digital supply pin and soldered a wire to it for 3.3v - works like a charm but its messy.
Thank you for the feedback! We'll keep this feedback in mind if/when this product comes up for review.
A jumper for easy switching RATE between GND and DVDD would also be nice for experiments.
@jmaerki - Thanks for the feedback. Do check the schematic and the back of the PCB. There is a jumper that allows you connect RATE to GND or VCC.
Hi robwyatt, I have the same goal, to increase the voltage on the load cell excitation to 5V. The data sheet for the HX711 states that when using internal analog supply regulator, "The output voltage is equal to VAVDD=VBG_(R1+R2)/R1" where VBG is the "Reference bypass output" Using the resistor values from the break out board, I get 1.25_(20+8.2)/20=1.76V. So will changing the supply voltage 3V to 5V effect the excitation voltage, without the need to change the resisters? Or is there something I am missing? Thanks in advanced!
Hi robwyatt, Some interesting reading regarding ADC's and why the AVGND and DVGND are tied. Basically treat ADC's as analog devices, even though internally they are mixed systems. Tying the grounds reduces the noise that gets coupled from the digital side to the analog side from stray/parasitic capacitance. The article condemns connecting DVGND to your projects digital ground plane, as this will cause a noisy ground plane for the ADC! Consequently the interference from this may be greater then the 2X gain in sensitivity that is acquired by switching form a 3V to 5V excitation voltage for the strain bridge staying_well_grounded Analog Devices Grounding Guide.pdf .
Hi nseidle, Could you please clarify how the excitation voltage is calculated? Because from the data sheet of HX711, then there will be a constant 1.76V at the excitation, independent of the input voltage using VAVDD=VBG(R1+T2)/R1 (so long as the supply voltage is between 2.6V and 5.5V). And if I was to modify the breakout board circuit by replacing R2 with a 52K resister, what type of resister would I need? ie foot print on the PCB. Do you know what type of resisters technology is used for R1 and R2 currently? eg Metal oxide, thin film, Bulk metal foil? (Just thinking of the temperature coefficient) Many thanks
Hi all, I came to this post, looking for some infos about the HX711. I noticed some strange things...
On both boards i have (not the Sparkfun but similar), i measured 1.287 V on the VBG pin. The R1 and R2 resistors are respectively 20k and 8.2k
So the given formula gives : 1.287V *(28200)/20000 = 1.81V
BUT on the cell i measured the VAVDD and found 4.10 V on both boards.
I tried reversing R1 and R2, the formula gives : 1.287V *( 28200 ) /8200 = 4.42 V So, something is wrong somewhere !
Who else did measure the VAVDD voltage ?? What are the values ?
Another point, regarding the AVDD and DVDD voltages. The datasheet says that the common mode input is max AVDD-1.3. Can someone tell me (us) what that means ?
Many thanks, Patrick.
Old post but the old data sheet I believe had a wrong formula for calculating the excitation. Correct is AVDD = VBG(R1+R2)/R2
See here https://cdn.sparkfun.com/assets/f/5/5/b/c/SparkFun_HX711_Load_Cell.pdf
