Increasing sample frequency of HX711 amplifier

Sensors Force & Weight
1

Hello.

I am using the SparkFun HX711 load cell amplifier board (SEN-13879) to read force measurements from a load cell. I would like to increase the sample frequency so I have snipped the bridge between pins 14 and 15 (XI and RATE) and set RATE = VDD. This gives me 80 sps, however I would like at least 100 sps. Therefore, I am planning to attach a crystal across pins 13 and 14 (X0 and XI). It says in the data sheet (https://cdn.sparkfun.com/assets/b/f/5/a/e/hx711F_EN.pdf) page 4 “By connecting pin XI to Ground, the on-chip oscillator is activated” and then further down “When using a crystal or an external clock, the internal oscillator is automatically powered down”. Does that mean I can just solder the crystal across pins 13 and 14 and then I will get a sample frequency equal to f_clk/138,240? Or do I need to lift pin 14 from ground to deactivate the on-chip oscillator first?

Also, can you recommend which load capacitance the crystal should have? Does that relate somehow to the 20pF ac coupled capacitor, which is required if using an external clock?

Thank you very much.

Regards,

Jonas

2

Hi Jonas.

XI and XO are the correct locations to connect a crystal. You’d need to disconnect XI from ground to disable the internal oscillator though. We’ve not tried using a crystal so you’re on your own as far as using an external clock but I think you’d need roughly a 14MHz crystal. Load capacitance will depend on the crystal you use and should be in the data sheet, but if it’s not specified, 10pF to 12pF would probably work. (You may need to experiment to see what works best.)

3

Hi Chris.

Thank you very much for the reply. I will try with 16MHz since they seem to be most common and therefore I can choose between different load capacitance. I cannot find anything in the data sheet, so I will try with 10 to 12pF as you suggest.