Changelog - Hardware version 0.2
- Fixed the problem of the offset signal amplification before the compensation.
- Increase the value of the resistors to reduce the electric current consume (Dr. do Lago).
- Changed the charge pump inverter capacitors from 1000uFx6.3V to 100uFx25V.
- Removed zener diode and trimpot used to adjusts the voltage reference by a resistor of fixed value.
- Added a ADC channel to reads the reference voltage.
Friday, August 21, before to get sleep, I had an idea to resolve the offset signal compensation problem with gain. On Saturday and on Sunday, I changed the PCB layout and did a clean-up on the firmware code.
On Monday, 24, I changed the electronic circuit of the first version to the new circuit. Dr. do Lago helped me selecting the right value of the resistors to reduce the current consume.
On Wednesday, 26, my friend Mr. Vidal, realized a comparative test between the pHduino and a pHmeter from a commercial brand using a titration of HCl with NaOH in a didactic laboratory. He said me the pHduino worked fine. The results I will show soon.
On Friday, 28, I made a PCB with the new layout, on Saturday, 29, I bought more electronic components and I mounted the circuit at my home toolshop.
Today, Monday, 31, I took some new shots of the circuit mounted.
The basic idea to resolved the main problem is apply an adjustable electric potential, using a trimpot, at the place of the voltage divider ground of the non-inverter amplifier (Stage1). Dr. do Lago deduced for me the equation for the Stage1.
I know that this point does not have a low impedance. I should put a follower voltage. However, the current that flows by the R1 and R2 is about 100 times lesser than the current that flow by the voltage divider (defined by R3, R4, and R5) that applies the voltage offset at R1. This circuit is suitable for this application.