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i have purchased a small board to measure pH with a MCU.

I am using a ADS1115 as ADC that have an internal Vref, that should be precise enough for my purpose.

Now, if i connect my Vin to the ADC pin, and read the Vin, i see that it fluctuate quite a bit from time to time.

I saved the V from pH board and the Vin to check if the Vin fluctuation affect the V from pH board since Vin it's used to power this board.

as expected, it was :(

So, what i can do to solve this issue? what's the most stable Vin that i can use? does a simple 7805 powered with 9V do the trick? there are other options to avoid this fluctuation?

Separate the Vin can make any sense? (separating Vin mean to use one to power the arduino & i2c things and the other to power the PH, of course ground will be in common)

Currently my Vin it's a usb power adapter, unbranded, no idea what's inside :)

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closed as unclear what you're asking by Olin Lathrop, PeterJ, Voltage Spike, Dmitry Grigoryev, Dave Tweed Sep 22 '17 at 14:41

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Your question could do with a big tidy-up if you wish to be taken seriously. Punctuate properly. Capitalise your sentences, 'I', part numbers (ESP8266, etc.) , brand-names (Arduino, etc.), initialisations (USB, I2C, etc.) properly. You have used two variations - 'PH' and 'ph' - when the correct way is 'pH' (assuming you are referring to the potential of hydrogen, 'H'). Capitals matter for legibility, accuracy and credibility. \$\endgroup\$ – Transistor Sep 18 '17 at 18:01
  • \$\begingroup\$ You couldn't even bother to get the first letter of either the title or text right! If you don't care, there is no reason we should either. Not reading, just closing, and -1 for the sloppiness. \$\endgroup\$ – Olin Lathrop Sep 19 '17 at 11:15
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Use a reasonable shunt regulator such as the TI TL431 at a minimum to power your ADS1115.

If you really want accurate results then use a voltage reference chip to power the ADC:

  1. Intersil X60003 (my personal favorite high accuracy reference)
  2. Analog Devices ADR4540
  3. Analog Devices AD584 (though this needs a higher Vin than you have)

There are plenty of Ref modules on Ebay and a new recent one that uses the AD584 powered from 5 V.

Any of these solution will give you a stable supply for your A/D and improve your PSRR.

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  • \$\begingroup\$ Thanks for the suggestion, but the problem here it's that i have to power the pH shield and the ADS1115 too. Standing to the datasheet, i can take very few current from this, it's that correct? if i power the pH shield with a standard power supply, the analog read will still fluctuate, even if the ADS1115 doesn't (ADS1115 have an internal voltage reference standing to cdn-shop.adafruit.com/datasheets/ads1115.pdf) \$\endgroup\$ – user3804873 Sep 19 '17 at 10:14
  • \$\begingroup\$ I was not suggesting you use these as the ADS1115 voltage reference....but to power the convertor. You can run the A/D on less than 5 V and still have it work well on the Arduino. \$\endgroup\$ – Jack Creasey Sep 19 '17 at 16:22

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