I have a O2-A2 sensor oxygen that the out is in current and I want to use it with ADS1115 connected to esp32 with 4.2 v supply.
The output of sensor in 20.9% o2 is between 80 to 120 micro ampere.
Recommended load resistor is 47 to 100.
How to calculate and design circuit for?
Thanks in advanced.
You can use Ohm's Law (V = IR) to calculate the voltage across a resistor, given the current flow through it.
Right, and for voltage reading I should read the voltage of resistor?
Yes, you measure the voltage across the load resistor. You may have problems since the output will be in the 10 mV range (100uA*100 ohms). The lowest range on the ADS1115 is 256 mV so the reading will be in the low part of the scale.
This is a poor choice for that sensor. The sensor manufacturer will have a better recommendation for an interface, likely an instrumentation amplifier in the first stage.
What if I use a circuit like this? is it safe for sensor?
Yes, but not at all for the ADC.
If you insist on this poor choice, put the resistor in the sensor ground leg, and measure the voltage with one single ended ADC input.
Thanks. Could it damage ADC in long usage?
Can you please suggest an instrumentation amplifier circuit or IC?
Hi @itssadegh
Might I ask where in the O2-A2 sensor datasheet it says that it requires a 4.2V supply?
Aren't you meant to just measure the voltage across the sensor's electrodes and load resistor?
The input voltage can never be allowed to exceed the ADC power supply voltage.
For measuring tiny voltages, avoid large offset voltages. Forget that idea.
Hi,
Yes you're right, But the voltage of the load resistor is around 11mv.
If I use a external supply it will be solved but as Jremington said it is not a proper way.
Hi, @itssadegh
Welcome to the forum.
Can you please post a link to data/specs of the sensor?
Thanks.. Tom.. 
Hi,
Thanks 
Yes, this is the datasheet link.
Hi,
Thanks for the data.
That sensor is basically an Oxygen Battery, you cannot add extra potential to get any better response.
What is the application?
Have you noted the shelf life of 6 months and operating life of 3 months, once opened?
Tom...
According to the data sheet, the operating life is > 24 months after initial operation (85% of original output at 20.9% O2).
It is interesting that the sensor is a simple electrochemical cell, oxidizing lead. The manufacturer recommends using a 22 or 47 Ohm load resistor, which reduces the output voltage considerably.
So I should use an op-amp.
It's for analyzing gases of breath exhales. Yeah and I have about one month left. After that I should consider the Output drift.
We used a similar sensor for boiler stack emission monitoring. We had a system that would check the calibration daily. If the drift exceeded th EPA specifications we would have to run a calibration. This was just a matter of exposing the sensor to pure air for the full range value and pure nitrogen for the low range value. You will most likely have to do something similar with your instrument. Once the drift reaches a certain point you won’t get reliable results anymore. It looks like that may be at the 85% of original output point.
You will need an amplifier of some sort. A low noise aka LNA or an instrumentation amp as previously mentioned. Adafruit may have a product that will suit.
Edit: I looked at Adafruit. I found a 24 bit ADC module. It was made for measuring resistance but maybe they have on for milivolts too.
10mV is not a problem for the HX711.
Leo..
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