Hello,
i'm a student and i need a bit help to understand a circuit (attachment). Attached is the schematic of the DO2 meter from DFRobot (https://wiki.dfrobot.com/Gravity__Analog_Dissolved_Oxygen_Sensor_SKU_SEN0237#More).

The circuit gets attached to galvanic DO probe, which should output a current signal. So the op amp should work as IV converter, but it doesn't realy fit any examples i find. The signal from the probe goes to non inverting input, but the inverting input is in a feedback loop, so the signal from the probe never goes through any reference resistor. this circuit can't be an I/V converter right?
Is it just a non inverting amplifer?
Also the resitors R5 and R6 confuse me, how do they effect the input?

So the questions are:
- What kind of configuration is this and how would i calculate Vout?
- does the current of the input plays any role?
- what do R5 & R5?


Thank you very much in advance!
BR hans

 

Like a headphone amplifier the signal going in is small and the signal coming out has a gain. 10 times 100 times, some multiple.
Ask yourself How can I simply measure the gain of an op amp amplifier? How can I set the gain? could I replace the sensor with 10.23 mV?
could I mess with R5 and R6 to make it 10.00 mV? more important is what does DFRobot specify. should I work backwards?
When a fresh fish tank is aeriated for 20 minets or something it becomes saturated, the gain is then adjusted so the meter (uC) reads 100%
As time goes by the disolved oxygen drops until the fish have trouble. That is a simplified introduction.
When the circuit is built using the specified parts the probe should work. The assembly is necessary

There are LTspice LM741 audio amplifier circuits but also learning by checking your work having a simulation as an approximate reference.
Amplifier Gain | Amplifiers and Active Devices | Electronics Textbook

 

hi taf,
Welcome to AAC.
As this is Homework, I can only give you hints.

Look at this marked up image and explain to me how you think this OPA circuit can be considered as an IV converter, also post your Gain calculation.

I have chosen a value of 100uA current generated by the DOxy probe.

E

 

Hi Taf,

This is non inverting amplifier circuit with input voltage the same as voltage drop at R6, as shown by Eric if current from sensor is 100uA then then the input appeared at amplifier input is the same as 100 uA x 7K5 = 750 mV, the you can calculate the out output using standard formula Vo=Vin x (1+(R2/R1)).

R5 is functioning to minimizing bias current to op amp input, to avoid output offset that can be affecting accuracy of measurement.

 

Hi @Aan.jpg
Edit your 4th text line equation.
E
Done.

 

Hey everyone, thank you all for time and help. I appreciate this a lot!
I think I understand it better now.