-5V to 5V range with Arduino PWM, an op amp and a symmetric power supply

[rayou23]

As you may know the current measuring here was done by measuring AI0, then AI1, and thus doing AI0-AI1 to get the voltage at the resistance and retrieving the current with Ohm's law. It was before implementing this whole circuit to draw the current-voltage characteristic in the negatives too.
But here, my voltages are going to be negative. So I cannot do an analogRead(AI0) or analogRead(AI1) because it's going to damage my analog inputs right? I thought Arduino pins only accepted positive 0-3.3V voltages. Am I mistaken ?

From aarduino due doc : I/O Voltage 3.3V

Due | Arduino Documentation

[rayou23]

What are you referring to ?

[jim-p]

Duplicate the part I circled in red and connect it to AI1.
So now you have a positive voltage 0 to 3.3V for both AI0 and AI1

[rayou23]

Okay yeah I thought about this but hoped there would be other possibilities.
Thanks! I'll keep you updated with the final schemes and photos of the prototype when everything is finished and working!

[runaway_pancake]

"Duplicate the part I circled in red and connect it to AI1."

AI1 is at GND. I'm pretty sure that anything else connected to it will be at GND, too.

[rayou23]

Yeah you're right I now disconnected the ground there and changed R4 from 10k 270 ohms to get more precision on my current

[jim-p]

[rayou23]

You turned R4 back to a 10K res. Do you think that's better ? I thought a smaller resistance like 270 would bring a smaller voltage drop in this branch, meaning I'd enjoy more of my range in my SD pin. Because for the moment, my Up in my SD pin goes from -0.6V to 2.2V

[jim-p]

I just left it there from the last schemaic.
You will have to experiment to see what gives you the best range but 270 seem to be a good choice.

[rayou23]

Good news is, I have things that are really close to the actual result I'm supposed to have. But certain plots have very strange faces. For example, manually, I plotted the Chip Select (DAT3) pin and got this :

Then I did it with the arduino and here's the result :

I don't know why there are such discontinuities. Here's the Python script for those interested:

import serial
import matplotlib.pyplot as plt
import numpy as np

# Serial port connection part
port = "COM3"
baudrate = 9600 
timeout = 10

ser = serial.Serial(port, baudrate, timeout=timeout)

A0=[]
A1=[]
i=[]
u=[]
val_A1=0
val_A0=0
Ur=[]
n=0
m=0
l=0

while True:
    line = ser.readline().decode('utf-8').strip()  #read the line from the serial port
    pos=0
    if line == "": #if there no more lines, break
        break
    for i in range (len(line)):
        if line[pos] == " ": # the space character is used to seperate AI1 and AI0
            val_A1=line[0:pos]
            val_A0=line[pos+1:]
        else:
            pos+=1
    
    A0.append(float(val_A0))
    A1.append(float(val_A1))
    n+=1
    print(line)

ser.close()

#re-scaling of the range
for m in range (len(A0)):
    Ur.append(-0.0064453125*(A0[m]-A1[m]))
    i.append(Ur[m]*1000/270) #current values are in this list

for l in range (len(A1)):
    u.append(A1[l]*(-0.0064453125)+3.3) #voltage values are in this list

#plot part
Y=i
X=u

plt.plot(X,Y)
plt.grid()
plt.title("Current-voltage characteristic")
plt.xlabel("Voltage (V)")
plt.ylabel("Intensity (mA)")
plt.show()

[rayou23]

Also, I would like to make everything automatic, even the selection of the different pins. I thought I could use a multiplexer that is connected to all the different entrys and redirects the entry of only 1 pin, waits for the current-voltage characteristic to be drawn, then goes onto the next one... I'm going to see what different choices I have with these components and how they work. Never used one I don't know if that's even possible or if it's just going to work digitally