Read signal from Opamp Circuit to Arduino with different GND

I have a project that require power both Opamp circuit and Arduino with a single 9V battery. The opamp circuit create signal and Arduino will read it.

I create a simple voltage divider for my Opamp circuit turn 9V to ±4.5 V rail to rail. And I just take 9V supply directly to Arduino.

Now, the problem, since the Opamp circuit has different “GND” than Arduino, if feed the out put of the Opamp circuit directly to Arduino Analog pin I fear that it will fry the Arduino. Are there any solution for this ?

** Since the nature of my input singal, single power supply opamp configuration did not work

*** Here is my circuit diagram, my input signal is Electromyography (EMG) - Muscle signal

There are two issues, the different reference/ground voltage and the fact 9V is a wider span than 5V.

The latter is fixed by using a voltage divider, and the ground can also be fed to a similar voltage divider
to be measured on a different analog pin.

If the signal is AC and DC offsets don't matter, then the mean level can be found in software.

A 1:1 resistor ratio divides 9V down to 4.5V which is nice and simple. Two 10k resistors will work
nicely.

Also, consider using a non-inverting arrangement – along with an op-amp with an input range that includes the lower rail [“ground” and/or “-V”] – such as the LM321/LM358/LM324, etc.

And, to protect the Arduino from an over-voltage on an input, use a Schottky diode [such as the 1N5817] to 5V [anode to the Arduino input, and Cathode to +5V] – like this:

Figure01.png

R3 is only needed if D2 and D3 protection diodes are used – also optional.

Adjust the Gain so the output range is between 0V and 5V. D1 is there just in case this range is exceeded.

A 100nF ceramic capacitor across the V+ and V- terminals, on that LM321, also not a bad idea.

A better choice for D1 is RB751S40, but it’s an SMD part, so there’s that.

NeedToKhow:
Now, the problem, since the Opamp circuit has different "GND" than Arduino....

Then could probably make the op-amp and arduino have the same grounds?

NeedToKhow:
** Since the nature of my input signal, single power supply opamp configuration did not work

What's the nature of your input signal? Give examples of those input signals and/or voltages.

A circuit diagram of your existing system can be useful here.

Hi All,
What ReverseEMF suggests is a good start.
I would suggest that the opamp + input should be biased
up to half the 9V supply with a pair of resistors (voltage
divider). That way the opamp output would be at about
4.5V. The signal would swing up and down from there.
Then the output could also be divided by 2 with resistors
to bring it into range for the Arduino analog input. (The
battery - can be connected to Arduino GND.) The + input
resistors probably would require bypass caps across them.
Herb

herbschwarz:
I would suggest that the opamp + input should be biased
up to half the 9V supply with a pair of resistors (voltage
divider). That way the opamp output would be at about
4.5V. The signal would swing up and down from there.
Then the output could also be divided by 2 with resistors
to bring it into range for the Arduino analog input. (The
battery - can be connected to Arduino GND.)

Yet, with my version, the output is at 0 volts, when the input is at 0 volts, [plus or minus input offset effects] and the output rises with the input. So why all the circuit histrionics?

herbschwarz:
The + input
resistors probably would require bypass caps across them.

There's only one resistor on the "+ input". And why the heck a "bypass capacitor"? What exactly is being "bypassed"?!?

Hi,
Welcome to the forum.

Have you googled
arduino emg

You do not have 2 different gnds.
The opamp uses +4.5 and -4.5V supply with respect to gnd and the arduino.

Tom.. :slight_smile:

TomGeorge:
You do not have 2 different gnds.
The opamp uses +4.5 and -4.5V supply with respect to gnd and the arduino.

That's only one possible power arrangement. The Arduino's Gnd pin can also be connected to the Minus side of the 9V battery. In which case, the OpAmp supply would be +9V and Gnd -- as in the diagram I supplied in Post #2. But, as I also noted in Post #2, this is only feasible for an OpAmp that can utilize voltages, on it's input, all the way to it's V- rail [i.e. where the Arduino **Gnd** is synonymous with the OpAmp's **V-**].

Hi all, thank for all the help. I have tried out the voltage divider circuit but my signal became too weak after that. So I tried a High Pass filter configuration instead, it works great for me. Here is my circuit afterward. The capacitor I used is 1uF and 1MOhm resistor