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I am using a quad op-amp (LMC6484) in a non-inverting circuit. Using only one of the 4 op-amps I was able to amplify Vin = 80.3 mV to Vout =1470 mV. Rin=2.66k Rf=10k. V+= 9V… V- = -9V

My problem is that when I use the other 3 op-amps using the same resistances, the output voltages are not independent of each other. When I change the input voltage on one of the signals, it alters the signals on the other 3 output voltages. (I am using 4 photoresistors connected to 160k resistors, which is connected to the same 5V Arduino pin and ground in order to give me the initial low input signals. Also when I say changing the input voltage, I mean covering one of the photoresistors with my finger.)

I am running the output signals through an Arduino Mega and monitoring their values in real time.

When I run the signals from the photoresistors without an op-amp, I get independent signals.

How would I make it so the output signals from the op-amp are independent of one another?

Wiring diagram

(Image source)

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    \$\begingroup\$ Because something in your schematic is wrong. \$\endgroup\$
    – PlasmaHH
    Oct 10, 2018 at 21:26
  • \$\begingroup\$ I'm reasonably certain dual and quad op amps usually have very low coupling between the units, so I think it's likely that you've made a mistake in your design. Could you show schematics? \$\endgroup\$
    – Hearth
    Oct 10, 2018 at 21:31
  • \$\begingroup\$ Schematic has been posted \$\endgroup\$
    – Bennis
    Oct 10, 2018 at 22:30
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    \$\begingroup\$ @Bennis - Hi, (a) FYI those Fritizing drawings are really wiring drawings, and take extra effort (e.g. finding the pin-out of the op-amps) before readers can mentally create a true schematic diagram. For this reason, those particular wiring diagrams are not well-received by some people here. I've heard that Fritzing can produce true schematic diagrams, but since I don't use that software, I can't advise further how to produce them. (b) Also, the resistors on that diagram don't match your text. It seems that they are all 10k on the diagram (assuming that third band is orange, not red). \$\endgroup\$
    – SamGibson
    Oct 10, 2018 at 23:52

2 Answers 2

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The negative battery lead, which I assume is supposed to be ground, only connects to pin 11 of the op amp. This means that the circuit is floating with respect to the Arduino.

Directly connect the battery - to Arduino ground.

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    \$\begingroup\$ OP also needs to join the 'ground' rails on the opposite sides of his breadboard together. All the gain setting resistors are on one side but they're not connected to battery negative on the other. \$\endgroup\$
    – brhans
    Oct 11, 2018 at 3:26
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I can see two possibilities, based on the limited information. The amplifiers may be coupled through the common power supply connections. Make sure that the power supplies are capable of providing the necessary current while maintaining stable voltages.

Another likely culprit is the Arduino. I would guess that the Arduino has a single A/D converter and uses an analog multiplexer to select the desired input channel. You may be seeing crosstalk between the channels that happens inside this multiplexer. I suggest that you observe the amplifier outputs using a good voltmeter to determine if this is the problem.

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  • \$\begingroup\$ I posted my schematic. Not sure if that helps at all. \$\endgroup\$
    – Bennis
    Oct 10, 2018 at 22:31
  • \$\begingroup\$ How would I determine if my power supplies (9V bat for op amp and 5V supply from arduino) are providing the necessary current. I am assuming those power sources would be enough.. Also with it being the arduino, I tested the inputs without an op amp and they were all independent and working fine, which makes me think it has something to do with op amp. \$\endgroup\$
    – Bennis
    Oct 10, 2018 at 22:47

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