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I use a FOD260L optocoupler and here is its datasheet and diagram below enter image description here

pin 2 is connected to an atmega168a (supplied by 3.3V ) via 270 ohm resistor. pin 3 is connected to ground of the MCU. On the right side, Vcc is connected to a PSU that supplies 5V (max 1.5A), Ve is left opened, pin 5 is connected to GND of the PSU. Referring the figure 11 on the datasheet, I connected 330 ohm between Vcc and Vo (as a pull up resistor) then I expect monitor Vo output. I tried to draw the schematic below ( I hope it is correct).

schematic

simulate this circuit – Schematic created using CircuitLab

Here is the oscilloscope output

enter image description here

The blue one is the input signal which is clean, and the red one is the output (Vo) signal which I could not understand why it is not clean. Of course I connect blue's probe ground to MCU's ground and the red one's probe to the PSU's ground. PSU and MCU have different power sources.

I see that the red signal is inverted which I expected to see. But could anyone explain why it is not as clean as the blue one?

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    \$\begingroup\$ 270 Ohms is a bit high for the input current-limiting resistor. The datasheet recommends 6.5 -- 15 mA forward current for the "on" condition. 270 Ohms gives (3.3-1.8)/270 = 5.6 mA in worst-case conditions. This should "probably" work, but if you're having problems, why not design the circuit so it's guaranteed to work by the datasheet? \$\endgroup\$ – The Photon Sep 30 '13 at 16:05
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The data sheet says: -

The VCC supply to each optoisolator must be bypassed by a 0.1μF capacitor or larger. This can be either a ceramic or solid tantalum capacitor with good high frequency characteristic and should be connected as close as possible to the package VCC and GND pins of each device.

Is this the problem? Here's a picture showing the 100nF cap from the data sheet: -

enter image description here

For information the data sheet also says: -

Enable Input – No pull up resistor required as the device has an internal pull up resistor.

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You left one input of the AND gate floating. That can lead to all kinds of flaky behavior, unless the datasheet explicitly says that these inputs float high when left open.

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  • \$\begingroup\$ On this device Ve can float. \$\endgroup\$ – Andy aka Sep 30 '13 at 14:19
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Ve shouldn't be floating. It's an input to an AND gate. That means that if the other input (from the TL081) is low, you get a definite OFF (driving your output high), but if the other input is high you get an indeterminate state depending on what noise happens to be on the pin at the time. Tie it to VCC and you should see better results.

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  • \$\begingroup\$ On this device Ve can float. \$\endgroup\$ – Andy aka Sep 30 '13 at 14:19

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