Hi all,
I am using an optocoupler to isolate I/O pulses (200 KHz 50% DC) from a PIC18F2550 and I am noticing that the I/O voltage is not holding to a steady Vdd even when the current source limit is being withheld (25 mA for this PIC).
I am using a 4N33 optocoupler which has a pretty constant ~1.2V drop across its LED and I placed a series resistor between the 4N33's LED and PIC's I/O port to take some measurements:
\(
\begin{tabular}{|c|c|c|c|c|c|}
\hline
V_{DD} (V) & \bf{R_{series} (ohm)} & \bf{V_{IO} (V)} & \bf{V_{LED} (V)} & \bf{I_{IO} (mA)} & {\bf{I_{ideal} (mA)}} \\ \hline
5.00 & open circuit & 5.00 & - & - & - \\ \hline
5.00 & 340 & 4.48 & 1.16 & 9.7 & 11.1 \\ \hline
5.00 & 200 & 4.16 & 1.20 & 14.8 & 18.9 \\ \hline
5.00 & 100 & 3.48 & 1.22 & 22.6 & 37.8 \\ \hline
5.00 & 10 & 1.54 & 1.22 & 32.0 & 378 \\ \hline
5.00 & 1 & 1.26 & 1.22 & 40.0 & 3780 \\ \hline
\end{tabular}
\)
I understand why having 100 ohms or less are causing the I/O output to sag, but I am unsure why the cases with ideal currents much lower than the limit (25 mA) are drooping. I have tried this moving from high valued resistors to low, and this PIC is fully operational, so I don't suspect it is damaged.
I don't really want to put a buffer op-amp in between as the level isn't too important, but I was curious what people had to say.
Could it possibly be a frequency response issue?
I am using an optocoupler to isolate I/O pulses (200 KHz 50% DC) from a PIC18F2550 and I am noticing that the I/O voltage is not holding to a steady Vdd even when the current source limit is being withheld (25 mA for this PIC).
I am using a 4N33 optocoupler which has a pretty constant ~1.2V drop across its LED and I placed a series resistor between the 4N33's LED and PIC's I/O port to take some measurements:
\(
\begin{tabular}{|c|c|c|c|c|c|}
\hline
V_{DD} (V) & \bf{R_{series} (ohm)} & \bf{V_{IO} (V)} & \bf{V_{LED} (V)} & \bf{I_{IO} (mA)} & {\bf{I_{ideal} (mA)}} \\ \hline
5.00 & open circuit & 5.00 & - & - & - \\ \hline
5.00 & 340 & 4.48 & 1.16 & 9.7 & 11.1 \\ \hline
5.00 & 200 & 4.16 & 1.20 & 14.8 & 18.9 \\ \hline
5.00 & 100 & 3.48 & 1.22 & 22.6 & 37.8 \\ \hline
5.00 & 10 & 1.54 & 1.22 & 32.0 & 378 \\ \hline
5.00 & 1 & 1.26 & 1.22 & 40.0 & 3780 \\ \hline
\end{tabular}
\)
I understand why having 100 ohms or less are causing the I/O output to sag, but I am unsure why the cases with ideal currents much lower than the limit (25 mA) are drooping. I have tried this moving from high valued resistors to low, and this PIC is fully operational, so I don't suspect it is damaged.
I don't really want to put a buffer op-amp in between as the level isn't too important, but I was curious what people had to say.
Could it possibly be a frequency response issue?
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