From this basic equation, you can see that if you want a larger active area to catch more light, your silicon photodiode junction capacitance is going to skyrocket. This is where your high speed photodiode capacitance issues start. A larger area means more capacitance. In a galvo feedback system, you often need a decent-sized photodiode active area to ensure the light beam from the moving mirror doesn’t fly off the sensor. But as you scale up that active area, you are inadvertently increasing the high speed photodiode capacitance, which creates a massive low-pass filter with your transimpedance amplifier’s feedback resistor.<\/p>\n\n\n\n
Why does this matter? Because the high speed photodiode capacitance combines with the feedback resistor (Rf) of your transimpedance amplifier, forming a pole in your feedback loop’s transfer function. This pole limits the frequency response of your positional sensor. If your high speed photodiode capacitance is too high, the sensor cannot track the rapid transitions of the laser beam, and your galvo servo loop becomes blind to high-frequency vibrations. To keep the loop stable and fast, keeping high speed photodiode capacitance as low as possible is not just a nice-to-have; it’s a hard design requirement.<\/p>\n\n\n\n
Let’s be real: many designers ignore how silicon photodiode junction capacitance scales with active area, thinking they can just compensate for it in their software. That’s a classic mistake. If your analog signal is smeared by high speed photodiode capacitance, no amount of digital filtering can bring back those lost high-frequency details. You cannot filter out physical lag. If your feedback loop doesn’t account for high speed photodiode capacitance, you’re going to get overshoot and jitter.<\/p>\n\n\n\n
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