I see designers all the time slapping a massive reverse bias on their diodes because they think they need “speed.” Spoiler alert: for low-light, low-frequency medical applications, you probably dont. Applying a reverse bias just injects nasty shot noise and dark current into a system that cant afford it. You want to operate in photovoltaic mode. And in photovoltaic mode, Nebenschlusswiderstand<\/strong> dictates almost everything about your noise floor.<\/p>\n\n\n\n Most generic detector manufacturers are selling you garbage when it comes to true zero-bias performance. They quote you a “typical” spec on page four of a datasheet, but when you actually plug it into your transimpedance amplifier, your signal-to-noise ratio goes straight out the window.<\/p>\n\n\n\n Let’s break down what Nebenschlusswiderstand<\/strong> actually is, why it makes or breaks a precision medical circuit, and how you can stop your op-amp from amplifying pure noise.<\/p>\n\n\n\n If you look at the equivalent circuit of a photodiode, it\u2019s not just a perfect little current source. You\u2019ve got a current source in parallel with a junction capacitance (Cj) and a resistor. That parallel resistor is your Nebenschlusswiderstand<\/strong> (usually written as Rsh).<\/p>\n\n\n\n In plain English? It\u2019s the resistance of the zero-biased photodiode junction. If you look at the current-voltage (I-V) curve of a photodiode, the Nebenschlusswiderstand<\/strong> is literally the slope of the curve right at the origin, where V = 0.<\/p>\n\n\n\n In a perfectly ideal, physically impossible universe, a photodiode would have an infinite Nebenschlusswiderstand<\/strong>. No leakage, no problems. In the real world, actual values range from tens of Ohms (for some really terrible or specific infrared materials) to thousands of Mega-ohms (Giga-ohms) for high-quality silicon.<\/p>\n\n\n\n Why do medical equipment designers care so much about this single parameter? Because when you are operating at 0V (photovoltaic mode), you don’t have dark current from an applied bias. The dominant source of noise in your detector is the thermal noise generated by that exact Nebenschlusswiderstand<\/strong> . If your Rsh is low, your noise is high. It is what it is. Thermodynamics doesn’t care about your project deadlines.<\/p>\n\n\n\nWhat is Photodiode Shunt Resistance (Rsh), Really?<\/h2>\n\n\n\n
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