关于-Photodiode Circuit Design Wizard

我在使用贵公司的Photodiode Circuit Design Wizard(https://www.analog.com/designtools/en/photodiode/)

出现了如下警告:

选择的运放是一个 非完全补偿(Decompensated)运算放大器 LTC6268-10,只有在噪声增益在10以上才能稳定工作。

在我的设计中,光电二极管的结电容非常小Cd=100fF, 给出的反馈电容Cf=78.9fF,高频时的噪声增益(Cd+Cf)/Cf=2.268。

给出的warning提示电路可能会不稳定。

但是我有个疑问:计算噪声增益是不是应该把运算放大器的输入电容(450fF@LTC6268-10)也计算进去?

  • 以下是ADI工程师Matt Duff的回复:

    Thanks for asking.  You are correct.  This formula really should include the input capacitance of the in amp, both common mode and differential.  The actual formula is really (Cd + Cf + Ccm + Cdiff) / Cf.  You would add in both the common mode capacitance of 450 fF and the differential capacitance of 100 pF.  In our frequency response, pulse response, and noise plots, we simulate with these two capacitances. 

    However in the warning message, we simplified it to (Cd + Cf) / Cf.  We thought it would be easier to understand and it gives some extra margin of error.  In other words, the op amp capacitance adds stability for G>1 stable amplifiers. We had not anticipated a 100 fF photodiode capacitance where the op amp input capacitance so completely dominates the response.

    Can I ask you about your application?  What are you attempting to measure and what photodiode are you using that has such small capacitance?  Based on your question, we should reexamine the code for this warning.  It would be great to get a better understanding of the real world application before we do.

    (感谢提问。您说得对。此公式确实应包括输入放大器的输入电容(共模和差分)。实际公式其实是 (Cd + Cf + Ccm + Cdiff) / Cf。您需要将共模电容 450 fF 和差分电容 100 pF 加进去。在我们的频率响应、脉冲响应和噪声图中,我们使用这两种电容进行仿真。

     

    然而,在警告消息中,我们将其简化为(Cd + Cf) / Cf。我们认为此公式更易于理解,并给出一些额外的误差裕量。换句话说,运算放大器电容增加了G>1稳定放大器的稳定性。我们未预测到100 fF光电二极管电容,其中运算放大器输入电容会完全控制响应。

     

    我能问一下什么应用吗?您想测量什么?您用的哪种光电二极管具有如此小的电容?根据您的问题,我们应重新检查此警告代码。在考虑如何做之前,更好地了解实际应用才有利于做出正确决策。)