Learn: A Direct Method of Measuring Op-Amp Input Differential Capacitance
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Learn: A Direct Method of Measuring Op-Amp Input Differential Capacitance

Input capacitance can be a key specification for high impedance and high frequency operational amplifier (op amp) applications. Notably, when photodiode junction capacitance is small, the op amp input capacitance can dominate noise and bandwidth issues. The op amp input capacitance and the feedback resistor create a pole in the amplifier’s response, impacting stability and increasing the noise gain at higher frequencies. As a result, stability and phase margin could degrade and output noise could increase. In fact, some previous CDM (capacitance—difference mode) measurement techniques were based on high impedance inverting circuits and stability analysis, as well as noise analysis.
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