A transimpedance amplifier has been designed for scanning tunneling microscopy (STM). The amplifier features low noise (limited by the Johnson noise of the 1 GΩ feedback resistor at low input current and low frequencies), sufficient bandwidth for most STM applications (50 kHz at 35 pF input capacitance), a large dynamic range (0.1 pA–50 nA without range switching), and a low input voltage offset. The amplifier is also suited for placing its first stage into the cryostat of a low-temperature STM, minimizing the input capacitance and reducing the Johnson noise of the feedback resistor. The amplifier may also find applications for specimen current imaging and electron-beam-induced current measurements in scanning electron microscopy and as a photodiode amplifier with a large dynamic range. This paper also discusses the sources of noise including the often neglected effect of non-balanced input impedance of operational amplifiers and describes how to accurately measure and adjust the frequency response of low-current transimpedance amplifiers.

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We are aware of only one operational amplifier where the non-inverting input is not neighboring a supply pin, the femtoamp-sensitive ADA4530. Its input voltage noise and low speed make it less attractive for an STM preamplifier, except when working at extremely low currents and keeping the input capacitance very low. For some operational amplifiers, such as the OPA627/OPA637, a similar problem arises for the inverting input due to a neighboring offset trim pin; this is not the case for the AD8615 and OPA657.

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The input current noise of the AD8615 is given as 0.05 pA/Hz at 1 kHz in its datasheet. This value is probably a printing mistake. At 1 kHz, our amplifiers using the AD8615 show a noise limited by the Johnson noise of the 1 GΩ feedback resistor, indicating that the input current noise of the AD8615 is below 2 fA/Hz at room temperature. This is also in line with the usual expectation that the current noise is not much higher than the shot noise of the input bias current (0.25 fA/Hz for a typical bias current of 0.2 pA) or the shot noise of the leakage current of the input protection diodes, whichever is larger. Assuming that the input protection diodes to the negative and positive supply are well matched and their leakage currents differ by only 10%, and this difference causes the input bias current, they would leak 10× the input bias current (2 pA), resulting in a shot noise of roughly 1 fA/Hz.

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