Advanced low-current amplifier: improved accuracy for ultralow dc applications

Abstract We present a new instrument for measuring and generating small direct currents (dc’s), the advanced low-current amplifier (ALCA). It features a sophisticated two-stage amplifier design to achieve the highest accuracy at moderate complexity. In the first stage, the current is amplified using a resistor network comprising up to 640 identical chip resistors. This is followed by a current-to-voltage converter with a precision thin-film or metal foil resistor network in the feedback path. Four ALCA variants have been developed, together covering a wide current range of about ten decades from 1 fA to 10 µ A. By employing an external standard resistor, the maximum current can be further increased to about 1 mA. The lower end of the current range is addressed by the low-bias ALCA. A prototype demonstrated gain fluctuations of about 100 µ A A −1 over a 1 year period, combined with a low noise level of 0.55 fA Hz −1/2 and a typical input bias current of 100 aA. This enables the measurement or generation of a 1 fA dc with a relative standard uncertainty of 1% in less than one hour. Moreover, high-value resistors of up to about 1 PΩ can be characterized within a reasonable measurement time.