LIGO Document P1400103-v1

A low-noise transimpedance amplifier for the detection of ‘Violin-Mode’ resonances in advanced LIGO suspensions

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This paper describes the design and performance of an extremely low-noise differential transimpedance amplifier, which takes its two inputs from separate photodiodes. The amplifier was planned to serve as the front-end electronics for a highly sensitive shadow-displacement sensing system, aimed at detecting very low-level ‘Violin-Mode’ oscillations in 400 µm diameter by 600 mm long fused-silica suspension fibres. Four such highly-tensioned fibres support the 40 kg test-masses/mirrors of the Advanced LIGO (Laser Interferometer Gravitational wave Observatory) interferometers. This novel design of amplifier incorporates features which prevent ‘noise-gain peaking’ arising from large area photodiode (and cable) capacitances, and which also usefully separate the DC and AC photocurrents coming from the photodiodes. In consequence, the differential amplifier was able to generate straightforwardly two DC outputs, one per photodiode, as well as a single high-gain output for monitoring Violin-Mode oscillations—this output being derived from the difference of the photodiodes’ two, naturally anti-phase, AC photocurrents. Following a displacement calibration, the amplifier’s final Violin-Mode signal output was found to have an AC displacement responsivity at 500 Hz of (9.43 ± 1.20) , and, therefore, a shot-noise limited sensitivity to such AC shadow- (i.e., fibre-) displacements of (69 ± 13) picometres /SQRT(Hz) at this frequency, over a measuring span of ± 0.1 mm.
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