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Photographs of the PDA front end (a) and a 7-channel bootstrapped amplifier electronics board (b). The entire PDA front end is made from all nonferromagnetic materials and is placed inside a magnetically shielded science region. A bundle of 2 m long mini-coaxial cables, covered by an electronic shielding sleeve (not shown) connects the PDA to the amplifier, which is located outside of the magnetic shields.
One channel circuit diagram for the PDA. The primary section is a low noise, high gain, two-stage bootstrapped trans-impedance amplifier designed to handle large capacitances from both the large area photodiode and the coaxial cables. The subsequent section executes an integrate-and-reset function built around a sample-and-hold circuit, the LF398. All parts are chosen for linear signal integration and to minimize electronic noise.
Noise spectrum of the direct voltage output from the two-stage amplifier in the bandwidth of interest. Noise floors of the PDA connected to a shielded coaxial cable with various lengths are shown in red (0.2 m), yellow (0.6 m), magenta (1.0 m), cyan (1.5 m), and blue (2.0 m). Green: same as blue except the coaxial cable is unshielded. Inset: continuation of the 1.5 m curve, measured with a low frequency spectrum analyzer (Stanford Research Systems SR760). At low frequencies the noise is dominated by the Johnson noise from the 1 MΩ resistor, which gives a spectrally flat 1.3 μV/√Hz. The background noise of the preamplifier and the spectrum analyzer (not shown) is smaller than 0.2 μV/√Hz. Voltage noise of 1 μV/√Hz corresponds to a light-equivalent spectral noise density of 0.17 pW/√Hz.
(a-c) Frequency dependence of coefficients from a quadratic function v(x) = a + bx + cx 2 fit to the output noise (ν) of the amplifier vs. the shielded coaxial cable length (x). The data, v(x), are obtained for each frequency from smoothing the raw data in Fig. 3.
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