Description
The schematic is fairly straightforward. The circuit uses a single IC, a MAX4169 quad opamp. The magnetic pickup (L1) is a ferrite core "cotton-reel" style inductor which also functions, with capacitors (C1A-C), as a 5kHz resonator to provide some front end bandpass filtering. The signal is then further amplified, and bandpass filtered, with three opamp stages (IC1D, IC1C, and IC1B). At this point the signal is rectified with diode (D1), and capacitor (C8) then functions as a negative peak detector (storing the peak negative signal value). The bleed resistor (R8) allows the stored peak negative voltage to slowly decay, and resistor (R7) provides current limiting for the opamp (IC1B). The final opamp stage (IC1A) level shifts the signal and multiplies it by two, so that the final output signal can use the full range of the power supply. Resistor (R11) provides current limiting for opamp (IC1A) in the event of a short circuit in the interconnecting cable. The power supply section, which expects a 5V input, first provides reverse voltage protection with diode (D2), and then performs ample filtering so that the circuit can share a common 5V power source along with a digital microcontroller. The power supply section also uses a simple voltage divider (R12 and R13), to provide a signal ground (SGND) at a level halfway between (V+) and (V-). This simple voltage divider works fine since the rest of the circuit makes only high impedance connections to (SGND).
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