Art of Electronics ground noise treatment puzzle
A lock-in amplifier accept extremely small input signals, often below thermal noise levels. Groundloop currents can be a dominant noise source.
A low-noise JFET preamplifier and its BNC front panel input jack are isolated from ground with a small resistor of about 10 ohms. Ground-loop currents on input coax cables must flow through this 10-ohm resistor. The resistance is far larger than coax shield (braid) resistance. Ground currents develop a voltage across the 10 ohm resistor, while those same ground currents develop a much smaller voltage along the coax braid. The preamp rejects the voltage across the 10-ohm resistor, but must cope with the much smaller voltage drop along coax braid.



simulate this circuit &ndash; Schematic created using CircuitLab
 A differential amplifier rather than a single-ended JFET preamp could also be used, where the voltage difference between hot input and BNC ungrounded shell is amplified. But a differential amplifier is likely noisier than a discrete single JFET.