Sensitivity and low noise are dictated by where the low noise components and gain is set in the signal gain block. Firstly you get maximum depth and sensitivity the most important thing is to remove resistive loss before the first amplifier, this includes the resistive characteristics of the input mosfets. With a coil resistance of 0.5 ohm the resistance of the input fets at around 5 ohms eats up a fair amount of the signal that is available to be amplified. Other losses are the capacitance of the coil, the cable and capacitance of the input components that reduce the initial slew rate of the detector to be able to recover fast decay target signals. To recover all available signals the initial input stage should have low noise and adjustable gain to suit the ground conditions. The set gain of 33 gpx5000 or 47 gp series and gpx4500 is too much or too little depending on ground decay characteristics. The back gain takes place after multiplexing and as far as i can tell with the existing block gain there is signal loss and trying to boost it greatly increases the overall system noise and target signals can be swamped by the noise. The front end gain sets the input signal at a level that produces optimal signal levels in respect to ground noise the get an improved signal to noise ratio and get the most out of the detector. Many variables need to worked out properly and not just increase the gain, sometimes less is more and sometimes it is the opposite. I also found excess capacitance can cause an offset in the ground balance and thus needs to be greatly reduced as to get better ground balance.
