The preamplifier is necessary to avoid electrical noise from long wires to the main unit. (Applies to the systems with wires)
PreAmp2008.jpg

It consists of a simple instrumentation amplifier setup with a gain of 10. The AD623 has been found suitable with a good common mode rejection ratio allowing a good suppression of the interference from the mains.

Topview of the preamplifier
preamp_topview.jpg