After testing so many boards and trying out new settings to specific customers orders and can now say that obe board does not fit all detectors. Boards that were created for the GPX4500 do not work on the GPX5000 due to the different input gain profiles of each detector. I was baffled as to why the input stage gain was only a multiplier of 33 and the GPX4500 has a gain multipier of 47. It is easy to figure out, if you measure the resistors on both detectors you will find that both detectors have an inverting input connected to a 10 ohm resistor while the feedback resistor in the 5000 is set at 330 ohm and the 4500 is set at 470 ohms. When compared side by side in normal mode the 5000 shows significant depth loss on most solid gold targets. The point is that the 4500 is a better deep gold detector in both enhance and normal modes. So why is the 5000 taking a hit in the gain of the first input stage? The answer is the fine gold mode. Fine gold uses an output pulse that is much the same as enhance but what is differerent is when the receiver comes on in relation to the transmitter turn off and ground decay. I would be thinking that having the receiver turn on in fine gold with similar front end gain of a 4500 would cause too much residual ground decay to get into the detector and make it noisy. So the cure was to use less gain but this causes the other modes to lose performance. It is very simple to test this and prove it by using a 4500 and 5000 and trying depth tests by using a test bed with buried objects. Apart from this difference between the detectors one could increase the gain in a 5000 to match the deep gpld performance but fine gold would become noisy. The only other way is to use a variable gain input stage so you can run as much gain as the ground minerals will allow. We invented the low noise variable gain input stage some 5 years ago and have fitted well over 1200 detectors with this sub board. The next phase is nower noise input stages that can make use of the low impedances of the coil and low ohmic resistance of the input back emf blocking and conductance Fets, All the later model detector use the standard low noise AD797 as the input stage but after some observation i discovered that the very low noise of these components could be made even better. The 1Hz to 10Hz noise specification is a lot higher than the higher frequencies that the ic is designed for, it has very low noise of 1nv/hz but not at the very low frequencies that the pulse induction detector looks at, the AD797 jumps up to 7nv/hz at these low frequencies and as this low frequency component drive the integration stages it seems that some sensitivity can be lost in the noise. The mission was to keep the noise floor under this level and keep the low frequency components under 1nv/Hz at the critical 1Hz to 10hz range. Well it has been done with a choice of 2 sub boards 1 of 0.82Nv/Hz and one of 0.52Nv/Hz. The greatest trick was to keep the slew rate very high,dampen any overshoot and keeping common mode noise under control by using a differential 2 stage amplifier and converting back to single ended output. The whole design was built from scratch and it is a serious upgrade to install in any pulse induction detector, be it a SD2000 or later detector.
