50 OHM OR 1K OHM SHAPING AMPLIFIER INPUTS.  WHAT IS BEST?

This is a subject that seems to be unnecessarily controversial and confusing.  I suggest that charge sensitive preamps should be terminated with the standard 1K Ohm shaping amp input impedance.

People commonly believe that since the interconnecting cables are 50 Ohm the load has to 50 Ohm. Because signal reflections will cause problems if cables are not properly terminated.

This is not necessarily true.  The charge sensitive preamps output is a pulse with a risetime typically greater than 10 ns.  If the cable propagation delay is near or greater than the pulse risetime, then the only choice is to properly terminate the line.  However, in some situations like driving only slow shaping amps for energy measurement, reflections donít usually cause problems.

When charge preamplifiers are used with fast and slow shaping amplifiers for fast timing applications, it is best to eliminate the need for long preamplifier output cables.  If the system is configured as in the diagram below it achieves: 1) Lowest parasitic capacitance in the detector to preamplifier connection minimizing preamplifier noise.  2) The best time resolution due to the greatest signal to noise ratio divided by risetime.  3) Greatest EMI immunity because a compact shielded system eliminates the ground loop antenna between the detector and preamplifier and between the preamplifier and shaping amplifier.  4) A significant decrease in preamplifier power disipation at high charge rates when the preamplifier output level rises.

Consider preamplifiers with 50 Ohm and 1k ohm output loads below the charge rate limit.  Assume a 12V power supply and a dc level of 6V at output node before the 50 Ohm output resistor.

With a 1k Ohm load we can save over .5 Watt per preamplifier in the power budget.  Remember the SR CHARGE8V is only .75" square.  In this case total power dissipation of the CHARGE8V is ~ 300 mW and use in vacuum environments requires no special considerations.

With a 50 Ohm output load total power dissipation for the CHARGE8V is close to 1 Watt and special consideration must be given to provide a sufficient thermal path.  This is true for all high density preamplifiers.