Thanks Matt, Karl and Chris for your responses.

I am currently using a modified version of the PSD described on page 4 
of Linear Tech's Application Note #3.  I use an amplitude stabilized 
Wien bridge oscillator (5000 Hz) for excitation.  The reference signal 
goes to an LF1011 comparator that drives an LT1034.  The amplified 
signal goes to the - input of  an LT1007;  the LT1043 switches the + 
input between ground and the signal so that the LT1007 acts as a 
synchronous detector.  (This circuit is also described in US patent 
#3940693.)  This is followed by a 2 pole LPF.

This arrangement works fine.  The LPF eliminates the excitation 
artifacts.  But the switching pulses still come through.

Shielding does not help the pulse problem, so I conclude that what I'm 
seeing on the downstream part of my boards is magnetic or EM propagated 
pickup.  Steel enclosures and/or ferrite beads might help, but I haven't 
tried either.

On the argument that eliminating a noise source is better than trying to 
filter or shield it, I wondered if other PSD designs might be 
intrinsically quieter.

I will try slowing the rise-time of the clock signal going to the LT1043 
and I have a Maxim DG419 switch on order to try as well.

I'll report in the event I have any success.


Chuck
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