Hello again,

Thanks for all the responses. I gather from the responses that it would be
best to keep the input impedance as high as possible, so that's what I will
try to do. I did forget to mention that the input buffer op amp that I am
using has a significant input bias current, several uA, so as far as I can
see, this will severly limit how high I can make my input impedance.
It will probably not be able to be high than a few K, in parallel with the
abt. 20pF or so of cap. due to the op-amp and the board, etc. However, if
the input atten. network is set to a higher atten, this impedance will go
up, of course. If anyone can see any way around the problem of input bias
current, please let me know.

As for input protection, I had looked at various diodes and TVS devices,
and they all had a rather high capacitance, so I rejected using them. The
op-amp that handles the input is a bi-polar op-amp, so I hope it won't be
too sensitive to esd. Now that I am beginning to realize that my input
impedance must be lower than I expected, I guess that the few added pF of
diode cap. won't make that much difference so I might go back to using,
say, two back-to-back 6.2V 500mW zeners from op-amp input to ground. These
will come after the input atten. network so I will actually be able to give
the scope inputs higher than 6.8 V.

I had also planned on attempting to make my own probe. Maybe this is not
cost effective? Last time I checked, probes were rather expensive, >$30. I
was just going to take a standard meter probe tip, place the appropriate
value resistor with a trim cap. across it, in the tip in series with the
center conductor of some coax, attach an aligator clip to the sheild, and
then solder a BNC onto the other end. Right idea, or wrong idea??

I sure do appreciate everyone's advice, you are saving me a lot of time,
money, and hassle. I'll let you know how it works/publish schematics on the
web when I am finished.

Thanks,

Sean




+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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