Michel,

   Linear Technology has an excellent application note in their 1993 "Linear
Applications Handbook Volume II". This is AN47; "High Speed Amplifier
Techniques. A designer's Companion for Wideband Circuitry". This covers a
wealth of information including the "ABC's of Probes" contributed by
Tektronix. One application is a 100Mhz servo controlled FET input amp with
a 10M/3pf input and 100pa bias current. The example has a gain of 10. It's
based on the LT1223 and is intended for probe and ATE pin amplifier
applications as well as A/D buffers.

   After running several simulations using a variety of vendor SPICE models,
I've decided on the above FET input amp using a MAX4100, LT1097, and FET.
Depending on the load capacitance, the 1db bandwidth was 60 to 100Mhz. It's
DC response was also very good. I'm using a 1M/20pf input impedance to
support standard X10 probes. As far as the input attenuator, I'm leaning
towards a divider and precision, low R-On, analog switches. I'm looking
at several 8-10 Bit, 40-60MPS, A/Ds. This is on hold as I'm focusing on
the logic analyzer right now.

   - Tom

At 09:09 AM 3/3/98 -0500, you wrote:
>Tom,
>Timing IS everything!!!, I was just about to start such DSO project for
>my own. Just bought myself a 100Msample/sec A/D.
>I was planning to use some 256K high speed "cache" chips I've got in my
>basement for sample storage. I am wondering how did you solve the
>analog variable atenuator part of the DSO. I am not much into analog and
>that is a bit of a problem for me. I would like it to be
>fully controlled by the processor (not hardware switch or
>potentiometer). Any good references ???
>
>Michel Tremblay
>Network specialist
>Sidus Systems Inc.
>mtremblay@montreal.sidus.ca