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Thread: Oscilloscope input amplifier designs sought
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face BY : Russell McMahon email (remove spam text)



Just received this email from a friend. He's highly competent
electronically. If he's trying to build one from scratch there will be a
good reason to do so. Any ideas?

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It is likely I will need to design a standard scope-type input stage
(attenuator and amp) at some time in the not too distant future.

The system will need a bandwidth of 0-200MHz (minimum), nominal input
impedance of 10Mohm (at some nominal shunt capacitance), and a standard
1:2:5 sequence covering gains from 500x to 0.05x.  The output of the
attenuator and input amp should be a 1V pk signal (preferably differential).

The attenuator needs to be electronically controlled (so attenuation can be
selected using 3V3 logic level signals).

Ideally it should use no moving parts and should consume as little power as
possible.

What I need is obviously in any number of modern 'scopes however I can't
seem to find any schematics.  I do have a schematic for a rather elderly
HP1741 which is helpful but most of the clever stuff is done in a thick-film
hybrid for which they only provide a block diagram.

What I would like to do is to collect as many examples of how the "real"
guys do this as possible.  It is probable that current scope designs will
all be proprietary black-box implementations but I would think if you were
to go back far enough there should be examples using discrete components for
which there are full schematics.

These days with very fast op-amps etc. there will probably be easier ways
but the basic principles should still be the same.

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