Sean

You're making a DSO so I think you should make the input "look" like a CRO.
I think it would be more marketable in that form. Personally, I wouldn't
purchase anything that didn't.

Good luck with the project.
Bill

At 01:46 AM 18/01/98 -0500, you wrote:
>Hello all,
>
>I am continuing work on my PIC-based digital storage osc. and I am actually
>in the building phase right now, although the front end is not completely
>finished. This brings me to my question, what should I make the input
>impedance? I can remember when I asked a few other questions about this
>device on the list before, at least one person said something like "if you
>do [blah blah blah, etc.] it will then have a nice 50 ohm input impedance"
>While 50 ohms is good for many devices where I want the load matched to the
>generator, lets say I want my scope to see the output of an oscillator with
>Zout = 1K, I will see little or nothing if my Zin(scope) = 50 ohms!
>Besides, what happens when my 50 ohm scope gets attached to a signal which
>is riding on a 12 V DC level? My scope will have to dissipate 2.88 Watts!
>It is true that the cable going to the probe will probably have a
>characteristic impedance of around 50 ohms, but if I compensate the
>probe(or the internal input nework), I should be able to compensate for any
>added reactance due to the cable, right?
>
>I expect the scope's front end to have abt a 50 MHz BW. My input network
>must connect the BNC input connector to a MAX4107 300MHz BW op amp which
>has an input which looks like a 1Meg resistor in parallel with 2pF of cap.
>This input network will have to provide variable attenuation.
>
>I would appreciate any suggestions that anyone has regarding what input
>impedance is best. I would think the higher, the better.
>
>Thanks,
>
>Sean
>
>+--------------------------------+
>| Sean Breheny                   |
>| Amateur Radio Callsign: KA3YXM |
>| Electrical Engineering Student |
>+--------------------------------+
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>