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aaron_do Senior Fellow Offline Posts: 1398	2 MHz Instrumentation Amp May 30^th, 2008, 1:58am Hi all, I need some kind of buffer with SMA inputs with an input capacitance of no more than 2 pF to work up to 10 MHz. Does such a buffer exist, or do i need to design on-chip buffers? thanks, Aaron
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pancho_hideboo

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Re: 2 MHz Instrumentation Amp
Reply #1 - May 30^th, 2008, 6:51am

aaron_do wrote on May 30^th, 2008, 1:58am:

I need some kind of buffer with SMA inputs with an input capacitance of no more than 2 pF to work up to 10 MHz.

As far as you connect an amplifier to DUT via 50ohm coaxial cable, a capacitance loaded to DUT is far larger than 2pF even though an input capacitance of amplifier is 2pF.
Or you expect 50ohm impedance of amplifier ?

What signal do you want to observe ? Oscilloscope ? Spectrum Analyzer ?

aaron_do wrote on May 30^th, 2008, 1:58am:

Does such a buffer exist, or do i need to design on-chip buffers?

I use active high impedance probe both for Oscilloscope and Spectrum Analyzer.
There are many excellent probes available.
Or I use OP Amp of ADI. Then connect DUT to OP Amp with very short line on board.
I often use following.
http://www.analog.com/UploadedFiles/Data_Sheets/AD8138.pdf
Maybe AD8138 is too over spec for your purpose. You can find reasonable one in ADI's products.

But maybe on-chip buffer is most cheap.

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« Last Edit: May 31^st, 2008, 3:32am by pancho_hideboo »

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aaron_do Senior Fellow Offline Posts: 1398	Re: 2 MHz Instrumentation Amp Reply #2 - Jun 1^st, 2008, 8:19pm Hi, thanks for the reply. BTW, do you know what kind of input-referred noise I could expect from an active high impedance probe? thanks, Aaron
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pancho_hideboo Senior Fellow Offline Posts: 1424 Real Homeless	Re: 2 MHz Instrumentation Amp Reply #3 - Jun 1^st, 2008, 10:04pm aaron_do wrote on Jun 1^st, 2008, 8:19pm: thanks for the reply. BTW, do you know what kind of input-referred noise I could expect from an active high impedance probe? You might be able to get such informations from data sheets of active probe. By why don't you measure DUT ground with probe. That is noise floor.
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aaron_do

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Re: 2 MHz Instrumentation Amp
Reply #4 - Jun 1^st, 2008, 11:15pm

Oh i don't actually have the probe at the moment. My university bought a very high-end (up to 20+ GHz) one from picoprobe but there were compatibility issues. So i was thinking of suggesting to my supervisor to try and get a lower-end active probe for use up to only a few hundred MHz. If i have to design a buffer just because the active probe's noise floor is too high then i would be a little more reluctant...

I have a data sheet for the high end probe but it says nothing about noise...

thanks anyway,
Aaron

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