Reply by January 8, 20152015-01-08
> >typical scope probe-> IN -> res divider+varcaps (1/10 - 1/100?) -> fet opamp(s)? -> some amp with single input and dif output -> VGA -> ADC > > Are you designing an oscilloscope? SE hiZ in, then a differential ADC? > Do you need switchable gain ahead of the ADC?
I need only one channel similar to what is in oscilloscope. VGA can be controlled by the SPI or analog.
Reply by John Larkin January 8, 20152015-01-08
On Thu, 8 Jan 2015 05:39:02 -0800 (PST), ucy193@gmail.com wrote:

> Need more details about the application to say much more. > > >typical scope probe-> IN -> res divider+varcaps (1/10 - 1/100?) -> fet opamp(s)? -> some amp with single input and dif output -> VGA -> ADC
Are you designing an oscilloscope? SE hiZ in, then a differential ADC? Do you need switchable gain ahead of the ADC? -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by January 8, 20152015-01-08
 Need more details about the application to say much more.


typical scope probe-> IN -> res divider+varcaps (1/10 - 1/100?) ->  fet opamp(s)? -> some amp with single input and dif output -> VGA -> ADC 

Reply by Tim Williams January 8, 20152015-01-08
"Gerhard Hoffmann" <ghf@hoffmann-hochfrequenz.de> wrote in message 
news:ch60ohFnt0vU1@mid.individual.net...
>> 1 pF has an impedance of 1.6K at 100 MHz! > > So what. 1Meg || 10 pF is not unusual for 100 or 200 MHz scopes. >
But that's a resistance of 1M in parallel with a capacitance of 10p. The _impedance_ ain't 1M at 100MHz! The OP is neither specific enough (does it literally have to be an impedance |Zin| = 1M, and if so, at what frequency(ies)?) nor very practical (14 bits at 100MHz, no really?). Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Reply by Gerhard Hoffmann January 7, 20152015-01-07
Am 08.01.2015 um 00:52 schrieb John Larkin:
> On Wed, 7 Jan 2015 14:46:21 -0800 (PST), ucy193@gmail.com wrote: > >> Hello, >> I'm looking for a schematic of the amplifier with band from DC to 100 MHz and impedance 1Mohm for 14bit ADC converter. I googled only simple solutions based on eg AD8138 with trafos and imput impedance only 50 ohm >> >> Any sugestion? >> >> >> Peter > > 1 pF has an impedance of 1.6K at 100 MHz!
So what. 1Meg || 10 pF is not unusual for 100 or 200 MHz scopes. regards, Gerhard
Reply by John Larkin January 7, 20152015-01-07
On Wed, 7 Jan 2015 14:46:21 -0800 (PST), ucy193@gmail.com wrote:

>Hello, >I'm looking for a schematic of the amplifier with band from DC to 100 MHz and impedance 1Mohm for 14bit ADC converter. I googled only simple solutions based on eg AD8138 with trafos and imput impedance only 50 ohm > >Any sugestion? > > >Peter
1 pF has an impedance of 1.6K at 100 MHz! Need more details about the application to say much more. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by Lasse Langwadt Christensen January 7, 20152015-01-07
Den onsdag den 7. januar 2015 23.46.27 UTC+1 skrev ucy...@gmail.com:
> Hello, > I'm looking for a schematic of the amplifier with band from DC to 100 MHz and impedance 1Mohm for 14bit ADC converter. I googled only simple solutions based on eg AD8138 with trafos and imput impedance only 50 ohm > > Any sugestion? > > > Peter
http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise -Lasse
Reply by January 7, 20152015-01-07
Hello,
I'm looking for a schematic of the amplifier with band from DC to 100 MHz and impedance 1Mohm for 14bit ADC converter. I googled only simple solutions based on eg AD8138 with trafos and imput impedance only 50 ohm

Any sugestion?


Peter