Analog input to ADC

[hani_a]

Hi,

I'm trying to build a fast (100 to 200 MHz) digital storage oscilloscope. I was able to find RAM and ADC's with the speed I needed. I'm currently researching the input circuitry, which I believe is the limiting factor. I'm looking for the circuitry required to interface the raw input to the ADC. I believe that I need an op amp to scale the signal down. I simply can't find a fast enough scaling circuit. Is the op-amp bandwith the same as its propagation delay? Is an op amp necessary for a very fast scaling circuit? Is there an alternative?

Thanks in advance!

Hani A.

 

[William At MyBlueRoom]

Try building a 1MHz first, 100MHz to 200Mhz is beyond alot of TTL and you should be sampling at least double (or more) the waveform frequency you want to view. If you sample 1MHz at 1MHz you wont see much.

 

[Nigel Goodwin]

Try building a 1MHz first, 100MHz to 200Mhz is beyond alot of TTL and you should be sampling at least double (or more) the waveform frequency you want to view. If you sample 1MHz at 1MHz you wont see much.

It's also essential to low-pass filter the input, to remove anti-aliasing problems, and sampling at only double the highest frequency would be much too low for a scope, as you wouldn't see anything like the original waveform.

 

[picasm]

There is a published design on "circuit cellar" magazine for the original version of "Bitscope" before the company which made it produced a commercial version. It uses MAX477 opamps which have a 200-300Mhz bandwidth.
See https://www.electro-tech-online.com/custompdfs/2006/09/e043pdf1.pdf for the article and the MAX477 data sheet is at **broken link removed**
Another alternative is the AD8056 or AD8055 which also have 300mHz bandwidth.

 

[sci-3d]

I use ads7841

In my project, I use 8051 uC and ads7841 IC to convert analog to digital.
If you don't mind please see:

https://mcu-programming.blogspot.com/2006/09/adc-interfacing.html

 

[hani_a]

Scope..

Hi everyone,

First, I'm thankful for the info you all provided me. I understand that building such a scope will be at the very least challenging, but deserves the trouble. I understand a good portion of the signal conditioning that needs to be done (i.e. filter). In fact, I might try to incorporate filter software that's available online into the interface. I've seen many simple designs of Analog to digital conversions and found the best & most complete design at the following link:

http://www.cmccord.co.uk/FYP/final_report.htm

Again, thanks a million for your help!

Hani A.