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FAST A>D conversion
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nctnico:

--- Quote from: stj on April 06, 2019, 11:20:30 am ---to answer some questions,
no mutiplexing, just 30 leds wired direct to the binary>decimal convertor.
update rate of the leds would be the rate the data under the probe changes state - that's why the latch is between the dac and binary>decimal decoder.
now 20MHz is kind of worse case - most signals would be 4MHz or less - i picked 20 because it's kind of the max you would see a 74ALS series being run.
i know they can go higher but it's very rare.

--- End quote ---
I don't think this setup is very useful because your eyes are way slower compared to a 20MHz repetition rate. You really need to rethink this.
What if you would combine level and frequency into RGB leds. The number of LEDs on would be a measure for the amplitude and the color a measure for the repetition rate.
IDEngineer:

--- Quote from: stj on April 06, 2019, 05:44:55 pm ---no, that's a 2D display, i'm after a single row of 30led's. your on the right track though, imagine a VU meter with a 20MHz+ frontend.
--- End quote ---
I'm imagining that... and wondering why, and what you're going to do with it. Others have noted that your eyes don't have the temporal response to track changes in the 10's of MHz. So why do you need such a high A/D sample rate? Are you trying to display min/max voltages?

So far I'm missing the relationship between a relatively high bandwidth sample rate and a relatively low bandwidth user interface.
stj:
the high bandwidth is needed to capture the state before it changes,
you dont need to update the leds at that rate because you only need to see the high and low signal levels to evaluate the quality of the signal.
so as i said, the led's can be latched at a few Hz.
radiolistener:
I think you can get some parallel ADC, such as AD9200 which was recommended above and connect it to the cheap Chinese eval board for CY7C68013A based logic analyzer:
https://www.aliexpress.com/item/EZ-USB-FX2LP-CY7C68013A-USB-logic-analyzer-I2C-serial-and-SPI-core-board-Source-Code-development/32851287266.html

it can handle 24 MHz sample rate for 8 lines or 12 MHz for 16 lines. And there is ready to use library to work with it even with no need to write program for CY7C68013A controller (this is a fast version of 8051-based architecture microcontroller with a high speed hardware implemented USB).
joeqsmith:
So you want to know the voltage level before the signal starts to transition?   Normally, my projects are not that clean and I break out a scope to get a feel for the signal integrity.   

Are you planning to use a fixed threshold to trigger the device and just use some fixed precious time as the criteria?   

Maybe not the best example of messy signals but here I am playing with an eval board.   
https://www.eevblog.com/forum/microcontrollers/typical-speed-of-fpgas/msg1282290/#msg1282290
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