Skip to main content
Topic: USB Scope (Read 19274 times) previous topic - next topic

Re: USB Scope

Reply #15
that's no problem at all, I'll keep in mind not to plug anything while the scope is working

can you share the schematic and program?

thanks

Re: USB Scope

Reply #16
my program is semi-working, it captures 1 second of data and opens that in excel.
its not a real scope and its rather buggy.
so i recommend you to write you're own program for it.
i have 2 PCB's, i got a PCB for the FT2232 board from FTDI and a PCB with the ADC.
warning, i used an 741 opamp in there, i was not sure what opamp was needed so its a placeholder.
replace with any fast opamp (20Mhz+)
and the PCB's are a total mess, there pure test PCB's, USE AT OWN RISK.
http://www.MegaShare.com/3462810

Re: USB Scope

Reply #17
And i am a bit further, got a schematic of the full inputstage, so inputstage + ADC's
warning, picture is BIG:
http://http://img18.imageshack.us/img18/9009/scopeinput08.png
PCB with CPLD and memory will take care of all power supply's.
the opamps will be the LT6200, LT6200-10  and probably an AD8065.

Re: USB Scope

Reply #18
after a little thinking i decided to drop the FT2232H for the FT232R.
the FT232R can get 3Mbaud, ~300Kbyte per sec while still using the easy virtual com port.
for the buffer, even 4Kbyte per channel is enough.
with 4Kbyte of buffer and 50MSPS sampling we get:
4Kbyte / 50MSPS = 80 uS sampling time.
looking at 20Mhz you can save 80uS / 50nS = 1600 sine waves in the buffer!
at 2Mhz its still 160 and at 200Khz you can fit 16 sine waves in it.
and its always possible to divide the sampling speed, heck, else the CPLD gets bored.

with a 4Kbyte buffer and 300Kbyte/Sec speed you can update the screen 300KB per Sec /4Kbyte * 2 channels = 37,5 times a second.
thats plenty :D
the FT232 is cheaper, doesn't need the eeprom and crystal and all other parts that the FT2232H needs.
the opamps are expensive enough.
schematic for CPLD board should be done tomorrow.

Re: USB Scope

Reply #19
If you want time base accuracy you still might want to use an XTAL. The internal oscillator will be very temperature sensitive. You cannot beat the phase noise of a high Q mechanical oscillator.

 

Re: USB Scope

Reply #20
a small update, first schematic of the CPLD board.
http://http://img840.imageshack.us/img840/2375/scopecpu.png
there are some obvious flaws, no transistor to switch the relays, no AGND.
next time i want to have the PCB's done and schematics updated :)