I can now digitize one signal at 100MHz using my A/D card, capture in the Digilent Xilinx FPGA board, and upload to a PC via the USB port. I'm using Octave to upload and display the data, making a sort of oscilloscope. So I decided it's time to create a sourceforge open-source project.
It's http://mhz100q.sourceforge.net . Code is all in Subversion. It's all there, but I still need to create some build instructions and other documentation.
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3 comments:
Hi Joe,
What is the input range on your A/D frontend? I see you have a 2x gain in your single ended to double buffer, but I'm not sure how your Vcc=Vcm+0.3 limits or improves the range. ?
Hi zbaker,
Yes, the single-ended to differential amplifier has a gain of 2, and the A/D has a range from -0.512 to +0.512 volts, so the input signal has a range of about 0.5 volts: -0.23 is a 0x00 output and 0.33 is 0xff. It's not quite centered on zero due to the feedback circuit.
The equation should be Vcm=Vcc*0.3. The 0.3 factor comes from the A/D converter spec sheet: its input signals are supposed to be biased around 0.3*Vcc, or about 0.9 volts with a 3 volt supply. Vcm is the voltage on the common mode input pin of the amplifier; it's used to bias the amplifier outputs so they are centered on 0.9 volts and keep the A/D happy.
Hope that helps.
Joe
Hi Joe, thanks for the response.
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