PC USB scope project.

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F2.jpg
 
Had a customer comment that on my cheapest scope there was bleed through from one channel to the other.
I thought it was just pickup from a floating channel but it seemed not when I tested it carefully.
Turned out I was clocking the PIC A2D a little too fast.
So one line of code to change to fix it.

Then noticed on most sensitive voltage mode a bit of an DC offset on the screen.
This was fixed with a slight resistor change on an op amp dc bias generator from 510r to 220r.
 
Have found a problem with the A2D on board the PIC micro I am using.
As it approaches 50KHz sine wave input it starts to duplicate numbers.
Instead of 2,4,6,8,10 I am getting 2,2,4,4,6,6,8,8 !
Microchip are looking into it.

So I decided to try a different tack and use interrupt routine to capture A2D data.
Took about a day to crack after searching high and low for interrupt routine example and interrupt setup example. Nothing in the datasheet ! and little in the compiler manual.

Still the same problem although I can capture data about 20% quicker.

To get around the problem I wrote a post capture routine that is a digital low pass filter.
Nothing quite like a good bodge !