How can I read accurate analog values on Arduino A2D?
I am running an 8x8 screen from the 5v supply on board an arduino (controlled by the arduino via shift reg's). I am also trying to read an analog value over one of the analog pins. I have noticed however large variations in the analog read value, enough to make it near useless. I get the same problem to a lesser extent without the screen on but still rendering it next to useless. Normal resistors seem to be absolutely fine giving a near constant value, but using photo-transistors produces the error, as does reverse bias LEDs. Any Idea's whats going on or how it may be fixed?
p.s. When the screen is on the interference is cyclic, looks like a relatively steady (frequency) sine wave. The rate of the interferences wave seems to vary with how long the display sequence is (basically its display, read, display, read loop). When doing a read I have tried with the screen both on and when the shift regs Output En is low (effectively off) - still getting the cyclic interference. Interference when all that I am doing is analogue reading appears random.