current sense
In 4 places on the oscilloscope, I put a series resistor just after the power supply so that I could measure the current being drawn by each load. For a battery-powered device like this one, the voltage drop across a small resistor isn’t enough to measure precisely with an 8 bit AtoD converter, so in each place, I used a fixed gain difference amplifier (with a gain of 50V/V) with inputs connected directly across the small resistor.
That’s all well and good, but I was getting 0V output from each one. It finally dawned on me what the problem might be. The data sheet said in one place that it amplified the magnitude of the difference between the two inputs, so I assumed the + and – inputs were interchangeable. When I drew the schematic symbol for the part, I happened to place the + pin where I should have placed the – pin. I looked at the data sheet again and in several other places it said that the + should be on the supply side of the resistor and the – on the load side and it is all completely obvious now that the data-sheet writers intended for it to be this way. So 4 pair of 3mm long crossed wires later and discussions of economic theory aside, the currents are now no longer reading zero.
analog input stage
So far, I have been working on the software for the oscilloscope by using the microcontroller’s onboard AtoD converter. I did some limited testing of the analog input stage last summer just to make sure it worked like I thought it should when I designed it, but have ignored it since. Yesterday, I tested it just a bit more so that I was confident it wouldn’t output any voltage that the microcontroller couldn’t deal with and then connected the output from the analog input stage to the analog input of the microcontroller. So now, putting a sine wave on the BNC input to my handheld oscilloscope puts an (inverted) sine wave on the screen.
