Hitting a Speed Bump

With all the progress made last week with the counter, we still are having trouble seeing the correct Vout signal pattern of the MUX. Prof. Battat returned today and looked at the setup himself, and we made a few changes. Today was filled with lots of bumps and hills, and, at one point, we even thought we had burned out the counter and solder another just in case. In the book Moonwalking with Einstein, Foer states that humans sometimes reach a "plateau" where their progress or betterment at a task no longer increases. Nonetheless, he finishes by writing that one can overcome this stage by being extra-meticulous in their search for error. With 3 weeks left, it is time to be extra diligent in our work.

The first change was to remove the 50 Ohm surface-mount, resistors on the MUX because it was affecting the counter's ability to count. Dr. Phillips and I were unsure how to address the issue earlier, but he suggested the use of buffers. The binary counter can drive 50 mA of current at about 3.3V, but, if we connect the MUX board with the A0 and A1 lines, the load from the resistor (connected to ground) drives up the current to approximately 66 mA, which the counter can't handle. 7400 series, LVC buffers require about the same amount of current as the counter (50mA) and it seems those would be less than useful. Hence, we decided to remove those resistors completely from the board, except the RF and RG resistors used for the Vout gain, and the counter corrected itself when the address lines were connected again. However, we discovered a bigger issue with the SD and EN lines. When they were grounded (as they should be for an active MUX), the entire signal from the LSBs would disappear or distort completely. Currently, we have no explanation for this scenario: there are no solder bridges, misplaced lines, etc. A single MUX works without the SD and EN lines connected. Yet, those lines are critical to the full 4 x (4:1) apparatus.

A second change revolves around our missing input signals from the signal simulator. Battat and I both checked to ensure our simulator produced the correct signals and that they were received directly on the ribbon cable input pins. Surprisingly though, the signals are not correctly distributed at the cable's ends or the MUX inputs. Using a voltmeter, we checked each connection on the MUX board and I found phantom voltage on the Vcc+ line, when the line was disconnected, meaning somehow the voltage from the ribbon cable lines were leaking to that area of the MUX. Battat stated the possibility that the error lies within the design of the board itself and we might have to solder another. Tomorrow, we'll see if we can find the flow of such phantom voltage.

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