in a previous post, i mentioned that the adc's on the cortex m4f microprocessor were unipolar, and i included a link to a simple op-amp circuit that could provide the appropriate voltage shift and scale operation. yesterday, i went from designing a layout to fabricating 9 boards in 24 hours. actually, i made 11 boards, but lost two in the process, because of issues relating to old pcb stock. in the future, i might design two layouts for each board, with the alternate layout applying wider tolerances, to account for older pcb stock. i used a positive photofabrication method for applying resist to the boards, which in my experience provides the sharpest lines. i used eagle cad to design the layouts, and gimp to beautify them. as per usual, the values for the components are etched right into the copper, so no silkscreen layer was necessary.
if you're going to use this layout, make sure it's oriented appropriately, and credit me. that's all i ask. 🙂 for details on my pcb fabrication process, check out this post
above, you can see the lot. the whole batch cost me under $20, including parts. this is a great argument against outsourcing, imho. i am not aware of any boardhouse that can put together this many pcb's in such a short amount of time, and for so little money.
i scratched my initials into the positive with a razor because i had forgotten to put those little square eddy-current baffles in my ground plane. not a big deal if it's a problem, considering the extremely low time and money overhead of these units.
and here's a closeup of a stuffed board! each unit actually has 2 parallel preamp stages, so my total is currently 18. i only needed 16 for the m4f, but it never hurts to have a few extras lying around.
i'm thinking 8 units should be stacked in a single chassis, with 1/4" ts jacks for input, and tip jacks for the output. for each channel, i'll include a "true" bypass switch (with indicator leds), in case i have a synth or something that can send out 0-3.3v unipolar signals. this 16-channel device will have its own power supply. maybe i should include a vu meter...