After a month or so of scrambling to redesign the oscilloscope, it is finally at the board house being fabricated. The board ended up being 2 layers and 99.5mm X 79.5mm. The 6mil spacing, 6mil width and 15mil minimum hole size ended up being a necessity (the last board I did was 8mil/8mil/20mil).
The oscilloscope is battery (rechargable LiPoly) or usb powered, has 2 channels of input (DC coupled only), has a 128×124 pixel color OLED display. The AtoD converter is called a “labrador3″ or just “lab3,” and it is an ASIC that was designed for use in ANITA, an antarctic neutrino detector experiment. It has 9 channels and holds 260 samples per channel in a switched capacitor array that can run at up to 3.7GSa/s. When a trigger comes, it converts all samples to digital values with 2340 parallel Wilkinson analog to digital converters (each one is a comparator + a 12 bit latch). The advantage of this way of doing it is the high-speed sampling using relatively low-power. The disadvantage of is that it can’t sample data continuously.
The input impedance of this scope is 1MOhm. The analog bandwidth is a big unknown at this point. The lab3 can sample billions of times per second, but I’ve never designed RF circuits before, so this first attempt will probably only be useful at a much much lower sampling speed. The input amplifier has a bandwidth of about 700MHz, and my sloppy, space-constrained board routing will undoubtedly limit that much further. My professor gave me the freedom to do whatever I wanted for the input stage (he suggested a different method for adding the 1.25V which I ignored) and for the routing, so I can’t blame anyone but me when it doesn’t work. The people in the IDLab did give me several pieces of advice for all this, so if it does work, credit will have to be shared with them.
The main reason to have anything but a resistive divider on the front-end of the oscilloscope is that the lab3 can only digitize voltages between 0.5V and 2.0V. In the neutrino detector experiment, the input was ac coupled and then a 1.25V DC bias voltage was set up on the input to the lab3, so there was no problem. In a general-purpose oscilloscope, however, one frequently wants to see the DC offset of a given signal, so the choice was made to make the input DC-coupled and use a summing amplifier to add 1.25V to the (attenuated) signal, knowing that that would severely constrain the analog bandwidth of the scope.
The input stage is high-impedance (4 sets of 4MOhm resistive dividers in parallel going into a 4to1 multiplexer) and has schottky protection diode pairs that limit the voltage going into the multiplexer to about +-(0.75V+the diode’s forward voltage), so a relatively high-voltage input should not destroy the device, since the current that develops through the high resistance is very small (~71uA for a 177V input signal across a 2.5MOhm resistor, the top of one of the resistive dividers). The IV curve for the schottkys that I picked doesn’t even show anything below 10,000uA and the forward voltage in that case is 0.24V, so the input to the multiplexer should never see more than +-1V.
At this point (3 weeks before I need to give a presentation on this project and hopefully show it in a functioning state), I still have to come up with a parts list (which is mostly complete), email it to the appropriate people in the IDLab to get the parts purchase ordered and then just wait about a week for the board and parts to come in, then scramble to get it soldered together, taking care at each step to see that each DC-DC converter and the input amplifier stages output the appropriate voltages before soldering down the delicate ICs.
After that’s done, and assuming it works, the rest is writing software to control the lab3 and DACs and to display the data and do some bandwidth & slew-rate testing before the presentation. Easy as pie. Three weeks to go? No problem. Remind me I said that in about 2.5 weeks.
edit 2008-04-25 9:21pm HST: Just FYI, there are 771 SMD pads on the top, 196 SMD pads on the bottom and 392 holes to be drilled, total and the board is ~12 square inches. For comparison, last semester’s board was ~4 square inches had 256 SMD pads on the top, none on the bottom and 126 holes drilled through.