Archive for the ‘arm7-labrador3-oscilloscope’ Category

jabberwocky repair and upgrade


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.


arm7-labrador3-oscilloscope board being fabricated


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.