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Step 10Board Layout
Once I had the design complete - including a working breadboard assembly of the prototype - it was time to lay out a printed circuit board for the scope. I did both the schematic capture as well as the board layout with a design tool called Diptrace. This is quite a capable tool that also comes with great customer support - the developers really listen to user feedback! - at a fraction of the price of so-called professional tools. In fact, they offer a free version that is restricted to two copper layers and a maximum of 250 component pins - which is actually more than the DPScope layout needs (it has two layers but only about 200 pins). So you could do this design without paying a dime for the tool.
The final layout is shown below. You can see that I placed the components in a logical order: To the left, close to the BNC probe connectors, is the input circuitry (attenuator, input amplifier, clampd diodes, probe compensation). In the middle is the offset DAC and the programmable gain amplifiers, follower by the dsPIC microcontroller. On the right finally you have the USB cable connector and the power supply. All the chips are placed in the same orientation (pin 1 is bottom left, notch is left) to minimize the chance for assembly mistakes.
As for the trace layout, the ground plane is a solid copper fill on the bottom plane - this minimizes supply inductance and provides some shielding against interference. The power supply traces are made pretty wide (50 mil / 2.25mm) since they carry considerable current, and again to minimize inductance. With some exceptions the top ayer carries the vertical traces, and the bottom layer the horizontal traces.
Now let's see how to build up this scope in practice!
The final layout is shown below. You can see that I placed the components in a logical order: To the left, close to the BNC probe connectors, is the input circuitry (attenuator, input amplifier, clampd diodes, probe compensation). In the middle is the offset DAC and the programmable gain amplifiers, follower by the dsPIC microcontroller. On the right finally you have the USB cable connector and the power supply. All the chips are placed in the same orientation (pin 1 is bottom left, notch is left) to minimize the chance for assembly mistakes.
As for the trace layout, the ground plane is a solid copper fill on the bottom plane - this minimizes supply inductance and provides some shielding against interference. The power supply traces are made pretty wide (50 mil / 2.25mm) since they carry considerable current, and again to minimize inductance. With some exceptions the top ayer carries the vertical traces, and the bottom layer the horizontal traces.
Now let's see how to build up this scope in practice!
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