SkillA weekend to learn, then evenings

KiCad to JLCPCB

Schematic, layout, and a fabricated, assembled board for about the price of a textbook. The workflow, the layout rules that matter for microvolt signals, and the mistakes that cost a reorder.

AssumesSoldering, breadboards, and the multimeter SpineAnalog / mixed-signal hardware

On this pageA weekend to learn, then evenings

The workflow
Schematic habits
Footprints and the parts library
Layout rules for microvolts
Design rule check and review
Ordering

The workflow

First PCB in KiCad, schematic to board to orderDIY Hideout23:38Open on YouTubeLonger than the videos elsewhere on this site, and worth it once: the whole arc from an empty schematic to a file JLCPCB accepts. Watch the first ten minutes before you open KiCad, and the rest with it open.

KiCad is free and it is what most open hardware, including OpenBCI, is drawn in. Five steps, each its own tool inside KiCad. Draw the schematic. Assign a footprint (the physical pad pattern) to each part. Lay out the board. Run the design rule check. Export the fabrication files. JLCPCB takes those files, makes the boards, and, if you use parts from their library, solders them on.

Schematic habits

Start by opening the OpenBCI Cyton schematic and TI’s ADS1299 evaluation board schematic side by side and reading them until you can explain every part. Then draw yours. Use hierarchical sheets: power, analog front end, digital, connectors. Label every net with a meaningful name (AVDD, DVDD, AGND, DGND, SCLK, DRDY). Put a text note on the sheet for every non-obvious value: “390k limits bias current to 23 µA (IEC 60601).” The schematic is documentation; six months from now you are the reader.

Add test points: a pad you can touch with a scope probe on every supply rail, the reference, the bias output, and one input. They cost nothing and they make bring-up possible.

Footprints and the parts library

Pick components that JLCPCB stocks and prefer the ones marked “basic,” which have no per-part loading fee. For passives, 0603 is the smallest you should hand-solder and a fine default; 0402 only if the assembler is placing it. Use the KiCad JLCPCB plugin (there are several) to attach the part number to each component so the assembly file is generated for you. The ADS1299 is an “extended” part; that is fine, it just costs a few dollars to load.

Layout rules for microvolts

These are the ones that matter. Every other rule is secondary.

Split the world. Analog inputs and reference on one region of the board, digital and microcontroller on another, with the ADS1299 straddling the boundary. Nothing digital routes under or beside the analog inputs.
One ground, joined once. A solid ground plane. If you separate analog and digital ground, join them at exactly one point, near the chip, as the datasheet shows. Two joins make a loop; a loop makes hum.
Symmetric inputs. Each channel’s positive and negative traces are the same length, same width, same layer, run as a pair. Any asymmetry is an impedance mismatch, and you know what that does to common mode.
Short inputs. The trace from connector to protection resistor to chip pin is as short as the geometry allows. Every millimetre is antenna.
Guard ring. A ground-connected trace surrounding the input section, on the same layer, so that leakage across the board surface goes to ground instead of into the input.
Capacitors at the pin. Each 0.1 µF bypass capacitor within 2 mm of its supply pin, with the shortest possible path to ground. The reference capacitors likewise.
Keep the switching away. The boost converter or regulator, if switching, lives as far from the inputs as the board allows, with its loop area minimized.
Isolation gap. If your board has an isolation barrier (an isolator chip, an isolated DC-DC), leave a clear gap of several millimetres in the copper across the barrier, on every layer, and route nothing across it but the isolator.

Design rule check and review

Run the DRC and fix every error; warnings you can judge. Then print the board at scale and lay real parts on the paper. Then, before ordering, have someone else look at it. A second pair of eyes has caught a reversed connector on every board I have ever ordered.

Ordering

Export Gerbers, drill file, bill of materials, and component placement. Upload to JLCPCB. Choose two layers (four if you want an internal ground, which is better and only a little more expensive), 1.6 mm, any colour, and assembly for the top side. Order five bare boards and two assembled. Check the assembly preview, where rotated or mirrored parts show up. The whole order for an eight-channel ADS1299 board is around $215 with the chips and takes about two weeks; JLCPCB itself takes little of that (about$ 2 for five bare boards, an $8.18 assembly setup, a$ 1.53 stencil, and fractions of a cent per joint), and the two ADS1299 chips at $69.82 each are most of the rest. Building it around the four-channel ADS1299-4, which is$ 37.10, brings the order to about $150.

What goes wrong

Connector pinout mirrored. The commonest reorder. Check the footprint’s pin 1 against the part’s datasheet, viewed from the same side. Wrong footprint for the chip. TQFP-64 with 0.5 mm pitch is the ADS1299; there are similar packages that are not. Check the pad dimensions against the datasheet’s drawing. Forgot the decoupling. It works on the bench and oscillates in the field. Every supply pin, every time. Ground plane split badly. Digital return current forced under the analog inputs. Look at the plane with the other layers hidden and trace the path from the microcontroller’s ground pin back to the regulator.

Recall

What is the single layout rule most likely to decide whether an EEG board records brain or hum?

Keeping the two input traces of each channel symmetric and short, run as a pair, so there is no impedance mismatch to convert common-mode hum into a differential signal.

Recall

Why join analog and digital grounds at exactly one point?

One join keeps digital return currents away from analog nodes while sharing a reference; two joins form a loop that picks up magnetic fields and circulates hum.

ExhibitI have ordered a board from KiCad through JLCPCB and checked the assembly preview before paying.