4. Decouple It Properly

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Problem 4

Put 100 nF across the TF-FAST supply pins and 10 µF near the connector, on short routed paths.

100 nF paths to the device supply and ground are each limited to 4.0 mm routed.
The 10 µF path back to the connector supply is limited to 15.0 mm.

Context

A decoupling capacitor is a local charge reservoir. The length that matters is along the copper to the pin and back, not the air gap on the screen. 100 nF belongs on the short loop at the IC. 10 µF is the bulk part nearer the connector.

Open the lesson Component guide

Components on this challenge

Decoupling capacitor. A small capacitor at a supply pin that supplies short current pulses.
Bulk capacitor. A larger capacitor that holds up the rail for slower changes in current.

Full brief

Put the small capacitor on the short loop at the device supply and ground, and the bulk capacitor nearer the connector. Measure length along the copper.

How PCBGrade measures this

U1 is a TF-FAST. Its VCC and GND pins are adjacent. J1 is the supply connector and is locked, as are J2 and U1. Move C1 (100 nF) so its routed path to U1 VCC and to U1 GND are each at most 4.0 mm, and move C2 (10 µF) so its routed path to J1’s supply pad is at most 15.0 mm. Route every net. Both capacitors can be given either value — a swap is graded, not rejected. Path length is pad centre to pad centre along traces and vias. Pours are not part of this board. Failing the C1 supply path, the C1 ground path, or required connectivity caps the score at 60. A wrong capacitor value caps the score at 60. A short between nets caps the score at 60.

capacitor connectivity/value · 30 pts
C1 supply path · 30 pts
C1 ground path · 20 pts
bulk placement · 10 pts
DRC · 10 pts
Cap 60: A short between nets caps the score at 60.
Cap 60: A C1 path over its limit, or an unrouted net, caps the score at 60.
Cap 60: A wrong capacitor value caps the score at 60.
Cap 69: Copper outside the outline cannot pass, and neither can a short between nets.

Reference

TF-FAST

A small digital IC whose VCC and GND pins sit next to each other so a local bypass capacitor can close a short loop.

1. VCC. Supply
2. GND. Return
3. IN. Input
4. OUT. Output

Trace · ratsnest · Drag a part or via. Drag a trace end to edit it. Drag empty board to pan.

Pass at 70. Check previews. Submit records.

Schematic2

C1 valueNot checked
Scored · 5 pts
The local part is 100 nF.
C2 valueNot checked
Scored · 5 pts
The bulk part is 10 µF.

Routing7

C1 netsNot checked
Scored · 10 pts
The local capacitor has to sit on the IC supply and the IC ground.
C2 netsNot checked
Scored · 10 pts
The bulk capacitor has to sit on the same supply and ground.
C1 supply pathNot checked
Scored · 30 pts
A long supply path is a poor local bypass.
C1 ground pathNot checked
Scored · 20 pts
The return to the IC ground pin has to be just as short.
C2 bulk pathNot checked
Scored · 10 pts
The bulk capacitor is measured from the connector supply pad.
Required connectivityNot checked
Mandatory gate
Every pin on VCC, GND, IN, and OUT has to be on one copper net.
No shortsNot checked
Mandatory gate
A supply-to-ground short makes the paths unmeasurable and caps the score.

DRC1

DRCNot checked
Scored · 10 pts
Width, clearance, and edge rules still apply.

PCBGrade

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