Build a gain-3 non-inverting amplifier with a low-pass near 995 Hz and a short feedback loop.
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Board 70 × 40 mm, two layers. J1, U1, J2, and J3 are locked. J1 pin 1 is the sensor and pin 2 is GND. The sensor is an ideal voltage, source resistance 0 Ω, swept from 0.2 V to 1.0 V. J3 pin 1 is 3V3 and pin 2 is GND. J2 pin 1 is the amplifier output. U1 is TF-OPA1: IN+, IN−, OUT, V+, V−. V+ is 3.3 V and V− is GND. The output clips at 0.10 V and at 3.20 V. This is a deterministic educational model, not SPICE. Gain is 1 + Rf/Rg. Each of Rf, Rg, the input resistor, and the input capacitor is one palette part. The model also reduces a series or parallel network of the allowed values when those parts share an intermediate net. Allowed resistors are 1 kΩ, 1.6 kΩ, 10 kΩ, 20 kΩ, and 30 kΩ. Allowed capacitors are 1 nF, 10 nF, 100 nF, and 1 µF. The reference, and the schematic points, are one Rf of 20 kΩ, one Rg of 10 kΩ, and an input low-pass of one 1.6 kΩ plus one 100 nF (about 995 Hz). Low-frequency gain, taken at the model’s 10 Hz operating point, must be from 2.94 to 3.06 (9). DC output at 0.2 V must be from 0.588 V to 0.612 V (6), and at 1.0 V from 2.94 V to 3.06 V (6). The −3 dB frequency is where the gain is the low-frequency gain over √2, and it must be from 800 Hz to 1200 Hz (6 each side). A 20 kΩ + 10 nF filter is 795.8 Hz and fails. Clipping is the smallest 0.01 V step of output slope from 0.2 V to 1.0 V, divided by the low-frequency gain, and it must be at least 0.90 (6). Roll-off at 10 times that −3 dB frequency, divided by the low-frequency gain, must be at most 0.125 (6). A capacitor from OUT to IN− shelves the gain and fails that check. Feedback length (15) is the sum of four zone-excluded copper paths: U1 OUT to the feedback network’s OUT-side pad, U1 IN− to that network’s IN− pad, U1 IN− to the ground-setting network’s IN− pad, and U1 V− to the ground-setting network’s GND pad. It must be ≤ 20.0 mm. The 100 nF from V+ to GND must have its 3V3 pad within 4.0 mm routed of U1 V+ (10). Zones do not count. Failing gain, either endpoint, either frequency edge, clipping, or roll-off caps the score at 60. DRC is clearance 0.20 mm, width 0.25 mm, and copper-to-edge 0.50 mm.
- analog behavior · 45 pts
- schematic · 20 pts
- layout · 25 pts
- DRC · 10 pts
- Cap 60: A gain, endpoint, cutoff, clipping, or roll-off miss caps the score at 60.
- Cap 69: Copper outside the outline cannot pass, and neither can a short between nets.