OPA2350PA Two-Stage Preamp for nerdaqII Seismometer 5V single supply | Rail-to-rail | ~10,000x max gain | DIP-8 dual op amp +5V GND STAGE 1: Input Amplifier STAGE 2: Output Amplifier OPA2350 PA (DIP-8) 1 OUT A 2 IN- A 3 IN+ A 8 OUT B 7 IN- B 6 IN+ B 4 V- 5 V+ C5 0.1uF ceramic C3 10uF + elect Geophone Coil (+) R1 1kΩ (-) R5 10k C4 22pF Rg1 100Ω pot C1 0.1uF film R3 100k 2.5V ref R4 100k C6 10uF + elect R6 10k C8 22pF Rg2 100Ω pot C2 1uF film R9 1kΩ R7 100k 2.5V bias R8 100k → A0 nerdaqII Start with both pots at 1kΩ (121x total gain). Increase slowly. Stop before clipping (trace hitting 0 or 65535). C1 blocks DC offset between stages. C2 blocks DC to ADC. Both critical at high gain. Keep geophone leads short and shielded. Gain = (1 + R5/Rg1) × (1 + R6/Rg2) = 101 × 101 = 10,201x max Component Reference Active Components • OPA2350PA (DIP-8) - Dual rail-to-rail op amp. Both amplification stages in one chip. Pin 5=V+, Pin 4=GND. • Rg1 100Ω trimpot - Stage 1 gain control. Adjusts feedback ratio. Multi-turn preferred for precision. • Rg2 100Ω trimpot - Stage 2 gain control. Same function as Rg1 for the second stage. Stage 1 Components • R1 1kΩ - Input resistor. Limits current from geophone coil into op amp input. Protects the chip. • R5 10k - Feedback resistor for stage 1. Sets gain together with Rg1. Gain = 1 + R5/Rg1. Stage 2 Components • R6 10k - Feedback resistor for stage 2. Sets gain together with Rg2. Gain = 1 + R6/Rg2. • R9 1kΩ - Series output resistor. Isolates op amp from ADC input capacitance. Prevents oscillation. Bias & Reference • R3 100k - Top half of 2.5V voltage divider for pin 6 (IN+ B). Biases stage 2 input at mid-supply. • R4 100k - Bottom half of 2.5V divider. Equal values give 2.5V from 5V supply. • R7 100k - Top half of output 2.5V bias divider. Keeps ADC input at mid-range for bipolar signal. • R8 100k - Bottom half of output bias divider. Sets the DC level that C2 AC-couples onto. • C3 10uF elec - Bulk power bypass. Smooths +5V supply ripple. Electrolytic, + on the top (straight line). • C5 0.1uF ceramic - Chip bypass cap. High-frequency noise on power pins. Mounted close to chip pins 4/5. • C6 10uF elec - Bypass on 2.5V reference line. Stabilizes the virtual ground for stage 2. + on top (straight line). • C1 0.1uF film - AC coupling between stage 1 output and stage 2 input. Blocks DC offset from stage 1. • C2 1uF film - AC coupling on final output to Arduino A0. Blocks DC so ADC sees only the AC seismic signal. • C4 22pF ceramic - Feedback cap in parallel with R5. Prevents high-frequency oscillation. Pole at ~723 kHz, well above seismic band. • C8 22pF ceramic - Feedback cap in parallel with R6. Same role as C4 for stage 2. Cheap oscillation insurance on perfboard.