OPA2350PA Two-Stage Preamp for nerdaqII Seismometer 5V single supply | Rail-to-rail | ~10,000x max gain | DIP-8 dual op amp | Both stages biased at 2.5V +5V GND STAGE 1: Input Amplifier STAGE 2: Output Amplifier OPA2350 PA (DIP-8) 1 OUT A 2 IN- A 3 IN+ A 5 IN+ B 6 IN- B 7 OUT B 8 V+ 4 V- C5 0.1uF ceramic C3 10uF elect + Geophone Coil (+) R1 1kΩ Cin 470uF elect + Rb1 100k (-) R5 10k C4 22pF Rg1 100Ω pot C9 100uF elect + C1 470uF elect + R3 100k 2.5V ref R4 100k C6 10uF elect + Rb2 100k R6 10k C8 22pF Rg2 100Ω pot C7 100uF elect + C2 470uF elect + R9 1kΩ R7 100k 2.5V bias R8 100k → A0 nerdaqII Both stages biased at 2.5V (Vcc/2). Stages DC coupled (C1 removed). Cin blocks geophone DC (coil is low-ohm path to GND). C9 and C7 give unity DC gain for AC amplification. Start with both pots at 1kΩ (121x total gain). Increase gain slowly. Stop before clipping (trace hitting 0 or 65535). 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 stages in one chip. Pin 8=V+ (+5V), Pin 4=V- (GND). • Rg1 100Ω trimpot - Stage 1 gain control. Connects to GND through C9 (DC unity gain, AC gain set by Rg1). • Rg2 100Ω trimpot - Stage 2 gain control. Connects to GND through C7 (DC unity gain, AC gain set by Rg2). Stage 1 Components • R1 1kΩ - Input resistor. Limits current from geophone coil through Cin into op amp input. Protects the op amp input. • R5 10k - Feedback resistor for stage 1. Sets gain together with Rg1. Gain = 1 + R5/Rg1. Stage 2 Components • C1 470uF elec - AC coupling stage 1 (pin 1) to stage 2 (pin 5 IN+ B). + faces stage 1. f_c = 0.0034 Hz into Rb2 100k. • 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 5 (IN+ B). Biases stage 2 non-inverting input at mid-supply. • R4 100k - Bottom half of 2.5V divider. Equal values give 2.5V from 5V supply. • Rb2 100k - Bias resistor for pin 5 (IN+ B). Biases stage 2 at 2.5V independently. High value keeps C1 cutoff low. • R7 100k - Top half of output 2.5V bias divider for ADC input. • R8 100k - Bottom half of output bias divider. Sets DC level C2 AC-couples onto. • C3 10uF elec - Bulk power bypass. Smooths +5V supply ripple. • C5 0.1uF ceramic - Chip bypass cap. High-frequency noise on power pins. • C6 10uF elec - Bypass on 2.5V reference line. Stabilizes virtual ground. • C2 470uF elec - Output coupling (pin 7 to A0). f_c = 0.0068 Hz into 50k. • C8 22pF ceramic - Feedback cap parallel to R6. Prevents HF oscillation. • C7 100uF elec - DC blocking in series with Rg2. Unity DC gain for stage 2. • C9 100uF elec - DC blocking in series with Rg1. Unity DC gain for stage 1. • C4 22pF ceramic - Feedback cap parallel to R5. Prevents HF oscillation. • Rb1 100k - Bias resistor for pin 3 (IN+ A). Biases stage 1 at 2.5V. • Cin 470uF elec - Input coupling. f_c = 0.0034 Hz. Was 0.1uF film (f_c = 15.3 Hz). • C1 470uF elec - Interstage (pin 1 to pin 5). f_c = 0.0034 Hz into Rb2 100k. • Cin polarity: + faces pin 3 (2.5V), - faces geophone (GND). Geophone coil is DC path to ground.