OP27 Two-Stage Preamp for nerdaqII Seismometer Dual +/-12V supply | ~10,000x max gain | 2x OP27 DIP-8 | Offset null trim | 3 nV/√Hz +12V +5V (nerdaqII) GND -12V STAGE 1: Input Amplifier (U1) STAGE 2: Output Amplifier (U2) OUTPUT OP27 U1 (DIP-8) 1 NULL 2 IN- 3 IN+ 4 V- 8 NULL 7 V+ 6 OUT 5 NC OP27 U2 (DIP-8) 1 NULL 2 IN- 3 IN+ 4 V- 8 NULL 7 V+ 6 OUT 5 NC C1 10uF elect + C2 0.1uF ceramic C3 10uF elect + C4 10uF elect + C5 0.1uF ceramic C6 10uF elect + Geophone Coil (+) R7 1k C11 1nF osc. suppressor (-) R5 10k C9 22pF Rg1 1kΩ trimpot null 10k U1 offset wiper→V+ C7 0.1uF film R8 10k C10 22pF Rg2 1kΩ trimpot null 10k U2 offset wiper→V+ C8 1uF film R10 1k 2.5V bias → A0 nerdaqII R11 100k R12 100k GAIN CALIBRATION: Start with Rg1 and Rg2 at max resistance (1kΩ each = 11x per stage, 121x total). Slowly decrease resistance on each pot to increase gain. At 100Ω per stage you get 10,201x total. Stop when trace clips (hits 0 or 65535). OFFSET NULL CALIBRATION: (1) Short R7 input to GND. (2) Adjust U1 null 10k pot until U1 pin 6 reads 0V DC. (3) Remove R7 short. Jumper U2 pin 3 (IN+) to GND. (4) Adjust U2 null 10k pot until U2 pin 6 reads 0V DC. (5) Remove jumper. Null wipers go to V+. C7 blocks DC offset between stages. C8 blocks DC to ADC. Keep geophone leads short and shielded. OP27 requires dual +/-12V supply. WARNING: OP27 pin 5 is NC (no connect). V+ is pin 7, V- is pin 4. Offset null wiper goes to V+ (pin 7 level), NOT V-. C11 FIX: Geophone coil inductance (~henries) forms LC tank with input capacitance. OP27 GBW (63 MHz) finds the ~34kHz resonance and oscillates. 1nF across input kills Q at HF, invisible at seismic freqs. Confirmed stable. Gain = (1 + R5/Rg1) × (1 + R8/Rg2) Rg at 1kΩ: 11 × 11 = 121x | Rg at 100Ω: 101 × 101 = 10,201x Component Reference Active Components • U1 OP27 (DIP-8) - Stage 1 ultra-low-noise input amplifier. 3 nV/√Hz noise. Pins: 1/8=NULL, 2=IN-, 3=IN+, 4=V-, 5=NC, 6=OUT, 7=V+ • U2 OP27 (DIP-8) - Stage 2 variable gain amplifier. Same pinout as U1. • Rg1 1kΩ multi-turn trimpot - Stage 1 gain control. Range: 11x (at 1kΩ) to 101x (at 100Ω). Use 10-25 turn. • Rg2 1kΩ multi-turn trimpot - Stage 2 gain control. Same range as Rg1. • null 10k trimpots (x2) - Offset null for U1 and U2. Connect across pins 1/8, wiper to V+. Single-turn is fine. Feedback & Input • R7 1k - Input series resistor. Limits fault current from geophone coil into U1 input. • C11 1nF ceramic - Oscillation suppressor across geophone input. Kills LC tank resonance at ~34kHz caused by geophone coil inductance + OP27 63MHz GBW. Transparent at seismic freqs (159Mohm at 1Hz). Feedback & DC Operating Point • R5 10k - Stage 1 feedback resistor. At Rg1=1kΩ: gain=11x. At Rg1=100Ω: gain=101x. • C9 22pF ceramic - Feedback cap in parallel with R5. Prevents parasitic oscillation with fast OP27. • R8 10k - Stage 2 feedback resistor. Same formula as R5/Rg1. • C10 22pF ceramic - Feedback cap in parallel with R8. Same role as C9 for stage 2. • R8 also provides DC feedback for U2 (sets DC operating point). C7 blocks DC from stage 1, but R8 closes the loop. • Total gain range: 121x (both pots at 1kΩ) to 10,201x (both pots at 100Ω). Output Conditioning • R10 1k - Series output resistor. Isolates U2 from ADC input capacitance. Prevents oscillation. Power Bypass • C1/C4 10uF elec - Bulk bypass on V+ for U1/U2. Smooths +12V ripple. + on top (straight plate). C4 optional. • C2/C5 0.1uF ceramic - High-frequency chip bypass on V+. Mount close to pin 7. • C3/C6 10uF elec - Bulk bypass on V- for U1/U2. + on GND side. C6 optional. Signal Coupling • C7 0.1uF film - AC coupling between stages 1 and 2. Blocks DC offset from U1 output. • C8 1uF film - AC coupling on final output to Arduino A0. Blocks DC from ADC. ADC Bias • R11 100k - Top of 2.5V bias divider. From +5V (nerdaqII) to output node. • R12 100k - Bottom of bias divider. From output node to GND. Equal values = 2.5V midpoint. Power • Dual +/-12V required. OP27 is NOT rail-to-rail, cannot run on single 5V. • +5V tap from nerdaqII for R11/R12 bias divider only (low current). Build & Calibration Sequence 1. Build the circuit. Do NOT connect geophone yet. Double-check all pinouts before applying power. 2. Set Rg1 and Rg2 to MAX resistance (1kΩ). This is minimum gain (121x). Apply power. 3. OFFSET NULL U1: Short R7 input to GND. Adjust U1 null pot until U1 pin 6 reads 0V DC. 4. OFFSET NULL U2: Remove R7 short. Jumper U2 pin 3 to GND. Adjust U2 null pot until U2 pin 6 reads 0V DC. Remove jumper. 5. Connect geophone. Check nerdaqII trace is centered around 32768 (the 2.5V bias midpoint). 6. GAIN: Slowly decrease Rg1 (counter-clockwise) to increase stage 1 gain. Repeat for Rg2. 7. Stop when trace clips (hits 0 or 65535). Back off slightly. Re-check offset null. 8. Final check: bypass caps C2/C5 must hug pins 7 and 4. Star ground at ADC input preferred.