The specimen
is the sensor.
A grounded-reference current amplifier, a slow precision ADC and your ESP32. The new detector reads charge from an insulated specimen holder, with the electronics outside vacuum.
One path from the holder to the image.
The chamber and stage stay earthed. Only the conductive specimen holder is insulated from the stage, and feedback keeps it near 0 V. A second ground wire to the holder would bypass the measurement.
| From | To | What the connection does |
|---|---|---|
| Conductive specimen / holder | Sealed feedthrough → U1A pin 2 | Sensitive current input. Keep the holder insulated from the earthed stage. |
| U1A pin 1 | 100 MΩ ∥ 10 pF → pin 2 | Negative feedback; 1 pA absorbed gives approximately +100 µV. |
| U1A pin 3 / guard | Quiet AGND | Sets the virtual-zero specimen reference. Guarding limits PCB leakage. |
| LMC662 pin 8 / pin 4 | +5 V / −5 V | Decoupled analog power. An on-board MCP1700 generates 3.3 V for the ADC and references. |
| U1A pin 1 | 40.2 kΩ → U1B pin 6 | 10 kΩ feedback from pin 7; pin 5 receives the filtered 1.32 V divider. |
| U1B pin 7 | Output protection / filter → ADC A0 | Attenuated signal centered near 1.65 V. A1 receives the 1.65 V reference. |
| ADS1115 SDA / SCL | ESP32 GPIO21 / GPIO22 | J3 pin 3 → GPIO21; pin 4 → GPIO22; pin 1 → GND. Leave ALERT pin 5 unconnected: default GPIO27 is scan-DAC CS; firmware polls I²C. I²C pull-ups use 3.3 V; ADDR at ground selects 0x48. |
| AGND / enclosure / chamber | Defined signal bond / protective earth | Give signal current one return path. The ESP32 uses its own USB power and shares AGND. J3 pin 2 is a ≤20 mA reference output, not ESP32 power. |
Pin and interface references: TI LMC662, pinout and layout · TI ADS1115, differential conversion and I²C. Component values and wiring shown here are this project's Rev A design.
See a small current become a voltage.
This calculator uses an assumed current. It does not derive a butterfly-wing signal from feature depth.
A guarded analog front end.
Mount the analog board in an earthed metal enclosure beside the atmospheric end of the feedthrough. The PCB design package contains the electrical schematic, copper layout, component list and fabrication outputs. Keep the 100 MΩ summing node clean and dry; flux residue and a long cable can dominate pA measurements.
Inspect before fabrication.
The downloadable board and schematic are the source of truth for footprints, net names and physical connector orientation.
Open PCB package index ↗Design files
Loading the design package index…
What Rev A still needs
Bench measurement of leakage, zero drift, current gain, input capacitance, step response and ADC scaling. A clean PCB layout and a passed connectivity check do not prove pA sensitivity or microscope image quality.
Open the editable ESP32 scan program ↗16-bit positioning, synchronized with the detector.
The selected DAC80502 board sends paired X/Y commands to the external scan amplifiers. Its buffered 1.25 V center is separate from this detector’s 1.65 V ADC reference. ESP32 GPIO18/23/27 carry SCLK/SDIN/SYNC; GPIO25 requests blanking. GPIO26 is unused. The LMC662 and ADS1115 signal path remains on its own board.
Open the scan-board wiring, calculator and PCB files ↗Calibrate the electronics before the beam.
- Assemble and clean the board.
Inspect the resistor values, op-amp orientation, guard clearance and connector nets against the design package. Clean the high-impedance region and let it dry fully. Fit the board in its shielded enclosure.
- Check the three low-voltage rails.
With the specimen disconnected, verify +5 V, −5 V and 3.3 V against AGND. Measure the U1B reference near 1.32 V and ADC A1 near 1.65 V. Check output limits before attaching the ADC.
- Measure zero and gain.
Use the design's known-current calibration procedure. Record the blanked input offset, polarity, output noise and gain; do not assume the nominal resistor or divider values are exact. A 10 pA input should change the TIA output by about 1 mV at 100 MΩ.
- Fit an isolated, conductive specimen.
Verify that no mounting screw or coating shorts the holder to the stage. Use the specified vacuum feedthrough and keep the input lead short. Start with a conductive reference; coated butterfly wings come after the detector is characterized.
- Establish the beam, then acquire slowly.
Follow the microscope's vacuum, filament and acceleration procedure. Measure incident beam current separately. The default is 5 ms settling plus a 10 ms acquisition window per pixel; one 128 SPS conversion alone takes about 7.8 ms.
- Validate contrast and repeatability.
Compare blanked and unblanked baselines, repeat scans and vary beam current. Watch for drift, saturation and charging. Shorten the dwell only after measuring the complete analog and ADC response.