Gas Sampling Pump PCB Design
Yes, create a separate pump project and start the full schematic there.
Thought for 3 sec
I’ll create a separate pump project, then start schematic capture there so the existing NH3 transmitter stays intact.
Worked for 39 sec
The new project is created: /mikaelle/gas-sampling-pump-pcb-design. I’ve started a dedicated schematic-capture thread there and will wait for its result before continuing verification/layout.
Worked for 4 mins 51 sec
Created the separate project and started schematic capture there: /mikaelle/gas-sampling-pump-pcb-design
Open items:
- The schematic-capture thread is still running in the new project.
- Layout/routing will wait until schematic/ERC is complete.
Suggested next steps:
- Open the new project to watch progress.
- Once schematic capture completes, ask me to review/fix ERC and continue to PCB layout.
Review then layout
Check schematic status
Run ERC fixes
Explain pump protection
Optimize for manufacturing
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Ask about this doc
Schematic Validation — Analog NH3 4–20 mA Loop Transmitter
Scope
Validation of the analog-only schematic for a DD-Scientific GS+4NH3-100 ammonia transmitter powered from the same 2-wire 12–30 V / nominal 24 V loop used for the 4–20 mA output.
Implemented Blocks
- 2-wire loop connector J1.
- Input protection: R1 surge/fusible impedance, D2 reverse-polarity Schottky, D1 SMAJ33A TVS, FB1 ferrite bead, C1/C2 input filtering.
- High-voltage low-Iq regulator: U1 TPS7A1601 adjustable LDO.
- 3.3 V analog rail: U1 with R2=2 MΩ, R3=1.13 MΩ, C3=10 µF input, C4=10 µF output, C5=10 nF feed-forward.
- Precision reference: U5 LM4040C25IDBZR with R4=33 kΩ shunt bias and C6 bypass.
- Sensor common-mode: R5/R6 100 kΩ divider and U3A buffer producing VCM ≈ 1.25 V from 2.5 V reference.
- Sensor connector J2 for WE/RE/CE plus spare NC pin.
- Electrochemical sensor front end: R7=10 Ω WE-RE datasheet load/depolarization resistor, R8/R9/R10 lead damping, U3B potentiostat CE driver, U2 LMP7721 TIA.
- TIA span network: R11=68 kΩ plus RV1=100 kΩ span trim and C11=100 nF feedback compensation.
- Zero/span conditioning: RV2 zero trim, R15-R18 matched 100 kΩ conditioning network, R21/C13 command low-pass.
- Output current sink: U4B servo, Q1 60 V MOSFET, R12=62.5 Ω precision current-sense resistor, R13 gate stopper, R14 gate pulldown.
Datasheet-Grounded Sensor Parameters
- GS+4NH3-100 sensor type: non-biased electrochemical ammonia sensor.
- Electrodes: Working, Reference, Counter.
- Output signal: 100 ± 30 nA/ppm.
- Range: 0–100 ppm.
- Nominal full-scale current: 10 µA at 100 ppm.
- Recommended load resistor: 10 Ω.
- Typical baseline range: ±1 ppm NH3 equivalent.
- T90: <60 s.
- Assembly note: use sockets/connector; do not solder directly to sensor pins.
ERC / Connectivity Review
- Schematic contains 47 BOM components and 25 named nets.
- No MCU, ADC, DAC, firmware device, digital logic, or programmable component was added.
- ERC review returned no listed electrical-rule categories/results in this run.
- No-connect markers were applied to unused sensor header spare pin, U1 NC/PG/DELAY, and U2 NC pins.
Current-Loop Scaling Check
Ideal current-loop servo relation:
Text
I_LOOP = V_CMD / R12 R12 = 62.5 ohm
Nominal mapping:
Table
| NH3 | Sensor current | TIA span | V_CMD | I_LOOP |
|---|---|---|---|---|
| 0 ppm | 0 µA | 0.000 V | 0.250 V | 4 mA |
| 25 ppm | 2.5 µA | 0.250 V | 0.500 V | 8 mA |
| 50 ppm | 5.0 µA | 0.500 V | 0.750 V | 12 mA |
| 75 ppm | 7.5 µA | 0.750 V | 1.000 V | 16 mA |
| 100 ppm | 10.0 µA | 1.000 V | 1.250 V | 20 mA |
The simulator confirmed the ideal relationship: 0.250 V across 62.5 Ω gives 4 mA, and 1.250 V gives 20 mA.
Power / Thermal Checks
- TPS7A1601 datasheet supports 3–60 V input, 5 µA typical ground current, 100 mA output capability, and requires at least 0.1 µF input and 2.2 µF output capacitance; schematic uses 10 µF input and 10 µF output.
- For the current-sink MOSFET, ideal dissipation is significant if little/no external loop load is present:
- 24 V loop, 20 mA: about 0.455 W.
- 30 V loop, 20 mA: about 0.575 W.
- Q1 uses a larger DPAK/TO-252-class MOSFET candidate rather than a tiny SOT-23 to support this dissipation, but layout must provide copper heat spreading.
Known Design Caveats / Next Checks
- The current-loop simulation is an ideal servo model, not a transistor-level macromodel simulation of the exact op-amp and MOSFET. Final prototype validation must check stability with real op-amp output swing, MOSFET gate threshold, loop cable capacitance, and load resistance.
- The signal-conditioning resistor network is wired as an analog scaling/level-shift stage with zero/span trim; final calibration should be done with NH3 calibration gas or certified sensor sensitivity.
- Electronics quiescent current is not included in the ideal current-sink simulation. The design target is <1 mA electronics draw and zero trim must compensate residual live-zero offset over temperature.
- Layout must guard the WE summing node with VCM, keep WE/RE/CE traces very short, keep the loop/MOSFET/power nodes away from the TIA, and provide thermal copper for Q1.
- If moving to PCB layout, first set board size/stackup and then place components by functional block: loop protection at J1 edge, regulator/power near input, sensor connector next to U2/U3, output MOSFET away from TIA.
Scope
Implemented Blocks
Datasheet-Grounded Sensor Parameters
ERC / Connectivity Review
Current-Loop Scaling Check
Power / Thermal Checks
Known Design Caveats / Next Checks
