ph-tester-dat
apps
display-dat and mcu-dat == stm8-dat
the control board
the sense board
What kind of op-amp IS required?
You need an electrometer-grade or FET-input op-amp.
Suitable op-amps for pH measurement
| Op-amp | Input bias current |
|---|---|
| LMP7721-dat | 3 fA |
| LMC6001-dat | 25 fA |
| TL072-dat / TL082-dat | ~65 pA (borderline) |
| OPA129-dat | 75 fA |
| AD8605-dat | 1 pA |
Rule of thumb:
Input bias current should be < 1/1000 of electrode current
Typical pH front-end architecture
pH electrode
│
│ (very high impedance)
▼
[ FET-input buffer ]
│
├─ Gain & offset
│
├─ Temperature compensation
│
▼
ADC / MCU
- First stage must be unity-gain buffer
- Guard rings + clean PCB required
- Shielded cable mandatory
Ibias
- Ibias ≤ 1 pA → good pH front end
- Ibias 1–10 pA → acceptable with care
- Ibias 10–50 pA → hobby / lab demo only
- Ibias >50 pA → no