Inconsistent readings do not automatically indicate a defective LMP91200. Isolate the chain in this order: probe and sample, cable and connector, guarded high-impedance PCB, LMP91200 configuration, ADC/reference, then firmware. A low-impedance millivolt source can pass while a real electrode fails because it does not reproduce electrode impedance, reference-junction behavior, leakage, grounding, or settling time.
Start by classifying the symptom
| Observed behavior | First areas to investigate |
|---|---|
| Stable raw voltage but incorrect pH | Calibration, ADC scaling, sign, gain, reference voltage, or temperature compensation |
| Rapid random fluctuation | Leakage, noise, grounding, connector faults, reference junction, or digital interference |
| Slow movement after immersion | Thermal or ionic settling, probe condition, sample chemistry, or inadequate stabilization time |
| Works with a millivolt source but not a probe | High-impedance leakage, guarding, cable, VCMHI loading, grounding, or electrode condition |
| Error changes with temperature | Electrode slope, buffer/sample chemistry, thermal lag, or temperature-dependent bias current |
| Only one board or connector is poor | PCB contamination, assembly residue, moisture, connector insulation, or layout variation |
Record raw LMP91200 output voltage, ADC code, calculated electrode millivolts, displayed pH, temperature, time since immersion, probe identity, and solution identity. This separates an analog problem from a conversion problem before any hardware is changed.
What the LMP91200 and pH electrode actually measure
A combination pH electrode produces a differential electrochemical potential between its glass sensing membrane and reference electrode. The LMP91200 buffers and conditions that very high-impedance signal for an ADC; it does not directly measure pH. At 25 °C, the theoretical Nernst slope is about 59.16 mV per pH unit, but the practical slope and offset belong to the individual electrode and measurement setup. TI describes an approximate output of +415 mV to −415 mV from pH 0 to pH 14 at 25 °C and electrode impedance of roughly 10 MΩ to 1,000 MΩ; these are design guidance, not universal specifications for every probe. See the LMP91200 datasheet.
Temperature changes the theoretical slope, the buffer’s stated pH, the sample’s chemistry, and the temperature relationship between probe and sensor. A generic correction cannot convert every process liquid to a universal 25 °C pH.
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LMP91200 specifications that matter in troubleshooting
| Parameter | Published detail and qualification |
|---|---|
| Supply | 1.8–5.5 V |
| Typical supply current | Approximately 50 µA in the stated pH-measuring configuration; total system current can be higher |
| Input bias current | ±125 fA maximum at 25 °C and ±445 fA at 85 °C for one powered condition; another zero-supply/common-mode condition lists ±600 fA at 25 °C and ±6.5 pA at 85 °C |
| Input offset | ±200 µV; offset drift ±2.5 µV/°C |
| Operating temperature | −40 °C to +125 °C for the IC, not necessarily the probe or sample |
| Guarding | Two guard pins support high-parasitic-impedance wiring and must be implemented as shown in the datasheet |
The basic bias-current error is Verror = Ibias × Relectrode. At 1,000 MΩ, 125 fA is about 0.125 µV, while 6.5 pA is about 6.5 mV—roughly 0.11 pH at a 59.16 mV/pH slope. That is an illustrative calculation, not a prediction of system accuracy. External leakage through a board, connector, cable, protection device, test point, or instrument can dominate the IC’s specified current.
For current documentation and product information, use TI’s LMP91200 product page. TI lists datasheet revision E dated February 29, 2016; confirm the revision used by your design.
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The fastest isolation procedure
- Verify the digital path with a low-impedance source. Inject a calibrated voltage within the permitted input and common-mode range. Check gain, polarity, VCM, ADC conversion, calibration equations, and temperature-code handling.
- Repeat with a realistic high-impedance source. A suitable pH-electrode simulator or high-resistance source exposes leakage, guard, connector, VCMHI, and loading problems that a low-impedance source hides.
- Measure the analog chain directly. Measure VDD and ground at the IC, VREF at both LMP91200 and ADC, VCM, and VOUT with an instrument that will not load the node. Compare VOUT, ADC code, calculated millivolts, and final pH.
- Swap probe and board systematically. Test suspect and known-good probes on suspect and known-good boards in the same fresh buffer. If the failure follows the probe, prioritize the electrode; if it follows the board, prioritize leakage, layout, configuration, or ADC.
- Test installation grounding. Compare an isolated laboratory vessel with the actual metal tank, pipe, or grounded vessel. Move the probe away from grounded metal and temporarily change shield termination only in a controlled test.
- Inspect and clean the physical design. Clean and dry the PCB, inspect the input route and connector under magnification, disconnect instrumentation from sensitive nodes, and compare short and production cables. Repeat under controlled humidity if possible.
Probe condition and calibration
Practical TI support guidance emphasizes keeping conventional glass electrodes wetted during storage; a dried probe may not recover. Inspect the bulb for cracks, coating, bubbles, or contamination, and check that the reference junction is wet and unobstructed. Confirm that the probe is suitable for the sample’s temperature and chemistry.
- Hydrate and clean the probe according to its manufacturer instructions.
- Measure buffer and probe temperature.
- Use fresh, traceable buffers covering the intended range.
- Rinse between solutions without returning rinse liquid to a buffer bottle and without aggressively wiping the glass bulb.
- Immerse the probe consistently, avoid vessel contact, gently agitate, then stop movement.
- Wait for stabilization; the required time depends on probe condition, temperature difference, stirring, sample composition, and electrode type.
- Record raw voltage, temperature, and calculated pH, not just the display.
- Use at least two points to establish slope and offset. Use a third buffer as an independent linearity check.
- Repeat the sequence or test the probe on a known-good commercial meter.
One-point calibration can correct an offset but cannot reliably correct slope. Three points can reveal nonlinearity; they cannot repair a contaminated, aged, or slow electrode.
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- Heating voltage: 5 plusmn 0.2V (AC middot DC)
- Working current: 5-10mA
- Response time: le5S;Settling Time: le60S
- Detection Temperature range: 0-80 ℃
- Component Power: le0.5W
TI’s practical guidance on hydration, rinsing, settling, and storage appears in its support discussion at this LMP91200 electrode thread.
Temperature and firmware checks
The LMP91200 provides a temperature-measurement mode for compensation, but it cannot infer the complete pH-versus-temperature chemistry of an unknown sample. Verify:
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- temperature sensor selection and wiring;
- Celsius/Fahrenheit conversion and signed values;
- the temperature used in the Nernst calculation;
- that calibration and measurement temperatures are comparable;
- that compensation is applied exactly once;
- that firmware uses calibrated slope and offset instead of assuming 59.16 mV/pH at every temperature;
- probe and temperature-sensor thermal lag.
PCB, connector, cable, and guarding
At tens to hundreds of megohms, ordinary PCB contamination can create a measurable error. Keep the INP route short and physically separate from VDD, clocks, SPI, PWM, switching regulators, displays, and exposed metal. Keep flux residue, fingerprints, dust, moisture, and solder-mask contamination away from the node. Remove unnecessary vias, test pads, resistor networks, switches, protection parts, and measurement connections from the input.
- Implement the LMP91200 guard pins and guard potential exactly as shown in the datasheet; a grounded copper pour is not automatically a valid guard.
- Use a guarded or triaxial connector/cable topology where required by the application.
- Clean the board with a process compatible with its materials and inspect it after drying.
- Test humidity and condensation effects; a dry office test does not validate a damp enclosure.
- Use low-leakage test equipment. A normal oscilloscope probe can load the electrode interface.
VCM, VCMHI, VOCM, grounding, and shields
This is a frequent LMP91200-specific failure area. TI support has described cases where a real electrode disagreed with a millivolt source because the liquid was electrically tied to ground, the common-mode relationship through VOCM was wrong, or VCMHI was loaded. A test point, oscilloscope probe, resistor, cable, or MCU input on VCMHI can disturb a sensitive node. One support response advised leaving VCMHI floating when unused for that design question; verify the exact requirement against the datasheet configuration and device revision.
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- Is the liquid, vessel, pipe, or probe body connected to protective earth or signal ground?
- Is the reference electrode making reliable ionic contact?
- Is a shield connected to the intended potential rather than ground by assumption?
- Does the reading change when the probe is moved away from a grounded vessel?
- Does disconnecting the cable shield change the result?
- Are VCM, VOCM, and VCMHI measured and stable under the actual cable and probe connection?
See the relevant TI support exchange at LMP91200 pH measurement not matching an mV equivalent. It is support evidence, not a substitute for the datasheet.
ADC and configuration verification
Read back and record supply voltage, VREF, VCM selection, PGA gain, output interpretation, pH-versus-temperature mode, diagnostic mode, ADC reference, and input range. Then check ADC reference accuracy and drift, resolution, clipping, code alignment, gain and offset, averaging, aliasing, and output settling. Capture raw analog output and ADC code simultaneously; do not mistake ADC quantization or mains pickup for electrode instability.
When to keep the LMP91200—and when not to add a buffer
Keep the LMP91200 when its low-power integrated pH front end suits the probe and the board can support clean guarding, calibration, and temperature control. TI’s TIDA-00561 reference design is a useful starting point, not a drop-in guarantee for every probe or sample.
An external buffer is not normally required at INP; adding one before identifying the fault can mask leakage, grounding, protection, or common-mode errors and introduce offset, noise, drift, and new protection limits. Consider a discrete solution only when the isolation tests show that the required gain, filtering, protection, or topology cannot be achieved with the integrated AFE. TI discusses LMP7721 and OPA928 as alternatives, but neither is a drop-in LMP91200 replacement: LMP7721, OPA928.
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Quick Recap
Final diagnostic checklist
- Classify the symptom and log raw voltage, ADC code, pH, temperature, time, probe, and solution.
- Prove firmware and ADC conversion with a low-impedance source.
- Repeat with a high-impedance simulator.
- Swap probes, boards, buffers, cables, and vessels one variable at a time.
- Inspect hydration, reference junction, contamination, aging, and sample compatibility.
- Verify calibration slope, offset, third-point check, and stabilization behavior.
- Measure VDD, VREF, VCM, VCMHI, VOCM, VOUT, and ADC reference under load.
- Clean and dry the PCB; inspect guarding, connector, cable, shields, and humidity sensitivity.
- Test grounded versus isolated vessels and remove accidental VCMHI loading.
- Apply filtering only after the raw measurement is proven valid.
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