OPA129U - Ultra-Low Bias Current Op Amp | Texas Instruments
MPN: OPA129U ✓ ActiveOPA129U Overview
An operational amplifier is a voltage-amplifying, active circuit component that uses negative feedback to control gain, bandwidth, and output behavior. Within the semiconductor hierarchy, the OPA129U belongs to the precision-amplifier category, specifically the ultra-low-input-bias-current amplifier segment. Unlike bipolar-input amplifiers, whose input currents rise rapidly with temperature, the OPA129U uses dielectrically isolated FET, or Difet, inputs to suppress gate leakage and reduce bias-current error.
The verified device data highlights a maximum input bias current of 100 fA, maximum input offset voltage of 2 mV, maximum offset drift of 10 µV/°C, and minimum open-loop gain of 94 dB. The 1 MHz gain-bandwidth product and 2.5 V/µs slew rate provide useful low-frequency precision performance rather than high-speed operation. The 8-pin SOIC package supports conventional surface-mount assembly and uses a standard single-amplifier footprint.
OPA129U is intended for instrumentation, photodiode and detector interfaces, high-impedance transducers, capacitive sensors, and feedback networks where input bias current dominates measurement error. Its ultra-low bias current is especially valuable when the source impedance is high enough that even picoamp-level bias current creates a meaningful voltage offset. The ±5 V to ±18 V supply range supports bipolar analog rails commonly found in laboratory and industrial instrumentation.
Typical applications include transimpedance preamplifiers, pH and electrochemical probe interfaces, photodiode amplifiers, precision current-to-voltage converters, and logarithmic amplifier modules used in scientific instruments. Because the bias current is exceptionally small, it also enables capacitive sensor front ends where the amplifier must contribute minimal error to the sensed charge.
When laying out a high-impedance design, keep the input leads short, guard sensitive nodes against surface leakage, and avoid contamination from flux residue and moisture. Bias-current errors scale with source impedance, so a 1 GΩ source can produce a 100 fA-induced error of 100 nV before amplifier offset and noise are considered. Confirm that the selected operating point remains within the verified supply and common-mode limits.
This page consolidates manufacturer and distributor findings for the OPA129U, documents the verified pricing reference as of 2026-10-29, identifies same-footprint drop-in alternatives, and adds practical bias-current and leakage guidance that is not presented as a simple distributor inventory summary.
Drop-in alternatives for OPA129U — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
OPA129UE4
✅ Drop-In📋 Reference alternative (not in catalog)
OPA124UAG4
✅ Drop-In📋 Reference alternative (not in catalog)
OPA277UA
✅ Drop-In📋 Reference alternative (not in catalog)
OPA227UA
✅ Drop-In📋 Reference alternative (not in catalog)
OPA627UA
✅ Drop-In📋 Reference alternative (not in catalog)
OPA129U Maximum Ratings & Electrical Characteristics
| Product Type | Single operational amplifier |
| Input Stage Technology | Dielectrically isolated FET (Difet) |
| Number of Amplifiers | 1 circuit |
| Maximum Input Bias Current | 100 fA |
| Maximum Input Offset Voltage | 2 mV |
| Maximum Input Offset Voltage Drift | 10 µV/°C |
| Minimum Open-Loop Gain | 94 dB |
| Gain-Bandwidth Product | 1 MHz |
| Slew Rate | 2.5 V/µs |
| Supply Voltage Range | ±5 V to ±18 V |
| Package | 8-pin SOIC |
| Mounting Type | Surface mount |
OPA129U Pin Configuration
| Pin 1 | NC — Not connected (per manufacturer pinout) |
| Pin 2 | -IN — Inverting input |
| Pin 3 | +IN — Non-inverting input |
| Pin 4 | V- — Negative supply connection |
| Pin 5 | NC — Not connected (per manufacturer pinout) |
| Pin 6 | OUT — Amplifier output |
| Pin 7 | V+ — Positive supply connection |
| Pin 8 | NC — Not connected (per manufacturer pinout) |
Typical Applications
OPA129U is suitable for 6 applications: Photodiode Transimpedance Amplifier, Electrochemical and pH Sensor Interface, Precision Current-to-Voltage Conversion, Capacitive Touch and Proximity Sensing, Laboratory Instrumentation and Logarithmic Modules, High-Impedance Buffer and Amplifier Stage.
Photodiode Transimpedance Amplifier
The OPA129U fits photodiode front ends because its maximum input bias current of 100 fA limits the DC error generated across high-value feedback resistors. A 1 GΩ feedback resistor would accumulate approximately 100 nV of bias-current offset at the stated limit, before amplifier offset, resistor noise, and photodiode dark current are included. Place the amplifier in the standard inverting transimpedance topology and use the device's 1 MHz gain-bandwidth product and 2.5 V/µs slew rate for moderate-speed optical signals. The main design trade-off is that capacitance compensation must be optimized around the total photodiode and board capacitance rather than assuming the amplifier is a high-speed part.
Recommended
Electrochemical and pH Sensor Interface
Electrochemical probes and pH sensors present very high source impedance, making input bias current the dominant amplifier error term. The OPA129U's 100 fA maximum bias current is therefore more useful than its 1 MHz bandwidth when designing a high-impedance noninverting or transimpedance input stage. Use guarding, clean connectors, and contamination-resistant PCB surfaces to prevent board leakage from overwhelming the specified input current. The 2 mV maximum input offset voltage remains relevant at the amplifier output, so it should be nulled or subtracted when the sensor output is evaluated against a small full-scale range.
Recommended
Precision Current-to-Voltage Conversion
The OPA129U is appropriate for measuring currents in the picoamp-to-nanoamp range where the feedback resistor must be extremely large. Its 100 fA maximum input bias current reduces the systematic error current relative to conventional bipolar-input amplifiers, while the 94 dB minimum open-loop gain supports high-gain feedback configurations. The trade-off is that resistor Johnson noise rises with the square root of resistance, so increasing feedback resistance to reduce bias-current error can eventually make resistor noise more important than leakage. Select the largest resistor that preserves adequate bandwidth, and include the resistor's temperature coefficient in the calibration plan.
Recommended
Capacitive Touch and Proximity Sensing
Capacitive sensors benefit from an input amplifier that does not itself disturb the sensed charge. The OPA129U's 100 fA maximum bias current minimizes the current drawn through high-value sensor bias paths, while the single 8-pin SOIC package simplifies analog board construction. Use the amplifier with the sensor's recommended charge-transfer or resistive network and account for the 2.5 V/µs slew rate when defining acquisition time. Layout quality remains critical: long input traces, contaminated surfaces, and nearby digital switching nodes can add leakage and parasitic capacitance that exceed the amplifier's own input errors.
Recommended
Laboratory Instrumentation and Logarithmic Modules
The OPA129U supports scientific instrument modules that log small currents across a broad measurement range, including log-amplifier front ends referenced in the manufacturer's application material. The 100 fA maximum bias current prevents the amplifier from becoming the limiting input error term at the low-current end of the scale. Its 1 MHz gain-bandwidth product and 2.5 V/µs slew rate are sufficient for many low-frequency logarithmic sensing loops, but the closed-loop feedback network must remain stable at every gain stage. Calibration should account for the 2 mV maximum offset and 10 µV/°C maximum offset drift.
Recommended
High-Impedance Buffer and Amplifier Stage
The OPA129U can buffer high-impedance sources before a subsequent measurement or control stage, reducing the loading imposed on the source by the input resistance and bias current. With a maximum bias current of 100 fA, it is useful where the source is a high-value sensor, reference junction, or leakage-sensitive electrochemical element. The 94 dB minimum open-loop gain is beneficial in a unity-gain follower configuration, although the exact input common-mode range was not present in the supplied excerpts and must be confirmed. For this use, guard the input, keep the layout symmetrical, and verify stability when driving capacitive cable or probe loads.
Recommended
Recommended Products Summary
Engineering reference data for OPA129U — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | OPA129UE4 | OPA124UAG4 | OPA277UA | OPA227UA |
|---|---|---|---|---|---|
| Brand | Texas Instruments | Texas Instruments | Texas Instruments | Analog Devices | Analog Devices |
| Package | 8-pin SOIC | 8-pin SOIC | 8-pin SOIC | 8-pin SOIC | 8-pin SOIC |
Key Differentiators
- Exceptionally low input bias current for precision sensing (vs OPA277UA)
- Direct same-family drop-in sourcing (vs OPA129UE4)
- Precision offset characteristics verified for the target part (vs OPA124UAG4)
- Low-noise input stage avoids isolation-junction leakage (vs MC1458D)
Design Notes
Route the OPA129U input directly to the sensor or feedback node and keep the loop area small. For high-impedance designs, surround the input with a grounded guard where the application permits it, keep the guard clear of the input itself, and avoid running traces parallel to switching-node wiring. Since the OPA129U can limit bias current to 100 fA, PCB leakage and contamination at the same order of magnitude can otherwise erase the benefit of the Difet input stage. Cleaning the assembled board and excluding moisture-absorbing materials near the input are part of the electrical design, not optional manufacturing extras.
Account for total input capacitance, including sensor capacitance, package capacitance, PCB parasitics, and cable loading, when closing the transimpedance feedback loop. The 1 MHz gain-bandwidth product and 2.5 V/µs slew rate describe the OPA129U, not the usable closed-loop bandwidth of a high-value transimpedance network. Compensation components should be selected from the manufacturer's recommended feedback configurations and validated for stability; adding a large compensation capacitor without a stability analysis can produce ringing or oscillation that is not apparent from nominal gain and slew-rate figures.
Do not select the OPA129U purely from its 100 fA maximum bias-current rating and ignore the signal source. A 1 GΩ source impedance combined with 100 fA produces about 100 nV of possible offset, but the feedback resistor's thermal noise, input-voltage noise, offset voltage of up to 2 mV, and drift of up to 10 µV/°C may dominate the final error. Calculate the complete error budget at the operating temperature and frequency, and calibrate offset only after accounting for drift and noise. Also verify the actual common-mode range because it was not included in the supplied verified excerpts.
The OPA129U operates from ±5 V to ±18 V, but the analog supply rails still require local bypassing. Place the manufacturer's recommended bypass capacitors directly across the positive and negative supply pins, with the shortest possible ground return, and avoid routing supply current through the sensitive input area. Decoupling does not create additional precision beyond the specified bias current; it prevents supply noise from modulating the gain and preserves the low-noise operating behavior expected from the OPA129 precision-amplifier family.
Compliance Information
The supplied verified excerpts identify the OPA129U as an 8-pin SOIC operational amplifier but do not provide explicit RoHS, REACH, halogen-free, or conflict-minerals declarations. Compliance must be confirmed from the current manufacturer product page or supplier declaration.