OPA129-W - 100 fA Ultra-Low Bias Current Op Amp | Texas Instruments
MPN: OPA129-W ✓ NakahandaOPA129-W Ulas
An operational amplifier is a high-gain differential amplifier whose inverting and non-inverting inputs are compared and whose output drives the difference between them, typically with feedback to set closed-loop gain. The OPA129 belongs to the hierarchy low-bias op amp -> precision op amp -> voltage-feedback amplifier -> amplifier IC -> integrated circuit. Unlike bipolar input stages whose bias current rises with temperature and with resistor mismatch, the Difet dielectrically-isolated FET input stage draws essentially constant current from the supply, which is the defining characteristic engineers select it for.
Key features of the OPA129 family include 100 fA maximum input bias current, 2 mV maximum input offset voltage, 10 uV/C maximum offset drift, a minimum open-loop gain of 94 dB, and low noise performance characterized at 10 kHz. Because bias current is the dominant error term in high-impedance front ends, the 100 fA rating translates to an input impedance in the hundreds of teraohms and to negligible loading on high source impedances.
The part is a monolithic device offered in 8-pin PDIP and SO-8 packages, with the OPA129-W suffix designating the SO-8 option. Because the input stage is dielectrically isolated, the circuit is not sensitive to surface leakage in the same way a guard-ring-free bipolar or MOSFET-input design can be, and the part is insensitive to temperature cycling effects that degrade ordinary junction FETs. The amplifier is usable on split supplies as well as single-supply operation subject to the datasheet rail limits.
Typical applications include photodiode and phototransistor transimpedance amplifiers, pH and chemical sensor instrumentation, picoammeter and electrometer front ends, precision current-to-voltage converters, high-impedance sensor interfaces, low-level current sensing, and capacitance-to-voltage front ends for touch and liquid-crystal sensing.
The dominant design consideration is guarding and layout: because the part is specified to 100 fA, board contamination, flux residue, and humidity leakage across the input pins can exceed the device's own bias current. Keep input traces short, clean, and free of solder-flux residue, and use guarding where the operating environment is not controlled.
This page synthesizes distributor availability, verified datasheet parameters, drop-in replacement candidates, and practical layout guidance for the OPA129-W, providing engineering context beyond the raw parameter table found on standard distributor listings.
Mga alternatibo na pwedeng ilagay na OPA129-W — parehas na package, pin-to-pin na katugmang. Hindi kasama ang mga bahagi ng iba't-ibang package na nangangailangan ng pagpapaayos ng PCB.
OPA129UA
✅ Drop-In ⚠️ Mga Talaan Hindi Naiwerikmo📋 Alternatibo na Reference (hindi sa katalogo)
OPA140UA
✅ Drop-In ⚠️ Mga Talaan Hindi Naiwerikmo📋 Alternatibo na Reference (hindi sa katalogo)
OPA627UA
✅ Drop-In ⚠️ Mga Talaan Hindi Naiwerikmo📋 Alternatibo na Reference (hindi sa katalogo)
AD549
✅ Drop-In📋 Alternatibo na Reference (hindi sa katalogo)
AD8613
✅ Drop-In📋 Alternatibo na Reference (hindi sa katalogo)
OPA129-W Mga Pinakamataas na Rating at Electrical Characteristics
| Input Bias Current | 100 fA maximum |
| Input Offset Voltage | 2 mV maximum |
| Input Offset Voltage Drift | 10 uV/C maximum |
| Open-Loop Gain | 94 dB minimum |
| Input Stage Technology | Difet dielectrically-isolated FET |
| Number of Channels | 1 (single amplifier) |
| Number of Circuits | 1 |
| Packhaging | 8-pin SOIC (SO-8), surface mount |
| Also Offered In | 8-pin PDIP (OPA129U) |
| Noise Characterization Frequency | 10 kHz |
| Tindminan RoHS | Compliant |
| Uri ng Pagpapasok | Surface Mount |
Pin Configuration ng OPA129-W
| Pin 1 | OS — Offset voltage null (trim) pin |
| Pin 2 | -IN — Inverting input |
| Pin 3 | +IN — Non-inverting input |
| Pin 4 | V- — Negative supply connection |
| Pin 5 | NC — No internal connection (offset null trim option) |
| Pin 6 | OUT — Output |
| Pin 7 | V+ — Positive supply connection |
| Pin 8 | NC — Not connected (per datasheet) |
Tipikal na mga Gamit
OPA129-W ay nararapat para sa 6 aplikasyons: Photodiode Transimpedance Amplifier, Picoammeter and Electrometer Front End, High-Impedance Chemical and pH Sensor Interface, Precision Current-to-Voltage Conversion, Capacitance-to-Voltage Touch and Liquid Sensing, Precision Instrumentation and Test Equipment.
Photodiode Transimpedance Amplifier
The OPA129-W is a natural fit for photodiode and solar-cell front ends because its 100 fA maximum input bias current places negligible error on the sense node while its 94 dB minimum open-loop gain holds transimpedance accuracy at high closed-loop gains. It is placed in the classic inverting configuration with the photodiode at the summing node and the feedback resistor defining the conversion gain. Compared with a bipolar-input amplifier whose bias current is typically in the nanoamp range, the Difet input stage removes the dominant error term and extends usable source impedance into the teraohm range. The practical trade-off is that the design remains sensitive to board leakage and stray capacitance, so guarding, clean assembly and a tight feedback layout are required.
Inirekomenda
Picoammeter and Electrometer Front End
Electrometer-style measurement of femtoamp and picoamp currents relies on amplifiers whose input bias current is smaller than the signal being measured, which is exactly the OPA129-W specification of 100 fA maximum. It is used as the first stage of a guarded current-to-voltage converter feeding a higher-gain or higher-accuracy second stage. Because the offset drift is limited to 10 uV/C, gain error over temperature stays predictable, and the 2 mV maximum input offset voltage keeps calibration residual low. Designers must still compensate for surface leakage with guarding and for stray capacitance with a compensation capacitor in the feedback loop to preserve stability in the high-gain transimpedance configuration.
Inirekomenda
High-Impedance Chemical and pH Sensor Interface
pH probes, ion-selective electrodes, humidity and capacitive sensors present source impedances in the hundreds of megohms to gigaohms, where input bias current directly becomes a measurement error. The OPA129-W's 100 fA maximum bias current means a 1 Gohm source produces well under 0.1 pA of error, keeping the readout dominated by the sensor rather than the amplifier. The device is used in a non-inverting or unity-gain buffer configuration ahead of an ADC, and its 94 dB minimum open-loop gain preserves signal amplitude into the converter. Key considerations are guarding the input, providing a driven shield or guard ring on humid boards, and ensuring the ADC front end does not leak more current than the amplifier itself.
Inirekomenda
Precision Current-to-Voltage Conversion
In shunt-based and current-sense measurement paths, the OPA129-W provides the very low bias current needed to avoid loading the shunt while offering the open-loop gain to hold the conversion accurate over temperature. It is commonly used in a transimpedance stage ahead of an ADC for 4-20 mA loop receivers or battery-monitoring shunt amplifiers. The 10 uV/C maximum offset drift limits temperature-induced conversion error, and the 2 mV maximum offset voltage allows a trimmed zero reference at the ADC stage. Because the amplifier does not add switching ripple, it is also used downstream of a switching converter where ripple would otherwise contaminate the measurement.
Inirekomenda
Capacitance-to-Voltage Touch and Liquid Sensing
Capacitive touch, proximity and liquid-level sensors are usually detected by converting a small change in capacitance into a voltage, and the front-end amplifier sets the accuracy of that conversion. The OPA129-W's 100 fA maximum bias current means the capacitive sensor node is effectively unloaded, so the sensitivity is determined by the sensor rather than by the amplifier's input impedance. It is used in an oscillator or charge-transfer front end followed by a buffer stage, where its low offset and low drift help keep the trip thresholds stable. Layout care is essential: keep the sensitive node physically small, guard it, and place the amplifier close to the sensor to minimise parasitic capacitance that would otherwise be indistinguishable from the target capacitance.
Inirekomenda
Precision Instrumentation and Test Equipment
Laboratory and field instrumentation frequently needs an amplifier that does not disturb the node it measures, which is why the OPA129-W appears in buffer stages for high-impedance probes, dielectric measurement setups and low-level current meters. Its 94 dB minimum open-loop gain and low drift let the buffer maintain signal integrity across a wide DC range, while the ultra-low bias current keeps probe loading negligible. In a typical design it sits between a guarded probe and a downstream ADC or meter, with a selectable gain network following. Designers must budget board-area cleanliness and optional guarding when building instrument-grade front ends, since contamination leakage can exceed 100 fA on a non-clean PCB.
Inirekomenda
Buod ng mga Inirekomendang Produkto
Impormasyon ng Talumpati sa Engineering for OPA129-W — ikabit, gabay sa disenyo, at impormasyon ng pagsunod sa kautusan.
Paunawang Hanapin
Ikabit sa Mga Alternatibo
| Parametro | Ito ang Producto | OPA129UA | OPA140UA | OPA627UA | AD549 | AD8613 |
|---|---|---|---|---|---|---|
| Package | SOIC-8 (SO-8) | SOIC-8 (SO-8) - same | SOIC-8 (SO-8) - same | SOIC-8 (SO-8) - same | SOIC-8 (SO-8) - same | SOIC-8 (SO-8) - same |
| Brand | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments | Analog Devices | Analog Devices |
| Number of Channels / Circuits | 1 / 1 | 1 / 1 | 1 / 1 | 1 / 1 | 1 / 1 | 1 / 1 |
Pangunahing mga Pagkakaiba
- Ultra-low 100 fA maximum input bias current with a Difet input stage (kumpara saOPA140UA)
- Tighter input offset voltage and drift ratings (kumpara saAD8613)
- High guaranteed open-loop gain for stable closed-loop accuracy (kumpara saAD549)
- Single-source family continuity and identical footprint for family members (kumpara saOPA627UA)
Talaan ng Disenyo
The OPA129-W is specified to 100 fA maximum input bias current, which is smaller than the leakage produced by many contaminated or humid PCBs. Keep the input traces short, route them away from high-impedance switching nodes, and avoid solder-flux residue around pins 2 and 3. Where the operating environment is not controlled, add a guard ring driven from the amplifier output around the input pads; this holds the input node at a defined potential and cancels surface leakage. The assembly process should include a cleaning step after soldering.
Do not assume a pin-compatible amplifier is an electrical equivalent. When substituting AD549 or AD8613 for the OPA129-W, verify offset-null pin availability, input common-mode range against the actual supply rails, and output swing capability. The OPA129 datasheet shows the offset null arrangement on the 8-pin package; if the original board wires a trim potentiometer to a specific pin and the substitute does not provide the same function, the trim must be removed or re-routed. Always re-verify stability with the actual feedback network.
In transimpedance configurations the OPA129-W is typically compensated with a small capacitor in parallel with the feedback resistor. Because the input capacitance of the photodiode plus PCB parasitic can be several tens of picofarads, the compensation capacitor value depends on the closed-loop resistor and the amplifier's gain-bandwidth product, which should be taken from the manufacturer datasheet rather than estimated. Keep the feedback resistor and its compensation capacitor within a few millimetres of pins 2 and 6 to minimize parasitic feedback-loop inductance.
The OPA129 is a dual-supply-capable precision amplifier; the exact rail limits must be taken from the manufacturer datasheet before design sign-off. Because the input stage is dielectrically isolated rather than junction-based, the device is less sensitive to temperature cycling than ordinary MOSFET inputs, which is one reason it remains specified for long-life measurement equipment. Decouple the supply pins with a short, low-inductance path to the ground plane and, in mixed-signal systems, feed the amplifier from a clean analog rail to avoid injecting supply ripple into a picoamp-level measurement.
Impormasyon ng Pagsunod sa Kautusan
RoHS compliance and lead-free termination are indicated by the manufacturer product and distributor listings. No explicit REACH, halogen-free or conflict-minerals statement was present in the retrieved data, and the part is a general-purpose precision amplifier rather than an automotive-qualified device, so AEC-Q100 does not apply.
