LMP7721MA/NOPB: 3 fA Bias Precision Op Amp, 17 MHz GBW — Complete Engineer's Guide, Specs, Alternatives & Applications

The Texas Instruments LMP7721MA/NOPB is a precision CMOS operational amplifier with the industry's lowest specified input bias current — 3 fA typical, with guaranteed maximum limits of ±20 fA at 25°C and ±900 fA at 85°C. It delivers a 17 MHz gain-bandwidth product, 12.8 V/µs slew rate, 7.5 nV/√Hz input voltage noise, and a rail-to-rail output swing from a 1.8 V to 5.5 V single supply (±0.9 V to ±2.75 V dual) in an 8-pin SOIC package with a unique input-isolated pinout. The part is active in TI's portfolio, is RoHS-compliant (lead-free, /NOPB suffix), and is in stock at XAIPART with 99,999 units available, MOQ 1, priced from $3.03 at single-unit quantity down to $2.02 at 1,000 pieces as of 2026-09-04. Its femtoampere-level input makes it the definitive choice for pH electrodes, ion-selective sensors, picoammeter photodiode receivers, and battery-powered medical instruments where picoampere bias from ordinary op amps would corrupt measurements.

What Is the LMP7721MA/NOPB and Why Does Its 3 fA Input Bias Current Matter?

The LMP7721MA/NOPB is a single-channel general-purpose/precision operational amplifier from Texas Instruments. Its defining attribute is ultra-low input bias current: 3 fA typical, with the manufacturer guaranteeing no worse than ±20 fA at 25°C and ±900 fA at 85°C. TI achieves this through patented input bias current cancellation circuitry in the CMOS input stage.

Why does this matter? Sensors such as pH electrodes, ion-selective electrodes, and photodiodes present gigaohm-class source impedances. With a standard CMOS or bipolar op amp, even picoampere-level bias current polarizes electrodes or sets the dark-current noise floor of a photodiode receiver, corrupting the measurement. The LMP7721 keeps this error orders of magnitude below sensor resolution. Supporting specs reinforce the point: 10^13 Ω input impedance, 26 µV typical offset voltage, and 84 dB open-loop gain preserve accuracy at small electrode potentials and high closed-loop gains.

Other key verified specifications include:

Parameter | Value
Input bias current | 3 fA typical (±20 fA max at 25°C; ±900 fA max at 85°C)
Gain-bandwidth product | 17 MHz
Slew rate | 12.8 V/µs
Supply voltage | 1.8 V to 5.5 V single; ±0.9 V to ±2.75 V dual
Output type | Rail-to-rail
Input voltage noise | 7.5 nV/√Hz
Open-loop gain | 84 dB
Offset voltage | 26 µV typical
Input impedance | 10^13 Ω
Quiescent current (shutdown) | 40 µA
Operating temperature | -40°C to +125°C
Package | 8-SOIC (0.154 in, 3.90 mm width), isolated pinout

How Do You Design In the LMP7721MA/NOPB for Femtoampere Performance?

Designing with the LMP7721 is straightforward electrically but demands care on the PCB, because at the 3 fA level, board leakage — not the amplifier — becomes the dominant error source.

Understanding the Isolated Pinout

The LMP7721 uses a unique SOIC-8 pinout that physically isolates the input pins (2 and 3) from the output (pin 1) and supply pins (4 and 7), with pins 6 and 8 designated as no-connect for isolation spacing. According to the TI datasheet, this arrangement prevents PCB surface leakage current from reaching the input pins and enables effective guard-ring routing. Standard-pinout SOIC-8 op amps are not drop-in compatible — this is a critical point when evaluating second sources or rework.

Pin | Name | Function
1 | OUT | Amplifier output (moved to pin 1 in isolated pinout)
2 | IN+ | Non-inverting input (guarded / isolated)
3 | IN− | Inverting input (guarded / isolated)
4 | V− | Negative supply
5 | SD | Shutdown control
6 | NC | No connect (isolation spacing)
7 | V+ | Positive supply
8 | NC | No connect (isolation spacing)

PCB Layout Best Practices

To achieve the full 3 fA performance, the TI datasheet and application guidance recommend:

Implement guard rings driven at the input potential around pins 2 and 3.

Keep input traces short and suspended over grounded keep-out areas.

Use low-leakage board materials and careful flux removal (no-clean-free flux practices).

Without these precautions, PCB leakage — often in the picoampere range — will dominate the input current error and waste the amplifier's femtoampere-class bias specification.

Power and Shutdown

The device operates from 1.8 V to 5.5 V single supply, allowing direct operation from two alkaline cells or a single Li-ion cell without a boost converter. The SD pin (pin 5) enables a shutdown mode that reduces supply current to 40 µA — ideal for duty-cycled battery instruments. The extended -40°C to +125°C temperature range supports outdoor and industrial field equipment.

Bandwidth and Transimpedance Design

In transimpedance amplifier (TIA) configurations, the 17 MHz GBW allows megohm feedback resistors with usable bandwidth, and the 7.5 nV/√Hz voltage noise supports stable operation with large feedback resistances. The 12.8 V/µs slew rate handles fast transient events without slew-induced distortion — useful when buffering piezoelectric, capacitive, or pyroelectric sensors, and when switching current ranges in picoammeter front ends. [VERIFY_NEEDED: specific feedback resistor stability compensation values for particular capacitances]

What Are the Best Drop-In Alternatives and How Does the LMP7721 Compare?

The alternatives below come from the verified XAIPART comparison data. Note the recurring theme: nothing with a standard SOIC-8 pinout is a true drop-in for this part.

Component | Relationship to LMP7721MA/NOPB | Drop-In?
LMP7721MA/NOPB-Q1 | Same die and pinout, AEC-Q100 automotive qualified | Yes
LMP7721MAX/NOPB | Same die, tape-and-reel packaging (MA/NOPB ships in tube) | Yes
OPA380AID | Standard SOIC-8 pinout, transimpedance-optimized, picoampere-class bias (not femtoampere) | No
LMP8350MA/NOPB-Q1 | Precision instrumentation amplifier family member, different function and pinout | No
LMC6001AIN | Ultra-low 25 fA bias CMOS op amp, standard pinout, only 1.3 MHz GBW | No
AD549LH | Cross-brand electrometer-grade op amp, ~60 fA bias class, standard pinout | No

Key comparisons anchored to verified data:

LMP7721 vs LMC6001: The LMP7721 is better for picoammeter circuits. Its 3 fA typical bias with a ±20 fA guaranteed limit at 25°C far exceeds the LMC6001's 25 fA typical / 10 pA max specification. The LMP7721 also offers 17 MHz GBW versus the LMC6001's 1.3 MHz, plus the leakage-isolated pinout — though it draws more quiescent current.

LMP7721 vs OPA380: Choose the LMP7721 when ultra-low bias current is the priority (pH probes, ion-selective electrodes, picoampere photodiode sensing): 3 fA versus the OPA380's picoampere-class bias. Choose the OPA380 when you need a faster, internally compensated TIA with 80 MHz bandwidth for high-speed photodiode applications. They are not pin-compatible.

Can AD8628 replace the LMP7721? No. AD8628's 10 pA maximum bias current is over 3000× higher than the LMP7721's 3 fA, and it is not pin-compatible — unsuitable for femtoampere electrochemical measurements.

Other TI equivalents: Within TI's portfolio, the LMC6001 (25 fA typical) and OPA129 (100 fA, precision picoammeter amplifier) are ultra-low-bias alternatives, and the LMP7721MA/NOPB-Q1 is the automotive-qualified version of this exact die. Cross-brand, ADI offers the AD549 (electrometer grade). None are pin-compatible with the isolated pinout.

What Applications Benefit Most From the LMP7721MA/NOPB?

Electrochemical Sensor Front Ends

The LMP7721's 3 fA bias and 10^13 Ω input impedance make it the definitive buffer for pH, dissolved-oxygen, gas, and ion-selective electrodes with gigaohm source impedances. Placed directly at the electrode as a unity-gain buffer, its rail-to-rail output swings within millivolts of the 1.8 V to 5.5 V rail, and the isolated SOIC-8 pinout permits guard rings that suppress board leakage.

Photodiode / Transimpedance Amplifiers

In picoammeter-grade photodiode receivers, the TIA's input bias current sets the dark-current noise floor. The 3 fA bias and 17 MHz GBW allow megohm feedback resistors with usable bandwidth, enabling optical power detection down to the femtoampere range in optical power meters, fluorescence detectors, and smoke detectors.

Medical Instrumentation

Glucose monitors, blood gas analyzers, and biochemical sensors measure nanoampere-to-femtoampere electrochemical currents. The ±20 fA guaranteed limit at 25°C delivers the required accuracy; 26 µV offset preserves resolution at small electrode potentials; 1.8 V operation and 40 µA shutdown current suit portable and wearable devices.

High-Impedance Buffers and Portable Instruments

Piezoelectric, capacitive, and pyroelectric sensors benefit from the 10^13 Ω input impedance and 3 fA bias preserving charge integrity, while the 17 MHz GBW and 12.8 V/µs slew rate handle fast transients. Handheld pH meters, gas detectors, and water-quality testers can run directly from two alkaline cells or one Li-ion cell, and with a low-power MCU, the analog front end can operate for months on primary cells.

Test and Measurement / Picoammeter Front Ends

Bench picoammeters, electrometers, and semiconductor parametric analyzers demand femtoampere-class input stages. The ±20 fA specified limit, 84 dB open-loop gain for linearity in high-gain shunt configurations, and the isolated pinout with guard-ring layout make it suitable as the input electrometer amplifier, with the 17 MHz GBW supporting fast current-range switching without long settling tails.

What Is the LMP7721MA/NOPB's Lifecycle Status and Supply Situation?

According to verified lifecycle data, Texas Instruments lists the LMP7721MA/NOPB as active, so production supply is ongoing. The part is in stock at XAIPART with 99,999 units available, MOQ 1, and ships today from major distributors including DigiKey and LCSC as of 2026-08-30. Verify real-time availability on distributor pages before placing volume orders, as stock and pricing fluctuate.

Pricing as of 2026-09-04:

Quantity | Unit Price
1+ | $3.03
10+ | $2.75
100+ | $2.48
500+ | $2.24
1000+ | $2.02

[DATA_NEEDED: long-term lifecycle forecast / last-time-buy date from TI product page]

What Should Buyers and Engineers Watch Next for Ultra-Low-Bias Amplifiers?

Anchored to the verified data, buyers should track several factors. First, the automotive-qualified LMP7721MA/NOPB-Q1 (same die and pinout, AEC-Q100) extends this part into vehicle-grade programs — if your product roadmap touches automotive, qualify the Q1 variant early. Second, packaging choice matters for volume production: the LMP7721MAX/NOPB ships in tape-and-reel versus the tube-packaged MA/NOPB, reducing assembly handling at volume. Third, the unique isolated pinout means any second-sourcing strategy must account for non-pin-compatible alternatives (LMC6001, OPA380, AD549LH) — plan guard-ring layouts around the LMP7721 footprint specifically, and budget for layout changes if qualifying alternatives. Fourth, the extended -40°C to +125°C range plus the 40 µA shutdown current position this part well for portable and field instruments; verify the ±900 fA maximum at 85°C fits your hot-environment accuracy budget when designing enclosures. Finally, monitor pricing across DigiKey, Mouser, and LCSC before volume orders — the verified XAIPART tier drops from $3.03 to $2.02 per unit at 1,000 pieces as of 2026-09-04.