The STMicroelectronics LMX321 is a single-channel, low-power operational amplifier for low-voltage analog signal paths. It is useful when a design needs ground-sensing inputs, an output that approaches both supply rails, and modest bandwidth without a large quiescent-current penalty.
This guide explains what the headline specifications mean in a real circuit, where the device fits well, and which limits should be checked before releasing a board.
Important: more than one manufacturer has used the LMX321 identifier. This guide covers the STMicroelectronics LMX321 family documented in ST datasheet DocID022943.
LMX321 key specifications
- Single general-purpose operational amplifier
- Supply range: 2.3 V to 5.5 V
- Typical supply current: 120 uA per channel at 2.7 V
- Gain-bandwidth product: 1.3 MHz
- Typical slew rate: 0.6 V/us at 2.7 V and approximately 0.7 V/us at 5 V
- Input common-mode range includes the negative rail and ground in a single-supply circuit
- Rail-to-rail output stage
- Unity-gain stable
- Operating temperature range: -40 degrees C to +125 degrees C
- Available as LMX321ILT in SOT23-5 and LMX321ICT in SC70-5
These numbers place the LMX321 between micropower amplifiers intended for very slow measurements and faster op amps that consume substantially more current. It is a practical fit for sensor conditioning, portable instrumentation, battery-powered equipment, active filtering, and low-frequency control loops.
Understanding the input range
The input common-mode range extends from slightly below the negative supply to approximately 1 V below the positive supply. With a 3.3 V single supply, inputs near ground are valid, but a signal approaching 3.3 V is outside the guaranteed common-mode range.
This distinction matters in high-side current sensing and sensors whose output rises close to the supply rail. A rail-to-rail output specification does not imply a rail-to-rail input stage. Check the maximum expected sensor voltage, supply tolerance, and transient conditions together.
The device is specified with no phase reversal when the input remains within the documented operating conditions. That behavior is useful in control and monitoring circuits, but input protection and series resistance are still required when a signal may exceed the supply rails.
What rail-to-rail output means here
The output can move close to both rails, but it does not reach an ideal 0 V or VCC under every load. ST specifies a maximum distance of about 180 mV from a rail with a 10 kohm load connected to mid-supply under the stated test conditions. Heavier loads reduce the available output swing.
For an ADC driver, calculate the required minimum and maximum output voltages before selecting gain. Leave margin for output loading, temperature, component tolerances, and the ADC input network. If the ADC uses a switched-capacitor input, a small isolation resistor and local capacitor may be needed to keep sampling transients away from the amplifier output.
Bandwidth and settling behavior
The 1.3 MHz gain-bandwidth product is sufficient for many low-frequency sensors and filters, but closed-loop bandwidth falls as noise gain increases. A useful first estimate is:
Closed-loop bandwidth approximately equals 1.3 MHz divided by noise gain
For example, a non-inverting gain of 11 gives a first-order bandwidth estimate near 118 kHz. Real performance also depends on source impedance, feedback components, output loading, and required phase margin.
Slew rate places a separate limit on large-signal behavior. At 0.7 V/us, a 2 V output step cannot complete faster than roughly 2.9 us even before small-signal settling is considered. Low-distortion sine-wave amplitude and frequency should therefore be checked against both gain bandwidth and slew rate.
Three practical applications
1. Ground-referenced sensor buffer
Use the LMX321 as a voltage follower when a resistive or analog sensor produces a slow signal near ground and the next stage needs lower source impedance. The unity-gain-stable architecture supports this configuration directly.
Confirm that the sensor's highest output remains at least about 1 V below the positive rail. Also include the amplifier's input bias current and source resistance when estimating offset error.
2. Non-inverting amplifier
For a ground-referenced signal, the familiar gain equation applies:
Gain = 1 + RF / RG
Choose resistor values that do not load the signal source excessively and are not so large that bias-current error and board leakage dominate. Verify that the amplified signal stays inside both the input common-mode range and the achievable output swing.
3. Low-pass active filter
The device can implement low-frequency Sallen-Key or multiple-feedback filters for sensor noise reduction. Keep the filter corner comfortably below the available closed-loop bandwidth. Capacitive loading affects phase margin, so compare the intended capacitance with the datasheet curves and add output isolation if bench testing shows ringing.
PCB layout checklist
- Place the supply bypass capacitor next to the power pins. ST specifically advises a 10 nF decoupling capacitor close to the device.
- Keep the inverting-input and feedback loop traces short.
- Route noisy digital clocks and switching nodes away from both inputs.
- Give low-level sensor signals a clean ground return instead of sharing a high-current path.
- Minimize leakage around high-impedance inputs; contamination can matter when resistor values are large.
- Test output stability with the real cable, ADC input, and capacitive load attached.
- Validate operation at minimum and maximum supply voltage, not only at nominal voltage.
LMX321 versus LMX358 and LMX324
ST uses the same LMX3xx architecture across three channel counts:
- LMX321: one amplifier in a compact five-pin package
- LMX358: two amplifiers for paired signal channels
- LMX324: four amplifiers when board density matters
Using the dual or quad version can reduce component count, but shared supply routing and channel interaction still need attention. The single LMX321 is often easier to place directly beside a sensor or ADC input.
When to choose a different op amp
Consider another device when the design requires an input range that reaches the positive rail, substantially higher bandwidth, faster large-signal settling, lower input offset, lower noise, or a shutdown pin. ST points designers to the TSV85x family when higher accuracy, standby capability, or smaller package options are priorities.
Do not substitute parts only because the package and first-page specifications look similar. Compare input common-mode range, output swing at the actual load, offset over temperature, bias current, stability with capacitance, supply current, ESD requirements, and package pinout.
Design review summary
The LMX321 is a sensible low-power choice for slow-to-moderate analog signals on 2.3 V, 3.3 V, or 5 V rails. Its strongest advantages are low current, ground-sensing inputs, compact packages, unity-gain stability, and an output stage that approaches both rails. Its main design constraint is the upper input common-mode limit, which remains about 1 V below VCC.
Before production, verify all limits against the latest manufacturer documentation and test the circuit across supply, load, and temperature corners.