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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Yes—an LM358 can be configured as a basic current-sense amplifier by measuring the voltage across a shunt resistor and amplifying it. It is usually a practical choice for low-side, one-direction DC measurements, but it is not rail-to-rail or a precision current-sense IC. High-side sensing, bidirectional measurement, very small shunt voltages, fast protection, and safety-critical designs generally call for a dedicated current-sense amplifier.
How an LM358 current-sense circuit works
A shunt resistor is placed in series with the load. Load current creates a small differential voltage across that resistor:
VSHUNT = ILOAD × RSHUNT
An LM358 section amplifies the voltage for an ADC, meter, control loop, or comparator:
VOUT ≈ ILOAD × RSHUNT × G
That relationship is only approximate in a real circuit. Shunt tolerance and heating, amplifier offset, resistor tolerance, bias current, noise, common-mode limits, and output saturation all affect the result. ST describes current-sense amplifiers as differential amplifiers whose output represents the voltage developed across a load-side shunt (ST current-sensing overview).
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Choose low-side or high-side sensing first
Low-side sensing
For the simplest LM358 design, connect the circuit like this:
Supply + ─── Load ─── RSHUNT ─── Ground
│
Sense voltage
Both shunt terminals are near ground, so a single-supply LM358 can usually handle their common-mode voltage. The trade-off is that the load ground rises by the shunt voltage. A ground-referenced circuit connected to the load can therefore see a small, current-dependent ground shift. A load short directly to ground may also bypass the shunt and escape detection. ST and TI discuss these low-side/high-side trade-offs in their sensing guidance (ST electrical sensing; TI current-sense topology note).
High-side sensing
Here the shunt is between the supply and load:
Supply + ─── RSHUNT ─── Load ─── Ground
│
Sense voltage
The load remains directly grounded, and a short from the load to ground can be detected. However, both amplifier inputs may sit close to the supply rail. The LM358 input common-mode range includes the negative rail but does not extend to the positive rail, and its output also needs positive-side headroom. A conventional difference-amplifier schematic is therefore not automatically valid on a 12 V or 24 V rail, especially when the LM358 is powered from 5 V. Check the selected device’s common-mode and absolute-maximum ratings (ST LM358 specifications; TI high-side sensing guidance). For high common-mode voltage, use a purpose-built current-sense amplifier.
Recommended circuit: low-side, unidirectional sensing
Use one LM358 section as a non-inverting amplifier:
+V
│
Load
│────────────── LM358 (+)
RSHUNT
│
GND
LM358 (−) ── RG ── GND
│
└── RF ── LM358 output
The gain is:
G = 1 + (RF/RG)
Consequently:
VOUT ≈ ILOADRSHUNT(1 + RF/RG)
Connect the shunt’s load-side terminal to the non-inverting input. Keep the feedback network connected exactly as shown; reversing the shunt leads produces a negative-going signal that a single-supply circuit cannot represent.
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Calculate the shunt, gain, and power
Worked 5 A example
Suppose the maximum current is 5 A, the shunt is 10 mΩ, and the desired output at maximum current is about 1 V.
- Shunt voltage: 5 A × 0.010 Ω = 50 mV.
- Required gain: 1 V / 50 mV = 20.
- Feedback values: RG = 10 kΩ and RF = 190 kΩ give G = 20.
- Shunt dissipation: I²R = 5² × 0.010 = 0.25 W.
A nominal 0.25 W resistor would be operating at its rating. Select a purpose-built shunt with thermal and pulse margin, and account for PCB heat spreading, tolerance, and temperature coefficient.
General design equations
Choose the shunt from the allowed voltage drop:
RSHUNT = VSHUNT,max / IMAX
Then calculate heating:
PSHUNT = IMAX²RSHUNT
For a target maximum output:
G = VOUT,max / (IMAXRSHUNT)
Finally choose RF = RG(G − 1), recalculate the gain using the actual standard values, and leave output headroom below the positive supply. A larger shunt improves signal-to-offset ratio but wastes more power and drops more voltage; a smaller shunt reduces loss but makes offset, noise, layout, and ADC resolution more important.
Shunt construction and PCB layout
- Use a current-shunt resistor, not an arbitrary small resistor, when current or accuracy is significant.
- Verify continuous and pulse ratings, temperature rise, tolerance, and temperature coefficient.
- Use Kelvin connections: take two separate sense traces directly from the shunt terminals.
- Route the sense traces symmetrically and away from high-current paths and switching nodes.
- Do not let one input return through a different ground-current path than the other.
- Place supply bypass capacitors close to the LM358 supply pins.
At low current, thermoelectric voltages, copper resistance, connector resistance, and ground drops can be comparable to a millivolt-level shunt signal.
LM358 limitations that set the usable range
Input common-mode voltage
The LM358 can sense down to its negative rail in appropriate conditions, but its input range does not include the positive rail. A 5 V LM358 therefore cannot accurately sense inputs sitting at 5 V. Even with a 12 V supply, do not assume accurate operation at the full 12 V rail. Check the exact manufacturer’s limits over supply voltage and temperature in the LM358 datasheet.
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Output swing and headroom
The output can approach ground more closely than the positive rail, but it normally stops short of the positive supply. Design for:
VOUT,max < VCC − VHEADROOM
Headroom depends on output load, temperature, supply voltage, and the specific LM358 grade. With a 3.3 V ADC, do not target a nominal 3.3 V output at maximum current without margin.
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Offset voltage
Input offset is especially damaging when the shunt signal is small. As a first estimate:
Offset error (%) ≈ VOS / VSHUNT × 100
For example, 2 mV of effective offset against a 50 mV shunt signal is about 4% before other errors. TI documents different offset limits for standard and B/BA LM358 variants; use the selected part number’s maximum, not a generic typical value.
Bias current and resistor values
Input bias current flowing through source or feedback resistance creates another voltage error. Very large feedback resistors also increase noise pickup, leakage sensitivity, and settling time. Choose values low enough that bias-current error is insignificant relative to the required accuracy.
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- Internal frequency compensation.
- The DC voltage gain is high (about 100dB).
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- Wide current and voltage range: single supply (3-30V).
Bandwidth
ST lists approximately 1.1 MHz unity-gain bandwidth for the LM358 product family (ST product information). A rough closed-loop estimate is:
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At gain 20, usable closed-loop bandwidth is far below 1.1 MHz and varies with load, compensation, and device. The LM358 can suit DC and slowly changing current. PWM measurement requires deciding whether you want instantaneous, average, or peak current. Fast overcurrent shutdown is usually better handled by a comparator or dedicated current-sense device.
High-side difference amplifier: what must be checked
A high-side design may use a difference amplifier:
VOUT = (R2/R1)(VSENSE+ − VSENSE−)
That equation assumes accurately matched resistor ratios. Four nominally equal resistors are not automatically a precision difference amplifier. Common-mode voltage, LM358 offset and bias current, input protection conduction, output swing, and PCB leakage also matter. The op-amp inputs must remain within their common-mode and absolute-maximum limits; powering the LM358 from a lower rail than the shunt voltage makes the circuit invalid. ST’s high-side application note explains why high-common-mode sensing commonly uses a dedicated architecture (ST AN4835).
Bidirectional current measurement
The basic low-side circuit measures only one current polarity. For bidirectional sensing, bias the output around a stable reference:
VOUT = VREF + ILOADRSHUNTG
Depending on polarity, the output moves above or below VREF. Use a buffered midpoint or other low-impedance reference, keep the output inside the LM358 swing range, and verify the input differential voltage. Offset near zero current can dominate the reading, so calibration is often required. Dedicated bidirectional parts such as TI’s INA181 are generally easier when zero-current accuracy matters.
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Filtering, protection, and PWM measurements
A first-order low-pass has an approximate cutoff of:
fC = 1/(2πRC)
Filter both sides of a differential input with matched RC networks. Filtering only one input converts common-mode switching noise into differential error. Keep the cutoff above the current waveform bandwidth you need, and check capacitor leakage and DC error.
- Instantaneous current: retain enough bandwidth and use careful layout.
- Average current: add a suitable low-pass filter or average digitally.
- Peak current: use a faster amplifier, peak detector, comparator, or controller feature.
- Add input series resistance or clamping only after checking that protection components do not distort the shunt signal.
- Provide a separate fast comparator path when an overcurrent event must disable hardware promptly.
A practical design workflow
- Define minimum and maximum continuous and peak current, direction, accuracy, bandwidth, supply range, ADC range, and allowable shunt drop.
- Choose low-side for simple unidirectional sensing when a ground lift is acceptable; choose high-side when load-ground integrity or short-to-ground detection matters.
- Calculate shunt resistance and I²R heating, then add thermal and overload margin.
- Calculate gain from the ADC range, leaving positive output headroom.
- Select practical feedback values and recalculate actual gain.
- Budget shunt, offset, resistor, bias-current, temperature, ADC, and noise errors.
- Add matched filtering, bypassing, and transient protection as required.
- Validate minimum and maximum supply, startup, disconnect, short circuit, maximum temperature, transients, and ADC behavior during saturation.
Troubleshooting common failures
Output stuck near ground
- Shunt polarity is reversed.
- The feedback network is wired incorrectly or the amplifier is unintentionally inverting.
- The shunt is bypassed by another ground path.
- The op amp is saturated or the signal is below the chosen reference.
Output will not reach the expected high voltage
- Positive output swing is limited by the LM358.
- Output load is excessive.
- Gain requires more voltage than the supply allows.
- Input common-mode voltage is out of range.
- Actual shunt current or resistance is lower than assumed.
Low-current readings are inaccurate
- Offset is large relative to the shunt voltage.
- The shunt is too small or the ADC lacks resolution.
- Bias current flows through overly large resistors.
- Noise, thermal gradients, or PCB ground errors dominate.
High-side output is wrong or unstable
- Input common-mode voltage exceeds the LM358 range.
- Resistor ratios are poorly matched.
- Protection diodes conduct during transients.
- The op amp supply is below the shunt common-mode voltage.
PWM readings are noisy
- The circuit measures switching current while the requirement is average current.
- Input filtering is asymmetric.
- Sense traces run near a switching node.
- Bandwidth or ADC sampling phase is unsuitable.
LM358 or a dedicated current-sense amplifier?
| Requirement | LM358 | Dedicated current-sense IC |
|---|---|---|
| Basic low-side DC | Suitable with error checks | Suitable |
| Tiny shunt voltage or very low current | Often requires calibration | Usually better |
| High-side near supply rail | Usually unsuitable without special design | Usually better |
| Bidirectional sensing | Extra reference and circuitry | Often integrated |
| Fast transients or protection | Application-dependent | Usually better |
| Lowest flexible component cost | Excellent | Often still inexpensive |
| Educational prototype | Excellent | Good |
Choose an LM358 when low cost, flexibility, low-side placement, slow-to-moderate bandwidth, and modest accuracy are acceptable. TI’s LMV358 can offer more useful output swing in low-voltage designs, but it remains a general-purpose op amp. For compact fixed-gain shunt measurement, compare the INA180 and bidirectional-capable INA181. ST also offers dedicated devices for low-side, high-side, bidirectional, automotive, and high-common-mode applications (ST current-sense portfolio).
Do not select a generic LM358 module as a precision sensor without verifying its shunt value, shunt power rating, calibration, output range, supply compatibility, and whether its stated current rating is continuous.
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