What is Current Sensing?

Current sensing refers to the measurement of electric current in a conductor. It is commonly done by measuring the small voltage drop across a precisely known resistance (the current sense resistor) that the current passes through. Ohm’s law (V=IR) allows us to calculate the current from the known resistance value and measured voltage drop.

Typical values of sense resistors range from a few milliohms to less than 1 ohm. The low resistance minimizes power loss and avoids affecting the circuit being monitored. However, the voltage drop across such a small resistor is also very small, often just a few millivolts for currents of interest. Accurately resolving these tiny differential voltages in the presence of noise and large common-mode voltages requires a specialized amplifier – the current-sense amplifier.

Key Characteristics of Current-Sense Amplifiers

Current-sense amplifiers (CSAs) are precision devices optimized for extracting small differential voltages across a shunt resistor while rejecting large input common-mode voltages. Key characteristics include:

Low Offset Voltage

Offset voltage is the differential voltage required at the input to produce zero volts at the output. Any offset voltage adds directly to the measured sense voltage and causes error. CSAs have very low offset, typically <100μV and some as low as 10μV.

High Common-Mode Rejection Ratio (CMRR)

The sense resistor may be at a high common-mode voltage with respect to ground. The CSA must reject this common-mode voltage to extract the small differential sense voltage. CMRR specs for CSAs are typically >80dB, with some over 100dB. The higher the better for minimizing common-mode error.

Adjustable Gain

CSAs usually have a fixed or manually adjustable gain to scale the small sense voltage to a level compatible with ADCs or comparators. Gains of 20V/V to over 500V/V are common. Higher gains improve resolution but reduce input range.

Wide Input Common-Mode Range

The input common-mode voltage range is the allowable voltage swing at the input. A wide range, approaching the supply rails, allows maximum flexibility in sense resistor placement. It’s not uncommon to see common-mode ranges of -0.2V to (Vs -0.2V).

Integrated Gain Setting Resistors

Some CSAs integrate the gain setting resistors. This saves space, guarantees precision gain, and reduces temperature drift compared to using external resistors.

Advantages of Current-Sense Amplifiers vs. Op-Amps or In-Amps

While an op-amp or instrumentation amplifier could be used to amplify the sense voltage, they are not optimized for the unique requirements of current sensing. Key advantages of current-sense amplifiers include:

• Precision: CSAs have much lower offset voltage and drift and higher CMRR than general purpose amplifiers. This improves current measurement accuracy.

• Ease of Use: CSAs are tailored for current sensing with integrated gain resistors, providing precision fixed gains. General purpose amplifiers require additional external gain setting components.

• Robustness: CSAs have over-current and reverse input protection, making them more robust to external faults that could damage a general purpose amplifier.

• Size: By integrating the gain resistors and using smaller packages, CSAs save considerable PCB space compared to using a general purpose amplifier with external components.

Applications of Current-Sense Amplifiers

Current-sense amplifiers enable current measurements in a wide variety of applications:

Overcurrent Protection

Monitoring current to detect faults or short-circuits is critical for safety and reliability. CSAs paired with a comparator provide a fast, precise overcurrent detection solution.

Battery Monitoring

Monitoring charge and discharge currents helps track battery state-of-charge and health. The high-side CSA configurations allow monitoring battery current without breaking into the power path.

Motor Control

Sensing winding currents enables torque control, stall detection, and commutation timing. The high bandwidth of CSAs allows sensing of PWM waveforms.

Power Supply Monitoring

Tracking load current gives early warning of overload conditions. High-side CSAs are ideal for monitoring supply rail currents.

Inrush Current Limiting

Limiting inrush current during hot-swap or supply startup prevents component stress. A CSA provides the rapid current feedback needed for closed-loop inrush control.

Configurations of Current-Sense Amplifiers

Current-sense amplifiers come in several configurations optimized for different use cases:

High-Side Sensing

In high-side sensing, the CSA and sense resistor are in series with the power supply rail. This allows load current monitoring without breaking into the ground plane. However, the CSA must handle the full supply voltage as a common-mode input.

Low-Side Sensing

In low-side sensing, the sense resistor is placed between the load and ground. This configuration has near zero common-mode voltage, relaxing CSA input range requirements. However, it does require an uninterrupted ground plane connection.

Bidirectional Sensing

Some CSAs can resolve current in both directions, allowing sensing of current into and out of a load. This is useful for monitoring battery charge/discharge or AC currents.

Example Current Sensing Circuits

Here are a few common current-sense amplifier circuits:

High-Side Current Monitoring

In this high-side sensing example, the INA180 CSA monitors current flow from a 5V supply to a load:

With a 10mΩ sense resistor, 3A of load current generates a 30mV drop across Rs. The INA180, with its gain of 100V/V, outputs 3V, which connects to an ADC for digital current readout.

Low-Side Current Monitoring

This low-side example uses the MAX4080 CSA to measure unidirectional load current:

The 50mΩ sense resistor generates 50mV at the 1A maximum current. With a gain of 100V/V, the MAX4080 outputs 5V at 1A for easy interfacing to a 5V ADC.

Bidirectional Current Monitoring

In this example, the LT1999 provides bidirectional current monitoring of a battery:

The LT1999 senses current in both directions across the 10mΩ sense resistor. Charging current develops a positive output voltage, discharging current a negative output voltage. The ±1.5A maximum current range maps to the ±1.5V output range for interfacing to an ADC.

FAQs about Current Sense Amplifiers

What gain should I use for my current sense amplifier?

The gain depends on your expected current range and the output voltage you need. Use Ohm’s law to calculate the sense voltage at your maximum current, then choose the gain to produce your desired output voltage. For example, if you expect 2A maximum across a 20mΩ sense resistor and need a 2.5V output, use a gain of 2.5V / (2A * 0.02Ω) = 62.5V/V.

Can I use a current sense amplifier for AC current sensing?

Yes, but you need a bidirectional CSA capable of handling both positive and negative sense voltages. Choose a gain that keeps the output within the CSA’s output voltage range for both current directions.

How does the sense resistor value affect current measurement?

A larger resistor value gives a larger sense voltage which improves measurement resolution. However, higher resistance also means higher power dissipation in the resistor (P=I^2R) and a larger voltage drop in the current path. The sense resistor is typically chosen as a compromise between resolution and acceptable voltage drop and power loss.

Where should I place the sense resistor?

The sense resistor location depends on what information you need. High-side placement allows monitoring supply current without breaking into the ground plane. Low-side placement has near zero common-mode voltage but does require an unbroken ground connection. The resistor should be as close to the CSA as possible to minimize noise pickup.

What is input bias current and how does it affect accuracy?

Input bias current is the small current that flows into or out of the CSA inputs. This current flows through the sense resistor, creating an additional unintended sense voltage that adds error. Lower input bias current improves accuracy. Most CSAs have input bias current of a few nA or less.

Conclusion

Current sensing is a critical function in many electronic systems, enabling power monitoring, fault protection, battery management and more. The tiny voltages developed across sensing resistors present unique amplification challenges. Current-sense amplifiers are precision devices tailored to accurately extract small differential voltages while rejecting large common-mode signals.

With their high CMRR, low offset, and wide input ranges, current-sense amplifiers outperform general purpose amplifiers in current sensing applications. Understanding the key specifications, configurations, and applications of current-sense amplifiers empowers designers to make precise, reliable current measurements in a wide variety of systems.