Achieving High-Performance Current Sensing Using a New Amplifier

The vast majority of analog chips (comparators, op amps, instrumentation amplifiers, references, filters, etc.) are designed to process voltage signals. When it comes to processing current signals, designers have fewer choices and more headaches. This is unfortunate because there are great advantages to directly monitoring and measuring current. Parameters such as motor torque, solenoid force, LED brightness, solar cell illumination, and battery energy are best monitored by observing the current. Therefore, a circuit is needed that can accurately sense the current and convert it into a voltage that can be easily amplified, regulated, and measured by common voltage-type devices (amplifiers, comparators, ADCs, etc.) .

Although a single resistor can convert current to voltage, a resistor alone does not provide a complete solution. The most common solution uses a sense resistor placed directly in series with the current path and an amplifier to isolate and condition the voltage across the resistor (VSENSE).

Figure 1: Principle of current sensing circuit.

Figure 2: Actual current sensing circuit.

Using an amplifier and a sense resistor together

At first glance, placing a resistor in series with ground may seem similar to the most straightforward current sensing scheme. This technique, known as low-side current sensing (Figure 3A), requires that there be no ground path, which would shunt current around the sense resistor or contribute current to adjacent circuits. In particular, if the mechanical enclosure is the system ground, it would be impractical to place a sense resistor in series. Also, because ground is not a good conductor, the ground voltage at different points in the system will be different, requiring a differential amplifier for precision measurements (Figure 3B).

Figure 3A: Low-side current sensing topology.

Figure 3B: Low-side current sensing circuit implementation.

There is a very serious problem when implementing low-side current sensing. Using a resistor in the ground path means that the ground potential of the load changes as the current changes. This will cause common-mode errors in the system and cause problems when interfacing with other systems that require the same ground potential. Because the amplitude of VSENSE will affect the resolution, the designer needs to make a trade-off between resolution and ground noise. A VSENSE full-scale of 100mV will translate into 100mV of injected ground noise. However, this ground level change problem can be avoided by placing the current sense resistor between the power supply and the load.

This alternative is called high-side current sensing. Again, the differential voltage across the sense resistor provides a direct current measurement, but there is a nonzero common-mode voltage across the resistor. This circuit therefore presents the technical challenge of distinguishing the small differential sense voltage from the common-mode voltage from the power supplies (Figure 4).

Figure 4: High-side current sensing.

For low-voltage systems, an instrumentation amplifier or rail-to-rail differential amplifier is sufficient for high-side sense resistor sensing. The amplifier output must be translated to ground without adding too much error. When the supply voltage is very high, circuitry is required to reduce VSENSE to within the amplifier's common-mode range or float the amplifier above the supply voltage. In addition to increasing board space and cost, this technique assumes that the common-mode voltage must be within a tight specified range. For most current sensing applications, it is useful to be able to predict large common-mode fluctuations. For example, if the current sensing circuit still works when the supply voltage drops, it can indicate whether the problem is with the supply or the load. Excessive current means that the current limit mechanism or the load is faulty, while too little current indicates a faulty supply. On the other hand, current sensing circuits may face common-mode voltages that exceed the supply voltage. Many current-sensing devices, such as motors and solenoids, are inductive, and rapid changes in the current flowing through them can cause inductive retracement, resulting in large voltage swings across the sense resistor. It is in these situations that amplifiers are most useful1.

Simple solution

To address the technical challenges of current sensing, high-side current sense amplifiers were developed. These special amplifiers are able to extract the low differential voltage generated by the current flowing through a small sense resistor from a high common-mode voltage. This sense voltage is then amplified and converted to a ground-referenced signal. Figure 5 shows the basic topology of a high-side current sense amplifier. In this case, the amplifier forces a voltage equivalent to VSENSE onto RIN. The current through RIN is forced through ROUT, which produces a ground-referenced voltage. Obviously, for a basic high-side current sense amplifier, the requirements are high input impedance, high gain with high accuracy, and a wide common-mode range with good common-mode rejection. Less obvious is that the accuracy of the amplifier is also important.

Figure 5: Basic high-side current-sense amplifier.

1For switching or rectifier loads, placing a sense resistor between the switch and the load will cause a large, possibly high-frequency common-mode voltage at the amplifier. Even amplifiers with high common-mode rejection will cause CMRR errors when large high-frequency common-mode voltages are present. To avoid this unnecessary difficulty, the sense resistor should be placed facing the power supply to avoid being affected by the rectified voltage.

Impedance is key

Ideally, neither current nor voltage sensing should affect the connected load. This means that the voltage sensing device should have an input impedance close to infinite to ensure that there is no significant current shunting to the load. Conversely, the current sensing should have an input impedance close to zero to ensure that the voltage applied to the load is not significantly reduced. A high-side current sensing circuit (amplifier + resistor) should meet both requirements. The amplifier used to sense the voltage on RSENSE must have a high input impedance, while the resistor used to sense the load current must be very small.

To further demonstrate this point, try using a large sense resistor. As the series resistance increases, the voltage across the load drops. The external series resistance is the cause of energy dissipation, and a sense resistor that is too large can also cause excessive heat dissipation, which can cause long-term reliability issues.

So, is there any reason to use large resistors? The main advantage of using large resistors is to increase the overall output voltage (Equation 1). This is useful when the amplifier has a fixed gain or limited gain configurability.

There is a limit on the size of the sense resistor. The input range of the amplifier and the maximum expected current will determine the largest usable sense resistor (Equation 2).

For example, if the maximum current through the sense resistor (ISENSE_MAX) is expected to be 50mA, and the maximum input voltage that the high-side current sense amplifier can accept is 250mV (VSENSE_MAX), then the maximum sense resistor is 5ohms (RSENSE_MAX).

In theory, designers should not be forced to compensate the amplifier by increasing the sense resistor. As long as the amplifier can operate with sufficient gain and accuracy, designers should use the minimum acceptable sense resistor. This can be calculated based on the input bias voltage of the current sense amplifier and the minimum current that it must handle.

For example, if 1mA resolution is required (IRES), and the bias voltage of the high-side current sense amplifier is 1mV (VOFFSET), the maximum sense resistor should be 1ohm (RSENSE_MIN). Equation 3 highlights a key point, that the minimum sense resistor is directly related to the bias voltage of the high-side current sense amplifier.

A Close Look at Advanced Current Sense Amplifiers

The new generation of high-side current sense amplifiers has significantly improved performance over the previous generation due to the accuracy of high-side current sensing. For example, the LTC6102 from Linear Technology is a new high-side current sense amplifier that incorporates zero-drift technology. The input bias voltage of this amplifier is only 10μV, and the maximum bias drift is only 50nV/℃. Compared with the previous generation of current sense amplifiers, the LTC6102 can use a smaller sense resistor2. If the system can allow for a larger VSENSE, the LTC6102 can accept a sense voltage as high as 2V. This combined bias plus this maximum sense voltage allows the amplifier to provide 106dB of dynamic range, which can handle microampere currents from the current amplifier. It can be used to sense smaller currents because any gain value can be achieved with external resistors. By using precision resistors, the gain accuracy can be better than 99%.

Figure 6: Linear Technology’s LTC6102 allows for straightforward high-side current sensing. Only one RSENSE and two gain resistors are required to configure the device. Designers can tailor power consumption, response time, and input/output impedance characteristics by selecting RIN and ROUT.

The LTC6102 does not sacrifice other important current sensing functions. High input impedance limits the input bias current to less than 300pA. The LTC6102 can still work under common mode input voltage conditions up to 105V. Common mode rejection reaches 130dB, and the deviation contributed within the common mode input voltage range of 100V is less than 32uV3. In terms of fault protection, the device has a response time of 1usec, so it can quickly shut down the power supply when an accident occurs in the load or power supply.

2 Compared to a typical high-side current sense amplifier with 1mV offset voltage and 1 uV/°C drift, the LTC6102 has a minimum theoretical sense resistor value (RSENSE_MIN, Equation 3) that is 99% smaller for any given current resolution (IRES).

3 Common-mode rejection equals 20 * Log [VCM / VOS].

Conclusion

High-side current sense amplifiers offer many inherent advantages for sensing and controlling current. Advanced battery management and motor control technologies are good examples of applications that place an urgent need for current sense amplifiers with higher common-mode voltage, higher accuracy, and higher precision. The industry-leading LTC6102 has been favored by the industry for its powerful functions and excellent precision. Today's high-side current sense amplifiers have reached the performance level of industry-leading precision op amps, providing designers with a simple, versatile and high-precision option that can completely replace the low-precision and complex current sense circuits of the past.