We often find customers using general purpose op amps like the LM321 for current sensing applications. This is one of the traditional op amps that have been in use for decades. These traditional op amps are low cost and used in countless applications. However, sometimes the same customers report back to us that these op amps are failing in their current sense circuits. When we looked at the returned op amp units, they worked as expected. So where is the problem?

Just because an op amp is "universal" doesn't mean "available for all uses". Current sensing applications require precision. Current sensing is commonly used in power management and overcurrent protection applications. Imagine an imprecise world. When your phone's battery is about to run out, the battery indicator may be 8%. You might design an overcurrent circuit that triggers at 100A , only to find that the protection circuit only kicks in at 150A and all downstream components are damaged. This is the difference between general and precise.

The key to a precision op amp is the input offset voltage. Its Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR) also have better specifications, but both parameters can be treated as input offset voltage as a function of common mode voltage or power supply voltage. What is input offset voltage? The input offset voltage is the inherent bias of every op amp input and is a slight mismatch of the input transistors due to the manufacturing process. In school, we learned that the ideal op amp has zero input offset voltage, but we know this is not the case in the real world.

Traditional general purpose op amps such as LM321 have VOS = ±7mV (max), modern general purpose op amps such as NCS20071 have VOS = ±3.5 mV (max). This maximum specification is distributed around zero. This means that most of the time a randomly selected device will exhibit a near-zero bias. You can be confident that your prototype circuit works flawlessly with the commonly used LM321, but when the circuit goes into mass production, you may find that a considerable percentage of it fails. This is because the manufacturing process creates part-to-part variation, and some devices are close to the limit. You should always design your circuit for maximum input offset voltage.

We sometimes see customers forget to check the worst-case limits of the circuit: input offset voltage limits, CMRR limits, resistor network tolerances, temperature effects, etc.

Compared to the LM321 and NCS20071 general purpose op amps, the new NCS21911 precision op amp has a maximum offset VOS = ±25µV (microvolts) due to its chopper-stabilized construction. How much difference does the offset voltage actually make? Let's consider a situation where the shunt drop is a fixed 50mV, as shown in Figure 1.

Figure 1. Comparison of input offset voltage and resulting output offset error.
Amplifiers with input offset voltages of 7 mV and 3.5 mV have significant output offset errors.

We can take a closer look at the example of Vos=7 mv in Figure 2.

Figure 2. Output Error due to Low-Side Current Sense and Input Offset Voltage

By choosing a precision op amp such as the NCS21911, the error due to the input offset voltage is almost negligible in this circuit example. Not only does it improve the output accuracy, it even has some margin to reduce the sense resistor size and still maintain the required accuracy.

Efficiency is greatly improved as the low offset voltage allows for lower sense resistor values while maintaining the same accuracy, as shown in Figure 3. What happens when the sense resistor size decreases? The sense resistor consumes less power, which means lower wattage and lower cost resistors can be used, and the physically smaller sense resistor ultimately takes up less space on the PCB, improving the overall energy efficiency of the system and reducing losses.

Figure 3. Comparison of input offset voltage and resulting shunt voltage drop for fixed accuracy requirements. The smaller the shunt pressure drop, the higher the efficiency.

In many applications, the load current flowing through the sense resistor is variable. Sometimes when customers try to make current measurements around 0A, they see a significant increase in error; this is normal and should be expected. When the current drops to zero, the error percentage goes to infinity. This current detection circuit is for measuring current; not for accurate measurement in the absence of current. Figure 4 shows how the accuracy improves as the current increases. Note the variation in error due to input offset voltage. The NCS21911's 25µV offset supports relatively accurate measurements even when the sense voltage is reduced.

Figure 4. Error due to input offset voltage

It appears that small improvements in efficiency and precision can save bills of materials, printed circuit board (PCB) costs, and electricity bills. While choosing a less expensive op amp may save you some money upfront, consider that the final system-level savings may be to your advantage by going with an affordable precision op amp.

In many applications a general purpose op amp will work fine. Even the traditional LM321 can work in current sensing applications for which the corresponding circuit has been designed. Remember, you should expect relatively high output errors. Alternatively, the size of the sense resistor should be larger to obtain a voltage drop that is sufficiently larger than the input offset voltage.

For low-side current sensing, turning to precision op amps improves accuracy and system energy efficiency. The NCS21911 precision op amp has a standard output pin, making it a simple plug-in replacement for general-purpose op amps such as the LM321 and NCS20071.