Current Sensing Using Nanopower Op Amps

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Designers implement current sensing for system protection and monitoring by placing a very small "shunt" resistor in series with the load and a current sense amplifier or op amp between the two. While dedicated current sense amplifiers work very well for current sensing, precision nanopower op amps are ideal when power consumption is a concern.
　　There are two locations to place the shunt resistor depending on the load: between the load and the power supply, or between the load and ground.
　　In both cases, the voltage across the shunt resistor is measured by an op amp in order to sense the current with a known value resistor. Applying Ohm's law (Equation 1), the current draw can be determined:
　　where V is voltage, I is current, and R is resistance.
　　The shunt resistor and op amp - HC1400P03 - are selected so that they have the least effect on the performance of the circuit.
When selecting the resistor, use low value resistors based on two criteria:
　　Keep the voltage drop across the resistor as low as possible and keep the negative terminal of the load as close to ground as possible for low-side sensing or as close to the power supply as possible for high-side sensing.
　　Keep power consumption low. From Equation 2, we can see that since you are measuring current, it is an independent variable, so the resistor should be as small as possible:
　　Here is a caveat: since you are measuring current rather than minimizing it (as I did in Part 1), you must minimize the resistor value to minimize power consumption—the opposite of the idea of power management in a DC gain configuration.
　　Ultra-low power current measurement technology is widely used in battery charging and monitoring of terminal devices such as mobile power supplies and mobile phones, and can also be used to ensure the normal operation of industrial IoT applications.
　　So how low can you go when choosing a resistor value? Simply put, the voltage drop across the resistor should be greater than the offset voltage of the operational amplifier you are using.
　　Example
　　Suppose you want to make a low-side differential current measurement to ensure that there are no short circuits and open circuits in the system. For simplicity, this example uses simple numbers and ignores parameters such as resistor tolerance.

The supply voltage is 3.3V. For proper operation, the system draws a maximum current of 10mA; you don’t want to effectively ground the load above 100V. The first thing you need to understand is that the voltage drop across the shunt resistor (due to the current) must be less than or equal to 100V.
　　If you use Equation 3 to determine the maximum shunt resistance: The
　　effective ground is 100V, as shown in Equation 4:
　　You must select an op amp that can detect changes in this voltage drop, indicating the presence of a fault. Since the load current is within ±10% of its typical value when the system is in normal operation. When the current changes by at least 10%, the op amp can detect the change in voltage across the sense resistor.
　　If there is a fault (such as an open circuit, undervoltage due to low current, short circuit or brownout due to high current), Equation 5 shows the change in current (IΔ):
　　Equation 6 calculates the change in the VSHUNT voltage drop:
　　For this example, I will choose the LPV821 zero-drift nanopower amplifier. Its zero-drift technology allows for a maximum offset voltage of only 10V, which can detect fault conditions. Zero-drift op amps are ideal for high-precision (<100V) measurements. In addition, the LPV821 is a nanopower amplifier that you can leave on all the time, continuously and accurately sensing current with minimal impact on the system power budget.
　　Thanks for reading part 2 of the “How to Make Precision Measurements with Nanopower Op Amps” series. We hope this series of articles has provided you with some insights into the benefits of using nanopower zero-drift op amps in DC gain and low-side current sensing applications.

一杯星巴克

Very good

Ouyangjunjie