High and low side current measurement

For most applications, the current is measured by sensing the voltage drop across the resistor. When measuring current, resistors are usually placed in two places in the circuit. This position is placed between the power supply and the load. This measurement method is called high-side sensing. The second place to place the sensing resistor is usually between the load and ground. This current sensing method is called low-side current sensing. These two methods for sensing the current in the load are shown in Figure 1.

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Figure 1. Current sensing method

Both measurement methods have their own advantages and disadvantages. One of the advantages of low-side current measurement is the common-mode voltage, that is, the average voltage at the measurement input is close to zero. This makes it easier to design the application circuit and to choose a device suitable for this measurement. Since the voltage measured by the current sensing circuit is close to ground, this method is preferred to measure current when dealing with very high voltages or in applications where the power supply voltage may be prone to spikes or surges. Because low-side current sensing can resist high-voltage spike interference and can monitor current in high-voltage systems, it is widely used in many automotive, industrial, and telecommunications applications. The main disadvantage of low-side current sensing is that the voltage drop across the sensing resistor will be different when the power supply ground terminal and the load/system ground terminal are used. If other circuits are based on the ground of the power supply, problems may occur. In order to avoid this problem to the limit, all circuits with interaction should be based on the same ground terminal. Reducing the current sense resistor value helps minimize ground drift.

When designing a circuit or selecting a device for current measurement, low-side current sensing is a simple method. Due to the low common-mode voltage at the input, a differential amplifier topology can be used. Figure 2 shows the classic differential amplifier topology using an operational amplifier (op amp).

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Figure 2. Operational amplifier configuration for low-side sensing

When using op amps for current sensing, to ensure correct calculations, multiple performance requirements need to be met. First, when working with a signal power supply, the operational amplifier needs to support a common-mode input voltage to ground. Since the differential amplifier usually increases the gain of the differential input signal, it must be swung to within the rail specification range of the op amp to ensure that the signal is properly transmitted to the output. For the above reasons, rail-to-rail input and output operational amplifiers are usually preferred for current sensing. Since the operational amplifier is not specified in the differential amplifier configuration, it is difficult to judge what kind of performance will be achieved in actual applications. If resistance is added around the op amp to build a current sensing circuit, the performance of parameters such as conversion rate, bandwidth, input current, common-mode rejection, and drift will be reduced. The degree of parameter degradation will depend on the closed-loop gain of the amplifier and the value of the gain setting resistor. When using a discrete solution, you need to consider the matching and tolerance of R1 and R2 in Figure 2, because changes in these components will directly affect the gain error of the circuit.

Another factor to consider when using a discrete current sense amplifier is the PCB layout. Need to place R1 and R2 as close as possible to the op amp and current sense resistor. After placing these components close to the op amp, the possibility of noise pickup at the non-inverting input of the op amp will be reduced. Since many current sense amplifiers are used in conjunction with DC/DC converters, it is necessary to carefully consider the placement of the entire current sense circuit to avoid radiated noise from the DC/DC power supply. The gain of the differential amplifier can be calculated by the equation shown in Figure 2. However, increasing or decreasing gain will affect the stability and bandwidth of the solution. If there is a capacitive load in the application, the stability of the op amp needs to be specially considered to avoid oscillation or severe output ringing.

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To overcome the shortcomings of the discrete implementation, an effective way is to use the current sense amplifier shown in Figure 3.

Figure 3. Low-side current sensing using INA199 current sense amplifier

The current sense amplifier integrates a gain setting resistor, which can reduce many layout problems in discrete implementations. The internal resistors are designed to reduce mismatches, thereby optimizing gain error specifications. The current sense amplifier is pre-configured to meet a variety of different gain requirements. For example, the gain of INA199 can be 50, 100 and 200V/V. Bandwidth and capacitive load stability are optimized for each gain setting using the capacitive load specified in the data sheet. The integrated gain setting resistor can reduce noise sensitivity, reduce PCB footprint, and simplify layout. The integration of these resistors does not necessarily mean that the package size will be increased. INA199 is available in 2 mm x 1.25 mm SC70 6-lead package and 1.8 mm x 1.4 mm ultra-thin quad flat no-lead (UQFN) package.

The current measurement of INA199 is higher than that achieved by a cost-effective discrete op amp design. The device has a gain error of 1.5% over the temperature range of -40°C to 105°C. The offset of INA199 is less than 150μV, and the drift is less than 0.5 μV/°C.

INA199 also has a REF pin. The voltage applied to the REF pin will increase the output voltage. If the downstream device needs to switch the current signal level, this pin can be used.

Alternative device suggestion

For applications with higher performance requirements, INA210-215 series devices have lower offset (35μV) and gain error (1%). If you need to use a digital interface to achieve high current monitoring, the INA226 has an offset of 10 μV and a gain error of 0.1%. If you need small digital current monitoring, you can choose the small 1.68 mm x 1.43mm package INA231, which is very suitable for portable applications or other applications with limited space. If you need to monitor the voltage output and current through the pin bindable gain setting, you can use the INA225.

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Table 1. Alternative device recommendations

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Table 2. Related Technology