Welcome to the training module on INA199Ax Series Current Shunt Monitors.
This training module will introduce basic knowledge of current sensing and TI’s INA199Ax current shunt monitors.
Current sensor is an electronic circuit that monitors the current flow by measuring the voltage drop across a resistor placed in the current path (while other technologies exist, such as magnetic, everything discussed here is limited to shunt resistor current measurement). The current sensor outputs either a voltage or a current that is proportional to the current through the measured path. A wide variety of applications benefit from the ability to measure current flow. Traditionally, current sensing was primarily for circuit protection and reporting. However, as technology advances, current sensing is becoming more and more important as a way to monitor performance. Here we list some applications that benefit from current sensing.
Low side current sensing techniques connect the current sensor element between the load and ground. Current is measured by looking at the voltage drop across a resistor placed between the load and ground. This method is straightforward, easy and rarely requires more than one operational amplifier to implement. But it may add undesirable resistance in the ground path and require an additional wire to the load that could otherwise be omitted.
High side current sensing techniques connect the current sensor element between the supply and the load. Current is measured by looking at the voltage drop across a resistor placed between the supply and the load. Current sensor is connected directly to the power source and can detect any downstream failure and trigger appropriate corrective action. Therefore, it won’t create an extra ground disturbance. However, this method requires very careful resistor matching in order to obtain an acceptable common-mode rejection ratio (CMMR) and must withstand very high, and often dynamic, common-mode voltages
Current shunt monitors are a unique new amplifier family that is solely dedicated to high side current sensing applications, and contains all the necessary functions needed to perform the measurement easily and economically. There are two types of current shunt monitors, current output and voltage output. The current output current shunt monitors have low quiescent current, have adjustable gains, and are available in both standard and high speed configurations. The ability of voltage output current shunt monitors to sense at or below zero is essential in sensing current from shorted supplies, during inductive kickback, and can be used for negative supply current sense.
The INA199Ax series of voltage output current shunt monitors can sense drops across shunts at common-mode voltages from –0.3V to 26V, independent of the supply voltage. Three fixed gains are available: 50V/V, 100V/V, and 200V/V. The low offset of the Zerø-Drift architecture enables current sensing with maximum drops across the shunt as low as 10mV full-scale. These devices operate from a single +2.7V to +26V power supply, drawing a maximum of 100µA of supply current. All versions are specified from –40°C to +105°C, and offered in both SC70 and thin QFN-10 packages.
Here shows the basic connections for the INA199Ax. The input pins, IN+ and IN–, need to be connected as closely as possible to the shunt resistor to minimize any resistance in series with the shunt resistance. Power-supply bypass capacitors are required for stability. Applications with noisy or high-impedance power supplies may require additional decoupling capacitors to reject power-supply noise. Connect bypass capacitors close to the device pins.
The zero-drift offset performance of the INA199Ax offers several benefits. Most often, the primary advantage of the low offset characteristic enables lower full-scale drops across the shunt. For example, non-zero-drift current shunt monitors typically require a full-scale range of 100mV. The INA199Ax series of current-shunt monitors give equivalent accuracy at a full-scale range on the order of 10mV. This accuracy reduces shunt dissipation by an order of magnitude with many additional benefits. Alternatively, there are applications that must measure current over a wide dynamic range that can take advantage of the low offset on the low end of the measurement.
the INA199Ax series common-mode rejection ratio is affected by any impedance present at the REF input. This concern is not a problem when the REF pin is connected directly to most references or power supplies. When using resistive dividers from the power supply or a reference voltage, the REF pin should be buffered by an op amp. In systems where the INA199Ax output can be sensed differentially, such as by a differential input analog-to-digital converter (ADC) or by using two separate ADC inputs, the effects of external impedance on the REF input can be cancelled.
With a small amount of additional circuitry, the INA199Ax series can be used in circuits subjected to transients higher than 26V, such as automotive applications. Use only zener diode or zener-type transient absorbers; any other type of transient absorber has an unacceptable time delay. Start by adding a pair of resistors as shown in the figure as a working impedance for the zener. It is desirable to keep these resistors as small as possible, most often around 10Ω. Larger values can be used with an effect on gain that is discussed in the section on input filtering. Larger values can be used with an effect on gain.
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