Electronic systems tend to manage their power consumption to reduce the production of heat as waste. This calls for optimizing the system efficiency by effectively distributing power. As the voltage applied to the circuit is usually a constant, engineers monitor power consumption by keeping track of the current drawn by the circuit—power being the mathematical product of the current and voltage fed to the circuit. Current sensing has additional advantages, mainly that of maintaining the health of the system, preventing circuit faults from turning disastrous, and preventing batteries from over-discharging.
Engineers use two basic methods to monitor electric current. The first method follows Ampere’s law, and engineers measure the magnetic field surrounding a current-carrying conductor. The second method follows Ohm’s law, and engineers measure the voltage drop across a small resistor inserted in series with the circuit. The first is a non-intrusive method, but useful only for regularly changing currents, such as alternating current. It is also an expensive method, rather prone to temperature coefficient errors and effects of non-linearity. The second method is simpler, but introduces an element of insertion loss.
The semiconductor industry offers resistive-sensing techniques that are cost-effective and accurate, while making measurements suitable for various applications running on direct current. The resistive-sensing technique relies on sensing the current on the low-side or on the high-side of the circuit, the optimal approach depending on the application.
In resistive sensing, engineers insert a low-value resistor in series with the current path. This produces a small voltage drop in proportion to the current the circuit is consuming and which passes through the resistor. An electronic sense amplifies this tiny voltage to make it easier to process further. However, the sense resistor’s placement depends on the environment of the application and this can present some serious challenges for the sense amplifier.
If the position of the sense resistor is between the load and the circuit ground, a single operational amplifier, acting as a sense amplifier, is adequate to amplify the resulting voltage drop. Engineers call this low-side sensing, and is different from high-side sensing, where they place the resistor between the positive lead of the supply and the load.
In both cases, the sense resistor must be of adequately low value to prevent it from dissipating high power, but its value must be high enough for it to generate a detectable voltage for the sense amplifier to multiply it accurately. The sense amplifier multiplies the difference of voltage across the sense resistor, but uses a common-mode voltage for the purpose.
For low-side sensing, the common-mode voltage is close to the ground, and the rest of the circuitry following the sense amplifier may run on low voltage. However, high-side sensing requires the common voltage to be close to the supply voltage, and sometimes this may be high enough to present supply voltage problems for the circuit following the sense amplifier.
Some applications are unable to tolerate the tiny voltage drop introduced by the sense resistor on the low-side. The situation aggravates as the load current increases. For them, engineers have to follow high-side sensing inevitably.