High-Side Current Sensing: Difference Amplifier vs. Current Sense Amplifier (Part 1)

In many applications such as motor control, solenoid control, and power management (such as DC-DC converters and battery monitoring), high-precision high-side current sensing is required. In such applications, monitoring the high-side current rather than the return current can improve diagnostic capabilities, such as determining short-circuit current to ground and continuously monitoring the return diode current, avoiding the use of sampling resistors, and maintaining ground integrity. Figures 1, 2, and 3 show typical high-side current sampling configurations for solenoid control and motor control, respectively.

　　

　　Figure 1. High-side detection in a typical solenoid valve control

　　

　　Figure 2. High-side sensing in a typical H-bridge motor control.

　　

　　Figure 3. High-side sensing in typical three-phase motor control.

　　In all of the above configurations, the pulse width modulation (PWM) common-mode voltage on the sampling resistor that monitors the load current swings from ground to the power supply. The period, frequency, and rise/fall time of this PWM input signal can be determined by the control signal from the power stage to the FET. Therefore, the differential measurement circuit that monitors the voltage on the sampling resistor requires extremely high common-mode voltage rejection and high voltage handling capabilities, as well as high gain, high accuracy, and low offset - in order to reflect the true load current value.

　　In solenoid control using a single control FET (Figure 1), the current always flows in the same direction, so a unidirectional current sensor is sufficient. In motor control configurations (Figures 2 and 3), the motor phases are split, meaning the current in the sense resistor flows in both directions, so a bidirectional current sensor is required.

　　Many semiconductor vendors offer a variety of solutions for high-side current sensing, and design engineers studying this type of application have discovered that these solutions can be classified according to two distinct high-voltage structures: current sense amplifiers and difference amplifiers.

　　Next, we will detail the important differences between these two architectures to help high-side current sensing designers choose the best device for their application. We will compare two high-voltage devices: the AD8206 bidirectional difference amplifier and the AD8210 bidirectional current sense amplifier. These two devices have the same pinout and both provide high-side current sensing and monitoring capabilities, but their performance specifications and architectures are different. So, how do you choose the right device?
How do they work ?

　　The AD8206 (Figure 4) is an integrated high-voltage difference amplifier that can withstand common-mode voltages up to 65 V by attenuating the input voltage to 1/16.7 through its built-in input resistor network to keep the common-mode voltage within the input voltage range of amplifier A1. However, its internal input resistor network also attenuates the differential signal by the same proportion. In order to achieve the AD8206's 20 V/V gain, amplifiers A1 and A2 must amplify the differential signal by approximately 334 V/V.

　　

　　Figure 4. Schematic diagram of the internal structure of AD8206

　　This device allows bidirectional input measurements by biasing the output amplifier to the appropriate voltage within the supply range. The bias is achieved by applying an external low impedance voltage to a precisely configured resistor divider network connected to the non-inverting input of amplifier A2. An excellent feature of the AD8206 is that it can correctly amplify the differential input voltage when the common-mode voltage is -2 V (the common-mode bias is 250 mV as shown in the figure).

　　The AD8210 (Figure 5) is a recently introduced high-voltage current-sense amplifier that is functionally identical to the AD8210 and pin-compatible. However, the AD8210 operates differently from a difference amplifier and has different performance specifications.

　　

　　Figure 5. AD8210 functional diagram.

　　One obvious difference is that the input structure does not rely on a resistor divider network to handle high common-mode voltages. The input amplifier includes a high-voltage transistor fabricated using the XFCB IC fabrication process, and since the VCE breakdown voltage of such transistors exceeds 65 V, the common-mode voltage at the input can be as high as 65 V.

　　Current sense amplifiers, such as the AD8210, amplify small differential input voltages in the following manner. The inputs are connected to a differential amplifier through R1 and R2. Using transistors Q1 and Q2, the currents through R1 and R2 are adjusted so that the voltage at the input of amplifier A1 is zero. When the input signal to the AD8210 is 0 V, the currents in R1 and R2 are equal. When the differential signal is nonzero, the current increases in one resistor and decreases in the other. The difference in current is proportional to the magnitude of the input signal and has the same polarity. The differential current through Q1 and Q2 is converted to a ground-referenced differential voltage by two precisely trimmed resistors. Amplifier A2 then amplifies this voltage using low voltage transistors—powered by its 5 V (typical) supply—to achieve a final output gain of 20.

　　Normally, current sense amplifiers of this architecture are useful only if the input common-mode voltage remains above 2 V or 3 V. However, the pull-up circuitry within the AD8210 keeps the inputs of amplifier A1 near the 5 V supply, even when the input common-mode voltage drops below 5 V, or as low as –2 V. Therefore, accurate differential input voltage measurements can be made at common-mode voltages below the 5 V supply of the device.

　　It should be apparent that current sense amplifiers and difference amplifiers perform the same function, although they operate in different ways. A difference amplifier attenuates a high input voltage to bring the signal down to a level that the amplifier can accept. A current sense amplifier converts a differential input voltage to a current and then to a ground-referenced voltage; its input amplifiers can withstand high common-mode voltages because they are made using a high-voltage process. The differences in the two architectures will undoubtedly result in performance differences that designers must consider when selecting a high-side current sensing solution. Typically, the manufacturer’s data sheet provides much of the information needed to make the right decision about the type of device based on accuracy, speed, power consumption, and other parameters. However, there are some significant differences inherent in the device architecture that are not immediately apparent in the data sheet, but are important design considerations. The following are some of the key points that engineers must consider to achieve the best solution.