The Current Sense Amplifier is one of Analog Devices, Inc.’s proprietary amplifiers designed to amplify small differential signals in the presence of large common-mode voltages. A typical application for a current sense amplifier is to amplify the voltage across a shunt resistor. ADI offers a variety of current sense amplifiers that operate from supply voltages as low as 1.8V and tolerate input common-mode voltages as high as 600V.

The Current Sense Amplifier is one of Analog Devices, Inc.’s proprietary amplifiers designed to amplify small differential signals in the presence of large common-mode voltages. A typical application for a current sense amplifier is to amplify the voltage across a shunt resistor. ADI offers a variety of current sense amplifiers that operate from supply voltages as low as 1.8V and tolerate input common-mode voltages as high as 600V.

A paper to analyze the common-mode step response of current sense amplifiers

Many applications use shunt resistors whose common-mode voltage is a function of time. Some examples of shunt applications with variable common-mode voltage are: H-bridge motor drivers, solenoid valve controllers, DC-DC switching converters. In these applications, the common-mode voltage obtained by the current-sense amplifier is PWM-varied from the battery voltage to ground.

An ideal current-sense amplifier would not react to input common-mode changes. But in fact, current-sense amplifiers have limited common-mode rejection, typically specified at DC, around 100V/V or 80dB.

A paper to analyze the common-mode step response of current sense amplifiers

In addition to the output error due to the dc common-mode rejection ratio (CMRR), there are errors related to the ac common-mode rejection ratio and the amplifier’s common-mode step response. This application note focuses on the common-mode step response of current sense amplifiers1.

Common Mode Step Response

Ideally, a current-sense amplifier produces an output based on the difference in its input, regardless of the actual value at the input (i.e., the common-mode voltage). However, in practical use, the amplifier output may change at different common-mode levels of its inputs. The changing output with a changing common-mode input is called the common-mode step response.

In applications where the input common-mode voltage varies widely, the common-mode step response of the amplifier may be particularly important; as the amplifier recovers from a change in the input common-mode voltage, the amplifier output may experience new changes due to the new common-mode level. misaligned and rendered ineffective. Therefore, an amplifier’s long settling time (and large errors during this time) can severely degrade the amplifier’s dynamic performance.

Common Mode Step Response Measurement

It is very difficult for a current sense amplifier to achieve a very fast, very common-mode step response. It requires a very stable and fast source, a fully shielded connector, and a properly designed circuit. The basic functional block diagram of this measurement is shown in Figure 1.

A paper to analyze the common-mode step response of current sense amplifiers

PWM input

A waveform generator is used to generate a PWM signal frequency of 0Hz to 100kHz and use it as the input signal for the MOSFET driver.

MOSFET driver

The driver injects high-level current into the MOSFET for very high-speed switching performance, eliminating excessive heat dissipation. The current range provided by the driver is hundreds of mA or even a few A.

MOSFET

The driver outputs a positive voltage, so an N-channel power MOSFET is used. These MOSFETs can withstand voltages up to 100V, with typical rise and reverse recovery times of 35ns and 115ns, respectively. Additionally, these MOSFETs feature 44mΩ RON (enough to maintain signal integrity) and can dissipate up to 130W. The outputs of these MOSFETs are used as the common-mode input voltage (VCM) of the current-sense amplifier.

Current Sense Amplifier

Current sense amplifiers can amplify small differential signals in the presence of large common-mode voltages. The current sense amplifiers tested in this application note have common-mode voltages up to 80V and operate from a single 5V supply.

Common Mode Step Response

The current-sense amplifier output produces a common-mode step-response waveform. The response may appear as a waveform with positive or negative spikes on the rising or falling edge, depending on whether the inverting or non-inverting input is dominant.

A simplified schematic of the common-mode step response measurement is shown in Figure 2. In this schematic, the current sense amplifier used is the AD8210.

Common Mode Step Response Results

Multiple Analog Devices current-sense amplifiers configured in shunt circuits were evaluated and compared to current-sense amplifiers offered by competitors. The AD8210, the first current shunt monitor evaluated, is a single-supply bidirectional current-sense amplifier that is tolerant to common-mode voltages from ?2V to +65V. The device’s voltage reference pin (VREF) is used to adjust the output offset with a fixed gain of 20.

In addition, the AD8207 bidirectional difference amplifier was evaluated; the amplifier is configured as a shunt amplifier. It can withstand ?4V to +65V common-mode voltage with +5V power supply; it can withstand ?4V to +35V common-mode voltage with +3.3V power supply. The device also features a zero-drift core that provides a typical offset drift of less than 500nV/°C, and a typical gain drift of less than 10ppm/°C. It also has a fixed gain of 20.

Additionally, the AD8418 and AD8418A were evaluated. Both current-sense amplifiers feature zero-drift cores that achieve a typical offset drift of 0.1µV/°C over the entire operating temperature range and a common-mode voltage range of ~2V to +70V. Both amplifiers are fully compliant for automotive applications (including electromagnetic interference (EMI) filters and circuitry) with high output at pulse width modulation (PWM) class input common-mode voltages.

Figure 3 shows a comparison of the waveforms of ADI’s various current-sense amplifiers with competitors’ products at an input common-mode voltage of +60V.

Common Mode Step Response Measurement Technique

To generate a common-mode step response for a current sense amplifier, the connections, components used, and component locations should be considered.

connect

Connector pins—such as those on power supplies, waveform generators, input/output, oscilloscope probes, and other interface connectors—should be as close as possible to the device under test (DUT) to avoid causing noise and interference in the wires.

The ground connections should only intersect at one point, called a single-point ground, to avoid ground loop problems caused by having different ground potentials in the system.

Instead of using alligator clips for oscilloscope probe grounding, take the probe tip ground (shaped like a coil) and plug it into the probe. If the probe tip is not available, make a coil of solid wire or solid wire and solder it next to the probing point (the input and output pins of the current sense amplifier) ​​to measure only the desired signal, eliminating possible unwanted ringing or spikes of induced noise.

Components used

A bypass capacitor should be added at the power supply to reduce the ripple voltage in the circuit; do not take it for granted. Ceramic capacitors do this job well because of their high stability, high efficiency, and low losses.

Since the +60V input common-mode voltage is used in this application note, the load resistance of the MOSFET driver should have a large power rating to withstand the high current.

The MOSFET should have a short reverse recovery time to minimize losses due to frequent charging and discharging of the MOSFET diode.

component placement

The MOSFET driver circuit consists of discrete components including MOSFETs, and the current sense circuit should be placed as close as possible to the MOSFET driver to minimize AC impedance and avoid noise or interference from long traces.

in conclusion

The ADI current sense amplifier has been tested and verified to have less than 700mV of overshoot or undershoot. Competitor products overshoot almost 2V. The ADI current-sense amplifier described in this application note settles faster than competing products for both rising and falling edges of the input common-mode voltage. In addition, these amplifiers can reject very high input common-mode voltages up to +60V. Due to these advantages over competitors’ products, ADI current sense amplifiers are useful for preventing circuit failures, preventing over-discharge of batteries, and maintaining the normal operation of certain systems; such as: battery monitors, power regulators, electric vehicles, Generator and Motor Control.

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