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Arek Kacprzak

Jul 10, 2015

sensors

current sensing
blog

DRV411

AMC1304L05

DRV5053

AMC1305M05

sensing

DRV421

AMC1200

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Related

Six ways to sense current and how to decide which to use

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High-precision current sensing is key to improve efficiency of closed-loop
control systems, i.e. motor drives. In this blog, I summarize the pros and cons
of different approaches to isolated current sensing and list some typical
applications in which they’re used.

Shunt resistors are used in wide range of industrial applications and
offer relatively high accuracy at low temperature drift. However, their use is
limited by the power dissipation caused by their own resistance value. In
applications with high common-mode voltages, shunt resistors require isolated
amplifiers such as the AMC1200
or, for the highest-performing systems, an isolated delta-sigma modulator like
the AMC1304L05. This device offers a low input voltage range of
±50mV allowing you to use smaller resistance shunts without compromising
performance.

Figure 1: AMC1304L05- Effective number of bits versus
oversampling ratio

Rogowski coils measure alternating current (AC) only and are wrapped
around a conductor that distributes the current to be sensed. They deliver a
voltage proportional to the rate of change of the AC current and therefore
require an integrator before being processed using an analog-to-digital converter (ADC).

Rogowski coils are suitable for retrofitting applications because the coil
can be mounted around the conductor without interrupting the current flow. They
don’t use a metal core, so the mechanical tolerances of the positioning both
influence and limit the achievable accuracy. For the same reason, they don’t
saturate and are thus used in high-current applications. Their low inductance
allows usage in systems with high slew rates.

In current transformers (CTs), the primary AC current generates a
field in a magnetic core. This magnetic field induces a proportional current in
the secondary winding. A burden resistor is required to convert the current to a
voltage signal for further processing in an ADC.

The accuracy of CTs depends on the mechanical tolerances of the setup, burden
accuracy and temperature drift of the magnetic core. The saturation level of the
magnetic core limits the dynamic range of a CT. On the other hand, dedicated
design allows you to tailor the CT for a certain use case. CTs are widely used
for sensing currents in power grids.

Magnetoresistive sensors change their resistance with the presence of
magnetic field, direct current (DC) or AC. Magnetoresistive sensors are small in
size and are typically used for position and angle sensing. They are
cost-effective alternatives for low-current applications that don’t require high
accuracy.

Depending on the material used, you can choose from two types of
magnetoresistive sensors:

Anisotropic magnetoresistance(AMR) sensors use ferromagnetic
materials in which a magnetic field influences the electrical resistance. The
resistance variation is very small; therefore, Wheatstone bridges are often
used to sense it.

Giant magnetoresistance (GMR) sensors rely on a significantly higher
impact of the magnetic field on the resistance of a structure built of
alternating ferromagnetic and nonmagnetic layers. There is no free lunch,
though – compared to AMR sensors, the production process is more complex and
expensive.

Hall-effect sensors deliver a voltage signal proportional to an AC or
DC magnetic field. They are inherently noisy, and the voltage level is highly
temperature-dependent. You can address both limitations using clever excitation
approaches like those used in the DRV411 sensor signal-conditioning integrated circuit
(IC).

Hall sensors can be used in open-loop applications that don’t require high
accuracy levels. For better accuracy, closed-loop approaches are best; these
include the Hall sensor, a magnetic core with compensation winding, and a
signal-conditioning circuit that is usually in the form of a complete module.
Closed-loop modules are available for wide range of accuracy, current and cost
levels. Other examples of Hall-effect sensors include the DRV5000 family.

Fluxgate sensors deliver the highest level of sensitivity, widest
dynamic range, and lowest noise and temperature-drift performance compared to
other current-sensing methods. The design of an external fluxgate sensor is
complex and requires low mechanical tolerances; only a few manufacturers
worldwide offer fluxgate-sensor modules. TI recently announced the DRV421, the industry’s first fully integrated fluxgate
sensor with all of the required signal-conditioning functions for closed-loop DC
and AC applications. With a magnetic core and a compensation coil, this solution
allows easy manufacturing of high-accuracy and low-level (leakage) current
modules.

Table 1 compares all of the methods described in this post.

Table 1: Comparison of current sensing methods

Stay tuned next month for more details on shunt-based current sensing.

Additional resources:

Read other posts on current sensing

Learn more about the DRV421
magnetic sensing IC

Learn more about the DRV5053
hall effect sensor

Shunt-based non-isolated current sensing

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