Engineers Deal With Drift in Many Ways. What About a “Zero Drift” Hall-Effect Current Sensor?
You may be familiar with current-sense amplifiers to mitigate drift. Now, Texas Instruments offers what they call the "industry’s first zero-drift Hall-effect current sensors."
Driven by the boom in electric vehicles and industrial automation, we've seen an increase in high-voltage and AC-connected electronics to monitor system performance. This need can often be met with current sensors that are used to measure, and subsequently, control systems.
Example of a current sensor being used to control external circuitry. Image used courtesy of Texas Instruments
However, modern current sensors still exhibit some tradeoffs between cost, accuracy, and isolation—giving engineers a series of difficult decisions when choosing the most practical device for their designs.
Types of Current Sensors: Open-Loop and Closed-Loop
Current sensors come in many different configurations, but all of them boil down to two flavors: open-loop and closed-loop. Both of these configurations depend on the manipulation of magnetic fields, specifically exploiting the Hall Effect (hence Hall-effect sensors).
Open-loop sensors generally consist of a Hall sensor placed in the gap of a magnetic core. The desired current produces a magnetic field, which is concentrated by the core and measured by the Hall sensor. These sensors are very cheap, but they lack accuracy.
One of the primary causes of inaccuracy in open-loop sensors is drift. Drift can either be thermal drift or time drift. Thermal drift denotes changes in normal operational behavior of a device due to changes in ambient temperature while time drift refers to changes in behavior due to age-related changes in the device.
Image showing an open-loop sensor. Image used courtesy of Digi-Key and Honeywell
Closed-loop sensors, on the other hand, utilize a feedback network with high-loop gain to prevent inaccuracies due to device variations (i.e thermal drift). They offer fast response, high linearity, and high electrical noise immunity—but they are relatively expensive. For these reasons, closed-loop sensors are often chosen in applications where high accuracy is vital.
How Engineers Deal with Drift
Engineers can mitigate the effects of drift in open-loop sensors to achieve a high-accuracy, low-cost current sensor. There are a few techniques currently employed.
One of these techniques is to use a current-sense amplifier that integrates a precisely matched, resistive gain network that will minimize the temperature-drift effects of the gain error.
Example of a current sense amplifier from ADI (the AD8410) to lower drift effects. Image used courtesy of Analog Devices
According to Jim Irish of MIT, another technique is to account for drift and calibrate results accordingly. Essentially, if one can understand the drift in a specific sensor, they could then correct for it.
TI's "Zero Drift" Hall-Effect Current Sensor
This week, TI announced what they claim to be “the industry’s first zero-drift Hall-effect current sensors.” These new chips, the TMCS1100 and TMCS1101, are said to allow high performance, even under the influence of temperature change and equipment aging.
TMCS1100 magnetic current sensor chip. Image used courtesy of Texas Instruments
These claims are supported by a total maximum thermal drift of 0.45%, and a 0.5% lifetime sensitivity drift. TI claims that these figures are respectively 200% and 100% lower than other magnetic current sensors.
Doors Opened With Zero Drift
Today, there are many demands for current measurement devices to monitor and control electrical systems, hopefully, to prevent failures. Having more accurate open-loop sensors, like the new Texas Instruments Hall-effect current sensors, will make it possible to detect smaller levels of performance variation, allowing for a deeper understanding of system performance.
This news offers significant improvements in current sensing accuracy and will hopefully aid in better system performance, protection, and cost in the future.
Featured image (modified) used courtesy of Texas Instruments
Do you know of other ways to mitigate the effects of drift? Let us know in the comments below.