I encountered my first “chopper” circuit in an old ignition spark coil. Basically, a buzzer chopped a dc current that passed through a coil. A separate step-up coil produced several thousand volts at low current. Good fun. Some old instrumentation systems chopped a low-amplitude dc signal to convert it to an ac signal. At the time, the dc characteristics of amplifiers weren't terribly good, so converting a signal to ac and running it through an amp made sense. A demodulator circuit at the amp's output produced an amplified dc signal.
The words “chopper amplifier”no longer refer to a device that chops dc signals. Instead, a modern chopper amplifier, better known as an auto-zero or auto-correction amplifier, provides excellent dc characteristics and readily handles low-level dc signals. An auto-zero amplifier (AZA) reduces offset voltage and drift in the signal path. An AZA does use internal electronic switches to perform functions that enhance dc performance, but they do not “chop” an input signal. For details of AZA amplifier design and operation, see Useful Links, right.
You often find AZAs in low-frequency-sensor applications that require low 1/f electrical noise — also called pink noise — in addition to low offset and drift. The 1/f noise affects mainly dc and low-frequency circuits because it produces an equal amount of energy within each frequency decade. Thus, the 1/f noise between 10 and 100 Hz has the same energy as the noise spread between 100 and 1,000 Hz. The concentration of energy at low frequencies can greatly influence measurements. By design, AZAs mitigate the effects of 1/f noise.
A load cell that uses a Wheatstone bridge (see figure, below, left) provides an example of where to use an AZA. When properly adjusted, the bridge produces a 0V difference between the two sides. Pressure on the cell changes the resistance of one of the four resistors and a small voltage then exists across the bridge circuit. A typical load cell might produce a full-scale output of 2 mV/V. So a 45-kg (max) load cell supplied with 10V yields an output of 0.44 mV/kg. That's a small signal. You measure the difference between the voltage on each side of the bridge and not a voltage with respect to ground.
Most load-cell applications have long measuring times and produce a dc output, thus AZAs easily and accurately amplify the bridge signal so instruments can measure it. Take a look at AZAs when you work with small low-frequency signals. If you cannot place an AZA circuit between a sensor such as a load cell, you can purchase signal conditioners and measurement devices that include auto-zero capabilities for low-level sensor interfacing. Like any device, AZAs have limits. And because they switch internal circuits, they can produce some higher-frequency noise of their own.
The increased adoption of wireless technology for mission-critical applications has revved up the global market for dynamic electronic general purpose (GP) test equipment. As the link between cloud networks and devices -- smartphones, tablets, and notebooks -- results in more complex devices under test, the demand for radio frequency test equipment is starting to intensify.
Much of the research on lithium-ion batteries is focused on how to make the batteries charge more quickly and last longer than they currently do, work that would significantly improve the experience of mobile device users, as well EV and hybrid car drivers. Researchers in Singapore have come up with what seems like the best solution so far -- a battery that can recharge itself in mere minutes and has a potential lifespan of 20 years.
Some humanoid walking robots are also good at running, balancing, and coordinated movements in group settings. Several of our sports robots have won regional or worldwide acclaim in the RoboCup soccer World Cup, or FIRST Robotics competitions. Others include the world's first hockey-playing robot and a trash-talking Scrabble player.
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