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Amplifiers Handle Small Signals
Jon Titus, Contributing Editor -- Design News, July 14, 2008
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.
Useful Links
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Kugelstadt, Thomas, “New zero-drift amplifier has an IQ of 17 µA,” Analog Applications Journal, Texas Instruments, 2Q 2007.
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Nolan, Eric, “Demystifying Auto-Zero Amplifiers — Part 1,” Analog Dialog, 34-2 (2000).
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Nolan, Eric and Reza Moghimi, “Demystifying Auto-Zero Amplifiers — Part 2,” Analog Dialog, 34-3 (2000).
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“Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers AD8551/AD8552/AD8554.” Rev. C. Analog Devices.
| Author Information |
| Jon Titus, a former designer and chief editor of EDN and Test & Measurement World magazines, remembers when “fast” signals operated at 10 MHz and programs came on paper tape. |
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