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Current mirrors and current sources offer advantages in a number of situations, including analog current-loop signaling in high-noise environments and level shifting an analog signal across a large potential difference without the use of optical or magnetic isolation techniques. This article summarizes some of the techniques available and suggests a number of useful circuits.
Noise is extremely important to designers of high-performance analog circuits, especially high-speed clocks, ADCs, DACs, VCOs, and PLLs. A key to reducing noise is keeping the noise gain of the low-dropout regulator (LDO) that drives the circuit close to unity without compromising ac performance or dc gain. A simple RC network can reduce the output noise of an adjustable-output LDO.
A Deeper Look into Difference Amplifiers - February 2014
The classic four resistor difference amplifier seems simple, but many circuit implementations perform poorly. Based on actual production designs, this article shows some of the pitfalls encountered with discrete resistors, filtering, ac common-mode rejection, and high noise gain. To achieve a solid, production worthy design, carefully consider noise gain, input voltage range, impedance ratios, and offset voltage specifications.
As the speed and resolution of data converters continue to increase, the need for higher-frequency sampling clock sources with lower phase noise is growing. The jitter presented to the clock inputs is one of the many performance bottlenecks facing designers when they create cellular base stations, military radar systems, and other designs that require high-speed, high-performance clock signals.
Successive-Approximation ADCs: Ensuring a Valid First Conversion - December 2013
Successive-approximation analog-to-digital converters (ADCs) with up to 18-bit resolution and 10-MSPS sample rates meet the demands of many data-acquisition applications, including portable, industrial, medical, and communications. This article shows how to initialize a successive-approximation ADC to get valid first conversions after power-up and initialization.
MEMS Microphones, the Future for Hearing Aids - November 2013
Driven by aging populations and increased hearing loss, the market for hearing aids continues to grow, but their conspicuous size and short battery life cause people to look for smaller, more efficient, higher quality devices. At the start of the signal chain, microphones sense voices and other ambient sounds, so improved audio capture can lead to higher performance and lower power consumption.
Some Tips on Making a FETching Discrete Amplifier - October 2013
Low-noise amplifiers for photodiode, piezoelectric, and other instrumentation applications typically call for extremely high input impedance, low 1/f noise, or sub-picoamp bias currents that may not be met with available integrated products. This article discusses the challenges of designing a low-noise amplifier using discrete components, with emphasis on input-referred noise and offset voltage trimming.
Minimizing Errors in Multiplexed 3-Wire RTD Data-Acquisition Systems - September 2013
In a programmable logic controller or a distributed control system, one data-acquisition module may monitor the temperature of many remotely located resistance temperature detectors (RTDs). The best accuracy will be obtained when each RTD has its own excitation circuit and ADC, but the module will be large, expensive, and power hungry. Multiplexing leads to a smaller, lower cost, lower power module.
A multiplexer routes the signal from one of several inputs to a common output. Many applications use amplifiers to condition the signal before the mux. In these cases, amplifiers with a disable function can be used to select the channel, eliminating the need for a mux—reducing cost, PCB area, and distortion. This article presents the challenges of using op amps with a disable function for channel selection.
Automotive dead reckoning navigation systems use gyroscopes to estimate the vehicle’s instantaneous heading. Combined with the distance traveled, this allows the navigation system to correctly determine the vehicle’s position when the satellite signal is blocked. If the satellite signal is lost for a long time, the accumulated angle error will become large. This article offers a simple way to minimize this problem.
Multipliers vs. Modulators - June 2013
Modulators are closely related to multipliers, but the output of a multiplier is the product of its inputs, while the output of a modulator is the product of the signal on one input and the sign of the signal on the other input. The modulation function can be modeled by an amplifier whose gain is switched positive and negative by a comparator on its carrier input; or a multiplier with a limiting amplifier between the carrier input and one of its ports.
Precision instruments that measure physical properties using a photodiode or other current-output sensor often include a transimpedance amplifier (TIA) and a programmable-gain stage to maximize dynamic range. This article shows the benefits and challenges of implementing a single-stage programmable-gain TIA to minimize noise while maintaining high bandwidth and accuracy.
Operational amplifiers are often used to make high-quality current sources. In industrial applications, they are widely used to provide 4 mA to 20 mA currents. The improved Howland current source is very popular because it can drive a grounded load. Its accuracy is determined by the amplifier and the resistors. This article shows how to choose the external resistors to minimize errors.
With their long life and low energy consumption, LEDs promise to change the lighting industry, but a key limiter to rapid adoption is the cost of the LEDs themselves. In well-designed systems, LEDs can reliably produce many more lumens per dollar of product cost, so to reduce the total luminaire cost we can drive the LEDs at a much higher current than that at which the device is specified.
ADCs provide optimal performance when the analog inputs are driven to the rated full-scale input voltage, but the maximum available signal often differs from the specified voltage and may need to be adjusted. A useful device for handling this requirement is a variable-gain amplifier (VGA). Understanding how the VGA affects the ADC’s performance will help in optimizing performance of the entire signal chain.
Simulation models for amplifiers are typically implemented with resistors, capacitors, transistors, diodes, dependent and independent sources, and other components—or as s-domain transfer functions. This article presents a method that models amplifiers as second-order analog filters, providing much faster time-domain simulations, especially for higher bandwidth amplifiers.
This article shows how compensating an amplifier that is normally stable with a gain higher than +9 to operate with a gain as low as +2 provides higher slew rate and faster settling time than an equivalent internally compensated amplifier. The two methods presented here trade complexity for total wideband noise. The ADA4895-2 dual high-speed, low-power operational amplifier is used as an example.
Implementing an Isolated Half-Bridge Gate Driver - November 2012
Many applications, ranging from isolated dc-to-dc power supplies to solar inverters, use isolated half-bridge gate drivers to control large amounts of power with high efficiency, power density, isolation voltage, and long-term reliability. This article discusses details of these design concepts to illustrate the ability of isolated half-bridge gate driver ICs to provide high performance in a small package.
Many applications, including driving modern ADCs, transmitting signals over twisted-pair cables, and conditioning high-fidelity audio signals, require differential signaling, which achieves higher signal-to-noise ratios, increased common-mode noise immunity, and lower second-harmonic distortion. This article offers a circuit block that can convert single-ended signals to differential signals.
Low power consumption is key for today's devices: portable instruments require reduced size and weight, longer operating life, lower cost, and more features—and line-powered systems can operate without heat sinks or fans, making them smaller, lower cost, more reliable, and greener. Lowering the supply voltage, and turning a SAR ADC’s fast clock off after each conversion can save power.
High-voltage switches, bipolar ADCs, and other devices with multiple power supplies often require that supply voltages be applied or removed in a particular sequence. This article proposes an easy method for determining the behavior of a system when subjected to supply transients, interruptions, or sequence variations. Available evaluation boards enable prototyping with a minimum of development.
As a vital part of modern cars, wiring harnesses containing thousands of assembly components connect various electronic systems, enabling them to work together. A single failure in any harness can affect the entire system. This article offers a circuit idea that provides a robust, cost-effective technique for implementing wire diagnostics on the video and audio transmission lines in automotive applications.
When using very low value current-sense resistors, the solder resistance becomes a substantial portion of the total resistance, adding to the measurement error. High-accuracy applications use 4-terminal resistors and Kelvin sensing, but this can be expensive. This article describes an approach that enables precision Kelvin sensing using a standard, low-cost, 2-pad sense resistor with a 4-pad layout.
Understanding Microphone Sensitivity - May 2012
Sensitivity, the ratio of the analog output voltage or digital output value to the input pressure, is a key specification of any microphone. This article will discuss the distinction in sensitivity specifications between analog and digital microphones, how to choose the best microphone for an application, how to get the fullest performance from that device, and why adding a bit (or more) of digital gain can enhance the microphone signal.
The controller area network (CAN) serial-bus topology allows devices and microcontrollers to communicate with each other without a host computer. Featuring arbitration-free transmission, it places a controller and a host processor at each device node, eliminating the more complex wiring harness that would be necessary to interconnect devices with a host computer.
Low-Noise, Gain-Selectable Amplifier - March 2012
Traditional gain-selectable amplifiers use switches in the feedback loop to connect resistors to an amplifier, but the switch resistance degrades the noise performance of the amplifier, adds significant capacitance on the inverting input, and contributes to nonlinear gain error. This article presents a gain-selectable amp that uses an innovative switching technique that preserves the noise performance while reducing the nonlinear gain error.
Designing a Low-Power Toxic Gas Detector - February 2012
Safety first! Many industrial processes involve toxic compounds, including chlorine, phosphine, arsine, and hydrogen cyanide, so it is important to know when dangerous concentrations exist.This article describes a portable carbon monoxide detector using an electrochemical sensor. CO is relatively safe to handle, but it is still lethal, so use extreme care and appropriate ventilation when testing the circuit described here.
This article describes how two synchronized DDS channels can implement a zero-crossing FSK or PSK modulator. In phase-coherent radar systems, zero-crossing switching reduces the amount of post processing needed for target signature recognition; and zero-crossing switching reduces PSK spectral splatter. Here, the AD9958 two-channel complete DDS is used to switch at the zero crossing.
How to Apply DC-to-DC Step-Up/Step-Down Regulators Successfully - December 2011
High efficiency dc-to-dc converters come in three basic topologies: step-down (buck), step-up (boost), and step-down/step-up (buck/boost). The buck converter is used to generate a lower dc output voltage, the boost converter is used to generate a higher dc output voltage, and the buck/boost converter is used to generate an output voltage less than, greater than, or equal to the input voltage.
Various software packages enable PC sound cards to provide oscilloscope-like displays, but the low-sample-rate, high-resolution ADCs and ac-coupled front ends have limited bandwidths. For repetitive waveforms, a sampling front-end stretches the time axis, allowing the PC to be used as a high-speed sampling scope. This article describes a front end and probe that provide an appropriate adaptation.
Data-acquisition systems and PLCs require versatile high-performance analog front ends that interface with a variety of sensors to measure signals accurately and reliably. Depending on the magnitude of the voltage or current being measured, the signal may need to be amplified or attenuated to match the full-scale input range of the ADC used for further digital processing and feedback control.
How to Apply DC-to-DC Step-Up (Boost) Regulators Successfully - September 2011
Battery-powered systems often stack cells in series to achieve higher voltages, but this is not always possible due to a lack of space. Switching converters use an inductor’s magnetic field to alternately store energy and release it to the load at a different voltage. With low losses they are a good choice for high efficiency. Boost, or step-up converters—covered here—provide higher voltage.
System design can be complex, but the ability to prototype and quickly demonstrate subsections of the solution can simplify the process and reduce the risks faced by designers. With the System Demonstration Platform (SDP), system designers can reuse central elements, allowing subsections of their designs to be evaluated and demonstrated prior to the final system implementation.
Simple Ambient Light Sensor Circuit - July 2011
Ambient light is increasingly considered as a source for harvesting energy to power heartbeat monitors, bathroom fixtures, remote weather sensors, and other low-power devices. At the heart of an energy-harvesting system is the ability to measure ambient light accurately. This design idea describes a simple, cost-effective circuit that provides a voltage proportional to the intensity of ambient light.
Typical low-power systems operate with a Li-Ion battery whose output varies from 4.2 V to 3 V, while the ICs require 0.8 V, 1.8 V, 2.5 V, and 2.8 V. A simple way to reduce the voltage is to use an LDO, but power not delivered to the load is lost as heat, making LDOs inefficient when VIN is much greater than VOUT. Switching converters store energy in a magnetic field, enabling high-efficiency regulation.
In Difference Amplifier Forms Heart of Precision Current Source, published in Analog Dialogue in September 2009, the AD8276 unity-gain difference amplifier and AD8603 micropower op amp were used to implement a precision current source. This article shows how the circuit can be simplified for use in low-cost, low-current applications—achieving ±1.5% accuracy from –40°C to +85°C.
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