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The modern electronics industry is driven by the goal of integrating more features into compact designs, a trend that is well-known. Mobile phones exemplify this, as many manufacturers now include MP3 players, digital cameras, and even satellite TV capabilities in their devices. The market for such multifunctional gadgets has seen significant growth over the past few years and continues to expand rapidly.
Design cycles for these products are typically shorter than the testing phase, which can often take longer. For example, design might take around four months, while testing could last six. To avoid delays and unnecessary revisions, designers must carefully select components that meet all performance requirements from the start.
This article outlines some practical design techniques, quick calculations, and general evaluation methods to assist engineers in making informed decisions during the design process. In the field of portable electronics, designers rely on their expertise to choose components based on factors like size, cost, and performance. However, these factors often require trade-offs, and the final choice depends heavily on the intended application.
The mobile phone market, in particular, offers both high-end and low-end options. High-end models come with advanced features, while budget-friendly ones focus on affordability. This diversity creates challenges for designers who must balance functionality with cost and performance.
Mobile phone motherboards contain various components, including operational amplifiers (op-amps), audio amplifiers, data converters, and application-specific integrated circuits (ASICs). When selecting an op-amp, designers need to consider package types. Smaller packages, such as SC70 or CSP, may compromise performance compared to larger ones like SOIC or MSOP. For instance, op-amps in plastic packages may exhibit lower performance and are more susceptible to damage during assembly.
In battery-powered applications, especially in PDAs and mobile phones, maintaining good power supply rejection ratio (PSRR) is crucial. An op-amp with PSRR around 80dB is recommended. Additionally, noise levels should be considered, particularly when using high gain configurations. Larger resistors tend to introduce more noise, so it's important to estimate Johnson noise based on resistor values. For example, a 100kΩ resistor generates about 40nV of noise.
When multiple op-amps are used, noise can be reduced by employing a technique where the output noise decreases by a factor equal to the number of amplifiers. For the LMV651, this could reduce the output noise to approximately 12nV. Limiting bandwidth through the use of small capacitors in parallel with feedback resistors is another effective strategy for noise reduction.
Selecting the right op-amp for an analog-to-digital converter (ADC) involves careful consideration of several parameters. Both the op-amp and ADC should operate at the same supply voltage. A low THD+N op-amp is preferable, and if distortion data is unavailable, checking the output impedance can provide useful insights. Lower output impedance usually correlates with better distortion performance.
Speed is another critical parameter. While faster op-amps offer more flexibility, they often come with higher power consumption and potential instability. Designers should choose an op-amp that is at least 50 times faster than the ADC’s sampling rate. Conversion rate is also important; it can be calculated using the formula 2πfVp, where f is the input frequency and Vp is the maximum output swing.
Settling time is another key factor. Manufacturers typically specify settling times within 0.1% or 0.01% of the input voltage, but for high-accuracy applications like 16-bit systems, tighter tolerances are needed. An approximation formula can help estimate settling time based on the ADC's bit resolution and the amplifier's open-loop bandwidth.
Noise is a critical factor that directly impacts the accuracy of the ADC. Estimating total output noise before detailed calculations can save time and resources. The formula for total RMS noise considers the noise gain, op-amp voltage noise, and closed-loop bandwidth.
Using a low-pass filter at the output helps control noise within a specific bandwidth. For example, a simple RC filter sets the bandwidth to 1/(2Ï€RC). If a second-order filter is used, the bandwidth increases slightly. Calculating the SNR after estimating noise allows designers to evaluate system performance effectively.
Reducing bandwidth reduces noise, while increasing it raises noise levels. Choosing a lower-noise op-amp can improve accuracy, but trade-offs like power consumption and package size must be considered.
Other specifications, such as DC parameters like input offset voltage and drift, become increasingly important in precision applications. These factors, along with AC performance, play a vital role in determining the overall reliability and accuracy of the design.