ANALOG INPUT BOARD

OVERVIEW

The Analog Input Board provides the very impor- tant link between the user's input signal and the DSP processor. From the front panel BNC, the user's signal passes through a low distortion front- end amplifier, gain stages, notch filters, anti- aliasing filter, and finally an A/D Converter. Once converted to digital form, the input signal is ready to be processed by the Digital Signal Processor.

INPUT AMPLIFIER

The goal of any measurement instrument is to per- form some given measurement while affecting the quantities to be measured as little as possible. As such, the input amplifier is often the most critical stage in the entire signal path. The design of the front end input amplifier in the SR850 was driven by an effort to provide optimum performance in the following areas: input voltage noise, input current noise, input capacitance, harmonic distortion, and common mode rejection (CMR). To provide such performance, a FET input differential amplifier with common-mode feedback architecture was chosen. The input signal is first passed through a series of relays to select input mode and input coupling.

The input FETs U100A and U100B are extremely low-noise matched FETs. To improve distortion performance, the input FETs are cascoded to maintain a constant drain-source voltage across each FET. This prevents modulation of the drain- source voltage by the input voltage. U109 senses the source voltages and maintains the same volt- age at the drains (via FETs U108A and B) with some DC offset determined by resistors N102 and N103. U105 provides common-mode feedback and maintains a constant drain current in each FET. The gain of the front end is fixed. U103 pro- vides the output. The DC offset is adjusted by P101 and the CMR by P102.

GAIN STAGES AND NOTCH FILTERS

Collectively, the front end amplifier and following gain stages provide gain up to about 2000.

The notch filters are simple single stage, inverting band pass filters summing with their inputs to remove 60 Hz or 120 Hz. Each filter has a depth

Circuit Description

and frequency adjustment. (60 Hz - depth:P222 and freq:P221 120 Hz - depth:P202 and freq:P201). The 120 Hz notch filter has a configur- able gain of either 1 or 3.17.

The notch filters are followed by two gain stages, each configurable up to a gain of 10.

Overloads are sensed at the input amplifier and the final amplifier outputs. Since there is no attenu- ation in the amplifier chain, this is sufficient.

ANTI-ALIASING FILTER

To prevent aliasing, the input signal passes through a low-pass filter so that all frequency com- ponents greater than half the sampling frequency are attenuated by at least 96 dB. This is accom- plished with an 8-zero 9-pole elliptical low pass filter. The pass band of this filter is DC to 102kHz. The stopband begins at 154 kHz. Stopband atten- uation is nominally 100 dB.

The architecture of the filter is based on a singly terminated passive LC ladder filter. L's are simulat- ed with active gyrators formed by op-amp pairs (U311, U321, U331, U341). Passive LC ladder fil- ters have the special characteristic of being very tolerant of variations in component values. Because no section of the ladder is completely iso- lated from the other, a change in value of any single component affects the entire ladder. The design of the LC ladder however, is such that the characteristics of the rest of the ladder will shift to account for the change in such a way as to mini- mize its effect on the ladder. Not only does this loosen the requirement for extremely high accura- cy resistors and capacitors, but it also makes the filter extremely stable despite wide temperature variations. As such, the anti-aliasing filter used in the SR850 does not ever require calibration to meets its specifications.

Following the anti-aliasing filter is the signal moni- tor buffer (U386) and A/D driver stage (U301).

A/D CONVERTER

The SR850 uses a dual channel A/D converter (U407). Each channel samples simultaneously at

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SRS Labs SR850 manual Analog Input Board, Input Amplifier, Gain Stages and Notch Filters, ANTI-ALIASING Filter, Converter
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SR850 specifications

The SRS Labs SR850 is a high-performance audio processor designed to enhance the listening experience across a variety of applications. With its advanced technologies, the SR850 delivers superior sound quality that is particularly noticeable in environments where audio clarity and fidelity are paramount. This device caters to audio professionals, audiophiles, and casual listeners who demand exceptional performance from their audio systems.

One of the standout features of the SR850 is its proprietary SRS 3D Audio technology. This innovative feature creates a more immersive soundstage, allowing users to experience audio as if they were in a live performance setting. By adding depth and dimension, it elevates the listening experience, making music, movies, and games far more engaging.

Additionally, the SR850 incorporates SRS dialog clarity technology, which enhances speech intelligibility in dialogues and vocals. This is particularly beneficial in film and television viewing, where dialogue can sometimes be drowned out by background music or sound effects. With dialog clarity, users can enjoy crystal-clear conversation tracks without adjusting volume levels continuously.

The device also supports various audio formats, ensuring compatibility with a broad range of media. It boasts inputs for both digital and analog sources, allowing connections to various devices such as TVs, laptops, gaming consoles, and more. The versatility of inputs makes the SR850 a convenient addition to any home entertainment system.

Another impressive characteristic of the SR850 is its compact and user-friendly design. The intuitive interface makes it easy for users to adjust settings to their preference, whether they are a novice or an audio expert. The device is equipped with programmable presets, enabling users to store their favorite sound profiles, making switching between settings quick and hassle-free.

Furthermore, the SR850 features advanced EQ capabilities, granting users the ability to tailor the audio output to their specific tastes. This personalized approach to sound adjustment enhances the overall experience, allowing users to get the most out of their audio content.

In summary, the SRS Labs SR850 is a sophisticated audio processor packed with features designed to significantly uplift sound quality and listening pleasure. With its cutting-edge technologies such as SRS 3D Audio and dialog clarity, an array of connectivity options, and customizable EQ settings, the SR850 stands out as an outstanding option for anyone looking to enhance their audio experience. Whether for professional use in sound engineering or personal enjoyment in home entertainment, the SR850 proves to be a valuable asset.
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