AD1870AR-REEL データシートの表示（PDF） - Analog Devices

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AD1870AR-REEL

Single-Supply 16-Bit ∑-∆ Stereo ADC

Analog Devices 

AD1870

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

NORMALIZED fS

TPC 7. Digital Filter Signal Transfer Function to fS

(Continued from Page 1 )

The flexible serial output port produces data in two’s complement,

MSB-first format. The input and output signals are TTL

compatible. The port is configured by pin selections. Each 16-bit

output word of a stereo pair can be formatted within a 32-bit

field of a 64-bit frame as either right-justified, I2S compatible,

word clock controlled, or left-justified positions. Both 16-bit

samples can also be packed into a 32-bit frame, in left-justified

and I2S compatible positions.

The AD1870 is fabricated on a single monolithic integrated circuit

using a 0.5 µm CMOS double polysilicon, double metal process

and is offered in a plastic 28-lead SOIC package. Analog and

digital supply connections are separated to isolate the analog

circuitry from the digital supply and reduce digital crosstalk.

The AD1870 operates from a single 5 V power supply over the

temperature range of –40°C to +85°C and typically consumes

less than 260 mW of power.

THEORY OF OPERATION

⌺-⌬ Modulator Noise Shaping

The stereo, internally differential, analog modulator of the

AD1870 employs a proprietary feedforward and feedback archi-

tecture that passes input signals in the audio band with a unity

transfer function yet simultaneously shapes the quantization

noise generated by the one-bit comparator out of the audio

band (see Figure 1). Without the ⌺-⌬ architecture, this quanti-

zation noise would be spread uniformly from dc to one-half

the oversampling frequency, 64 × fS.

؉VIN

VIN SINGLE-TO-

DIFFERENTIAL

CONVERTER

DAC

MODULATOR

BITSTREAM

OUTPUT

DAC

؊VIN

Figure 1. Modulator Noise Shaper (One Channel)

⌺-⌬ architectures “shape” the quantization noise-transfer function

in a nonuniform manner. Through careful design, this transfer

function can be specified to high-pass filter the quantization

noise out of the audio band into higher frequency regions. The

AD1870 also incorporates a feedback resonator from the fourth

integrator’s output to the third integrator’s input. This resona-

tor does not affect the signal transfer function but allows the

flexible placement of a zero in the noise transfer function for

more effective noise shaping.

Oversampling by 64 simplifies the implementation of a high

performance audio analog-to-digital conversion system. Antialias

requirements are minimal; a single pole of filtering will usually

suffice to eliminate inputs near fS and its higher multiples.

A fourth order architecture was chosen both to strongly shape

the noise out of the audio band and to help break up the idle

tones produced in all ⌺-⌬ architectures. These architectures

have a tendency to generate periodic patterns with a constant dc

input, a response that looks like a tone in the frequency domain.

These idle tones have a direct frequency dependence on the input

dc offset and an indirect dependence on temperature and time

as it affects the dc offset. The AD1870 suppresses idle tones 20

dB or better below the integrated noise floor.

The AD1870’s modulator was designed, simulated, and ex-

haustively tested to remain stable for any input within a wide

tolerance of its rated input range. The AD1870 is designed to

internally reset itself should it ever be overdriven, to prevent it

from going unstable. It will reset itself within 5 µs at a 48 kHz

sampling frequency after being overdriven. Overdriving the inputs

will produce a waveform “clipped” to plus or minus full scale.

See TPCs 1 through 6 for illustrations of the AD1870’s typical

analog performance as measured by an Audio Precision System

One. Signal-to-(distortion + noise) is shown under a range of

conditions. Note that there is a small variance between the

AD1870 analog performance specifications and some of the

performance plots. This is because the Audio Precision System

One measures THD and noise over a 20 Hz to 24 kHz band-

width, while the analog performance is specified over a 20 Hz to

20 kHz bandwidth (i.e., the AD1870 performs slightly better

than the plots indicate). The power supply rejection graph (TPC 5)

illustrates the benefits of the AD1870’s internal differential ar-

chitecture. The excellent channel separation shown in TPC 6 is

the result of careful chip design and layout.

Digital Filter Characteristics

The digital decimator accepts the modulator’s stereo bit stream

and simultaneously performs two operations on it. First, the

decimator low-pass filters the quantization noise that the modu-

lator shaped to high frequencies and filters any other out-of-

audio-band input signals. Second, it reduces the data rate to an

output word rate equal to fS. The high frequency bit stream is

decimated to stereo 16-bit words at 48 kHz (or other desired

fS). The out-of-band one-bit quantization noise and other high

frequency components of the bit stream are attenuated by at

least 90 dB.

The AD1870 decimator implements a symmetric finite impulse

response (FIR) filter that possesses a linear phase response.

This filter achieves a narrow transition band (0.1 × fS), high

stop-band attenuation (>90 dB), and low pass-band ripple

(<0.006 dB). The narrow transition band allows the unattenu-

ated digitization of 20 kHz input signals with fS as low as

REV. A

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