AD1870 Analog Devices, AD1870 Datasheet - Page 7

• Manufacturer Part Number: AD1870
• Description: Single-Supply 16-Bit Stereo ADC
• Manufacturer: Analog Devices

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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, I
Word Clock controlled or left-justified positions. Both 16-bit
samples can also be packed into a 32-bit frame, in left-justified
and I
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 cir-
cuitry 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
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 quantiza-
tion noise would be spread uniformly from dc to one-half the
oversampling frequency, 64 × f
Continued from Page 1
V
- Modulator Noise-Shaping
IN
DIFFERENTIAL
2
CONVERTER
S-compatible positions.
–100
–110
–120
SINGLE-TO-
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
0.0
0.1
0.2
)
V
V
IN
IN
0.3
NORMALIZED
0.4
S
.
0.5
0.6
f
S
0.7
0.8
DAC
DAC
2
S-compatible,
0.9
MODULATOR
BITSTREAM
OUTPUT
1.0
∑-∆ 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 f
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 indirect dependence on temperature and time as
it affects dc offset. The AD1870 suppresses idle tones 20 dB or
better below the integrated noise floor.
The AD1870’s modulator was designed, simulated, and exhaus-
tively 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 16 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 (TPC 5)
graph illustrates the benefits of the AD1870’s internal differen-
tial architecture. 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 bitstream
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 f
decimated to stereo 16-bit words at 48 kHz (or other desired
f
frequency components of the bitstream are attenuated by at
least 90 dB.
The AD1870 decimator implements a symmetric Finite Impulse
Response (FIR) filter which possesses a linear phase response.
This filter achieves a narrow transition band (0.1 × f
stop band attenuation (> 90 dB), and low passband ripple
(< 0.006 dB). The narrow transition band allows the unattenu-
ated digitization of 20 kHz input signals with f
S
). The out-of-band one-bit quantization noise and other high
S
. The high frequency bitstream is
S
and its higher multiples.
AD1870
S
as low as
S
), high