FUNCTIONAL BLOCK DIAGRAM

RCK

WCLK

BCLK

DGND1

RDEDGE

384/

256

CAPL1

CAPL2

AGNDL

REF

CLKIN

TAG

SOUT

RESET

MSBDLY

LJUST

AGND

CAPR1

CAPR2

AGNDR

REF

SERIAL OUTPUT

INTERFACE

THREE-STAGE FIR

DECIMATION

FILTER

DGND2

THREE-STAGE FIR

DECIMATION

FILTER

CLOCK

DIVIDER

VOLTAGE

REFERENCE

SINGLE TO

DIFFERENTIAL INPUT

CONVERTER

SINGLE TO

DIFFERENTIAL INPUT

CONVERTER

AD1877

REV. A

Single-Supply

16-Bit

Stereo ADC

FEATURES
Single 5 V Power Supply
Single-Ended Dual-Channel Analog Inputs
92 dB (Typ) Dynamic Range
90 dB (Typ) S/(THD+N)
0.006 dB Decimator Passband Ripple
Fourth-Order, 64-Times Oversampling

Modulator

Three-Stage, Linear-Phase Decimator
256 F

or 384 F

Input Clock

Less than 100 W (Typ) Power-Down Mode
Input Overrange Indication
On-Chip Voltage Reference
Flexible Serial Output Interface
28-Lead SOIC Package

APPLICATIONS
Consumer Digital Audio Receivers
Digital Audio Recorders, Including Portables

CD-R, DCC, MD and DAT

Multimedia and Consumer Electronic Equipment
Sampling Music Synthesizers
Digital Karaoke Systems

AD1877*

one-bit comparator's quantization noise out of the audio pass-
band. The high order of the modulator randomizes the modulator
output, reducing idle tones in the AD1877 to very low levels.
Because its modulator is single-bit, AD1877 is inherently
monotonic and has no mechanism for producing differential
linearity errors.

The input section of the AD1877 uses autocalibration to correct
any dc offset voltage present in the circuit, provided that the inputs
are ac coupled. The single-ended dc input voltage can swing
between 0.7 V and 3.8 V typically. The AD1877 antialias input
circuit requires four external 470 pF NPO ceramic chip filter
capacitors, two for each channel. No active electronics are
needed. Decoupling capacitors for the supply and reference pins
are also required.

The dual digital decimation filters are triple-stage, finite impulse
response filters for effectively removing the modulator's high
frequency quantization noise and reducing the 64

� F

single-bit

output data rate to an F

word rate. They provide linear phase

and a narrow transition band that properly digitizes 20 kHz signals
at a 44.1 kHz sampling frequency. Passband ripple is less than
0.006 dB, and stopband attenuation exceeds 90 dB.

(Continued on Page 6)

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

� Analog Devices, Inc., 2000

PRODUCT OVERVIEW

The AD1877 is a stereo, 16-bit oversampling ADC based on
Sigma Delta (

) technology intended primarily for digital

audio bandwidth applications requiring a single 5 V power supply.
Each single-ended channel consists of a fourth-order one-bit
noise shaping modulator and a digital decimation filter. An on-
chip voltage reference, stable over temperature and time, defines
the full-scale range for both channels. Digital output data from
both channels are time-multiplexed to a single, flexible serial
interface. The AD1877 accepts a 256

� F

or a 384

� F

input

clock (F

is the sampling frequency) and operates in both serial

port "master" and "slave" modes. In slave mode, all clocks must
be externally derived from a common source.

Input signals are sampled at 64

� F

onto internally buffered

switched-capacitors, eliminating external sample-and-hold ampli-
fiers and minimizing the requirements for antialias filtering at the
input. With simplified antialiasing, linear phase can be preserved
across the passband. The on-chip single-ended to differential signal
converters save the board designer from having to provide them
externally. The AD1877's internal differential architecture provides
increased dynamic range and excellent power supply rejection
characteristics. The AD1877's proprietary fourth-order differen-
tial switched-capacitor

modulator architecture shapes the

*Protected by U.S. Patent Numbers 5055843, 5126653, and others pending.

AD1877�SPECIFICATIONS

REV. A

�2�

TEST CONDITIONS UNLESS OTHERWISE NOTED

Supply Voltages

5.0

Ambient Temperature

�C

Input Clock (F

CLKIN

) [256

� F

]

12.288

MHz

Input Signal

991.768

�0.5

dB Full Scale

Measurement Bandwidth

23.2 Hz to 19.998 kHz

Load Capacitance on Digital Outputs

Input Voltage HI (V

)

2.4

Input Voltage LO (V

)

0.8

Master Mode, Data I

S-Justified (Refer to Figure 14).

Device Under Test (DUT) bypassed and decoupled as shown in Figure 3.
DUT is antialiased and ac coupled as shown in Figure 2. DUT is calibrated.
Values in bold typeface are tested, all others are guaranteed but not tested.

ANALOG PERFORMANCE

Min

Typ

Max

Unit

Resolution

Bits

Dynamic Range (20 Hz to 20 kHz, �60 dB Input)

Without A-Weight Filter

With A-Weight Filter

Signal to (THD + Noise)

86.5

Signal to THD

Analog Inputs

Single-Ended Input Range (

� Full Scale)*

REF

� 1.55

REF

+ 1.55

Input Impedance at Each Input Pin

REF

2.05

2.25

2.55

DC Accuracy

Gain Error

�0.5

2.5

Interchannel Gain Mismatch

0.01

Gain Drift

115

ppm/

�C

Midscale Offset Error (After Calibration)

�3

LSBs

Midscale Drift

ppm/

�C

Crosstalk (EIAJ Method)

�90

�99

p-p = V

REF

� 1.333.

DIGITAL I/O

Min

Typ

Max

Unit

Input Voltage HI (V

)

2.4

Input Voltage LO (V

)

0.8

Input Leakage (I

@ V

= 5 V)

�A

Input Leakage (I

@ V

= 0 V)

�A

Output Voltage HI (V

@ I

= �2 mA)

2.4

Output Voltage LO (V

@ I

= 2 mA)

0.4

Input Capacitance

DIGITAL TIMING (Guaranteed over 0

�C to 70�C, DV

= AV

= 5 V

� 5%. Refer to Figures 17�19.)

Min

Typ

Max

Unit

CLKIN

CLKIN Period

780

CLKIN

CLKIN Frequency (1/t

CLKIN

)

1.28

12.288

20.48

MHz

CPWL

CLKIN LO Pulsewidth

CPWH

CLKIN HI Pulsewidth

RPWL

RESET LO Pulsewidth

BPWL

BCLK LO Pulsewidth

BPWH

BCLK HI Pulsewidth

DLYCKB

CLKIN Rise to BCLK Xmit (Master Mode)

DLYBLR

BCLK Xmit to L

RCK Transition (Master Mode)

DLYBWR

BCLK Xmit to WCLK Rise

DLYBWF

BCLK Xmit to WCLK Fall

DLYDT

BCLK Xmit to Data/Tag Valid (Master Mode)

SETLRBS

RCK Setup to BCLK Sample (Slave Mode)

DLYLRDT

RCK Transition to Data/TAG Valid (Slave Mode)

No MSB Delay Mode (for MSB Only)

SETWBS

WCLK Setup to BCLK Sample (Slave Mode)
Data Position Controlled by WCLK Input Mode

DLYBDT

BCLK Xmit to DATA/TAG Valid (Slave Mode)
All Bits Except MSB in No MSB Delay Mode
All Bits in MSB Delay Mode

POWER

Min

Typ

Max

Unit

Supplies

Voltage, Analog and Digital

4.75

5.25

Analog Current

Analog Current--Power Down (CLKIN Running)

�A

Digital Current

Digital Current--Power Down (CLKIN Running)

�A

Dissipation

Operation--Both Supplies

255

315

Operation--Analog Supply

175

215

Operation--Digital Supply

100

Power Down--Both Supplies (CLKIN Running)

325

�W

Power Down--Both Supplies (CLKIN Not Running)

�W

Power Supply Rejection (See TPC 5)

1 kHz 300 mV p-p Signal at Analog Supply Pins

20 kHz 300 mV p-p Signal at Analog Supply Pins

Stopband (

0.55 � F

)--any 300 mV p-p Signal

AD1877

REV. A

�3�

AD1877

REV. A

�4�

TEMPERATURE RANGE

Min

Typ

Max

Unit

Specifications Guaranteed

�C

Functionality Guaranteed

�C

Storage

�60

+100

�C

DIGITAL FILTER CHARACTERISTICS

Min

Typ

Max

Unit

Decimation Factor

Passband Ripple

0.006

Stopband

Attenuation

48 kHz F

(at Recommended Crystal Frequencies)

Passband

21.6

kHz

Stopband

26.4

kHz

44.1 kHz F

(at Recommended Crystal Frequencies)

Passband

kHz

Stopband

24.25

kHz

32 kHz F

(at Recommended Crystal Frequencies)

Passband

14.4

kHz

Stopband

17.6

kHz

Other F

Passband

0.45

Stopband

0.55

Group Delay

36/F

Group Delay Variation

�s

NOTES

Stopband repeats itself at multiples of 64

� F

, where F

is the output word rate. Thus the digital filter will attenuate to 0 dB across the frequency spectrum except

for a range

�0.55 � F

wide at multiples of 64

� F

Specifications subject to change without notice.

ABSOLUTE MAXIMUM RATINGS

Min

Typ

Max

Unit

1 to DGND1 and DV

2 to DGND2

to AGND/AGNDL/AGNDR

Digital Inputs

DGND � 0.3

+ 0.3

Analog Inputs

AGND � 0.3

+ 0.3

AGND to DGND

�0.3

+0.3

Reference Voltage

Indefinite Short Circuit to Ground

Soldering (10 sec)

300

�C

ORDERING GUIDE

Package

Model

Temperature

Description

Option

AD1877JR

�C to 70�C

SOIC

R-28

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD1877 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

AD1877

REV. A

�5�

PIN FUNCTION DESCRIPTIONS

Input/

Pin

Output

Name

Description

I/O

RCK

Left/

Right Clock

I/O

WCLK

Word Clock

I/O

BCLK

Bit Clock

5 V Digital Supply

DGND1

Digital Ground

RDEDGE

Read Edge Polarity Select

Slave/

Master Select

384/

256

Clock Mode

5 V Analog Supply

Left Channel Input

CAPL1

Left External Filter Capacitor 1

CAPL2

Left External Filter Capacitor 2

AGNDL

Left Analog Ground

REF

Left Reference Voltage Output

REF

Right Reference Voltage Output

AGNDR

Right Analog Ground

CAPR2

Right External Filter Capacitor 2

CAPR1

Right External Filter Capacitor 1

Right Channel Input

AGND

Analog Ground

LJUST

Right/

Left Justify

MSBDLY

Delay MSB One BCLK Period

RESET

Reset

DGND2

Digital Ground

5 V Digital Supply

SOUT

Serial Data Output

TAG

Serial Overrange Output

CLKIN

Master Clock

DEFINITIONS
Dynamic Range

The ratio of a full-scale output signal to the integrated output
noise in the passband (20 Hz to 20 kHz), expressed in decibels
(dB). Dynamic range is measured with a �60 dB input signal
and is equal to (S/[THD+N]) 60 dB. Note that spurious har-
monics are below the noise with a �60 dB input, so the noise
level establishes the dynamic range. The dynamic range is speci-
fied with and without an A-Weight filter applied.

Signal to (Total Harmonic Distortion + Noise)

(S/(THD + N))
The ratio of the root-mean-square (rms) value of the fundamen-
tal input signal to the rms sum of all other spectral components
in the passband, expressed in decibels (dB).

Signal to Total Harmonic Distortion (S/THD)

The ratio of the rms value of the fundamental input signal to the
rms sum of all harmonically related spectral components in the
passband, expressed in decibels.

Passband

The region of the frequency spectrum unaffected by the attenu-
ation of the digital decimator's filter.

Passband Ripple

The peak-to-peak variation in amplitude response from equal-
amplitude input signal frequencies within the passband,
expressed in decibels.

Stopband

The region of the frequency spectrum attenuated by the digital
decimator's filter to the degree specified by "stopband
attenuation."

Gain Error

With a near full-scale input, the ratio of actual output to
expected output, expressed as a percentage.

Interchannel Gain Mismatch

With identical near full-scale inputs, the ratio of outputs of the
two stereo channels, expressed in decibels.

Gain Drift

Change in response to a near full-scale input with a change in
temperature, expressed as parts-per-million (ppm) per

�C.

Midscale Offset Error

Output response to a midscale dc input, expressed in least-
significant bits (LSBs).

Midscale Drift

Change in midscale offset error with a change in temperature,
expressed as parts-per-million (ppm) per

�C.

Crosstalk (EIAJ Method)

Ratio of response on one channel with a grounded input to a
full-scale 1 kHz sine-wave input on the other channel, expressed
in decibels.

Power Supply Rejection

With no analog input, signal present at the output when a
300 mV p-p signal is applied to power supply pins, expressed in
decibels of full scale.

Group Delay

Intuitively, the time interval required for an input pulse to
appear at the converter's output, expressed in milliseconds
(ms). More precisely, the derivative of radian phase with respect
to radian frequency at a given frequency.

Group Delay Variation

The difference in group delays at different input frequencies.
Specified as the difference between largest and the smallest
group delays in the passband, expressed in microseconds (

�s).

AD1877

REV. A

�6�

(

Continued from Page 1

)

The flexible serial output port produces data in twos-comple-
ment, 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

S-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 I

S-compatible positions.

The AD1877 is fabricated on a single monolithic integrated circuit
using a 0.8

�m 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 AD1877 operates from a single 5 V power supply over the
temperature range of 0

�C to 70�C, and typically consumes less

than 260 mW of power.

THEORY OF OPERATION

Modulator Noise-Shaping

The stereo, internally differential analog modulator of the
AD1877 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

DAC

SINGLE TO

DIFFERENTIAL

CONVERTER

MODULATOR
BITSTREAM
OUTPUT

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
AD1877 also incorporates a feedback resonator from the fourth
integrator's output to the third integrator's input. This resonator
does not affect the signal transfer function but allows the flexible
placement of a zero in the noise transfer function for more effec-
tive noise shaping.

Oversampling by 64 simplifies the implementation of a high per-
formance audio analog-to-digital conversion system. Antialias
requirements are minimal; a single pole of filtering will usually
suffice to eliminate inputs near F

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 indirect dependence on temperature and time as it
affects dc offset. The AD1877 suppresses idle tones 20 dB or
better below the integrated noise floor.

The AD1877'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 AD1877 is designed to internally
reset itself should it ever be overdriven, to prevent it from going
instable. It will reset itself within 5

�s 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 AD1877'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 AD1877 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 AD1877 performs slightly better
than the plots indicate). The power supply rejection (TPC 5)
graph illustrates the benefits of the AD1877'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

. The high frequency bitstream is

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

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

frequency components of the bitstream are attenuated by at
least 90 dB.

The AD1877 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

), high

stopband 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

as low as

44.1 kHz. The stopband attenuation is sufficient to eliminate
modulator quantization noise from affecting the output. Low
passband ripple prevents the digital filter from coloring the
audio signal. See TPC 7 for the digital filter's characteristics.
The output from the decimator is available as a single serial
output, multiplexed between left and right channels.

Note that the digital filter itself is operating at 64

� F

. As a

consequence, Nyquist images of the passband, transition band,
and stopband will be repeated in the frequency spectrum at
multiples of 64

� F

. Thus the digital filter will attenuate to

greater than 90 dB across the frequency spectrum except for a
window

�0.55 � F

wide centered at multiples of 64

� F

. Any

input signals, clock noise, or digital noise in these frequency
windows will not be attenuated to the full 90 dB. If the high
frequency signals or noise appear within the passband images
within these windows, they will not be attenuated at all, and
therefore input antialias filtering should be applied.

AD1877

REV. A

�7�

Sample Delay

The sample delay or "group delay" of the AD1877 is dominated
by the processing time of the digital decimation filter. FIR fil-
ters convolve a vector representing time samples of the input
with an equal-sized vector of coefficients. After each convolu-
tion, the input vector is updated by adding a new sample at one
end of the "pipeline" and discarding the oldest input sample at
the other. For an FIR filter, the time at which a step input appears
at the output will be when that step input is half way through
the input sample vector pipeline. The input sample vector
is updated every 64

� F

. The equation which expresses the

group delay for the AD1877 is:

Group Delay (sec) = 36/F

(Hz)

For the most common sample rates this can be summarized as:

Group Delay

48 kHz

750

�s

44.1 kHz

816

�s

32 kHz

1125

�s

Due to the linear phase properties of FIR filters, the group
delay variation, or differences in group delay at different fre-
quencies is essentially zero.

OPERATING FEATURES
Voltage Reference and External Filter Capacitors

The AD1877 includes a +2.25 V on-board reference that deter-
mines the AD1877's input range. The left and right reference
pins (14 and 15) should be bypassed with a 0.1

�F ceramic chip

capacitor in parallel with a 4.7

�F tantalum as shown below in

Figure 3. Note that the chip capacitor should be closest to the
pin. The internal reference can be overpowered by applying an
external reference voltage at the V

REF

L (Pin 14) and V

REF

(Pin 15) pins, allowing multiple AD1877s to be calibrated to
the same gain. It is not possible to overpower the left and right
reference pins individually; the external reference voltage
should be applied to both Pin 14 and Pin 15. Note that the ref-
erence pins must still be bypassed as shown in Figure 3.

It is possible to bypass each reference pin (V

REF

L and V

REF

with a capacitor larger than the suggested 4.7

�F, however it is

not recommended. A larger capacitor will have a longer charge-
up time which may extend into the autocalibration period, yield-
ing incorrect results.

The AD1877 requires four external filter capacitors on Pins 11,
12, 17 and 18. These capacitors are used to filter the single-to
differential converter outputs, and are too large for practical
integration onto the die. They should be 470 pF NPO ceramic
chip type capacitors as shown in Figure 3, placed as close to the
AD1877 package as possible.

Sample Clock

An external master clock supplied to CLKIN (Pin 28) drives
the AD1877 modulator, decimator, and digital interface. As
with any analog-to-digital conversion system, the sampling clock
must be low jitter to prevent conversion errors. If a crystal oscil-
lator is used as the clock source, it should be bypassed with a
0.1

�F capacitor, as shown below in Figure 3.

For the AD1877, the input clock operates at either 256

� F

384

� F

as selected by the 384/

256 pin. When 384/256 is HI,

the 384 mode is selected and when 384/

256 is LO, the 256

mode is selected. In both cases, the clock is divided down to
obtain the 64

� F

clock required for the modulator. The out-

put word rate itself will be at F

. This relationship is illustrated

for popular sample rates below:

256 Mode

384 Mode

Modulator

Output Word

CLKIN

Sample Rate Rate

12.288 MHz

18.432 MHz

3.072 MHz

48 kHz

11.2896 MHz

16.9344 MHz 2.822 MHz

44.1 kHz

8.192 MHz

12.288 MHz

2.048 MHz

32 kHz

The AD1877 serial interface will support both master and slave
modes. Note that in slave mode it is required that the serial
interface clocks are externally derived from a common source.
In master mode, the serial interface clock outputs are internally
derived from CLKIN.

Reset, Autocalibration and Power Down

The active LO

RESET pin (Pin 23) initializes the digital deci-

mation filter and clears the output data buffer. While in the reset
state, all digital pins defined as outputs of the AD1877 are
driven to ground (except for BCLK, which is driven to the state
defined by RDEDGE (Pin 6)). Analog Devices recommends
resetting the AD1877 on initial power up so that the device is
properly calibrated. The reset signal must remain LO for the
minimum period specified in "Specifications" above. The reset
pulse is asynchronous with respect to the master clock, CLKIN.
If, however, multiple AD1877s are used in a system, and it is
desired that they leave the reset state at the same time, the
common reset pulse should be made synchronous to CLKIN
(i.e.,

RESET should be brought HI on a CLKIN falling edge).

Multiple AD1877s can be synchronized to each other by using
a single master clock and a single reset signal to initialize all
devices. On coming out of reset, all AD1877s will begin sam-
pling at the same time. Note that in slave mode, the AD1877 is
inactive (and all outputs are static, including WCLK) until the
first rising edge of L

RCK after the first falling edge of LRCK.

This initial low going then high going edge of L

RCK can be used

to "skew" the sampling start-up time of one AD1877 relative to
other AD1877s in a system. In the Data Position Controlled by
WCLK Input mode, WCLK must be HI with L

RCK HI, then

WCLK HI with L

RCK LO, then WCLK HI with LRCK HI

before the AD1877 starts sampling.

The AD1877 achieves its specified performance without the
need for user trims or adjustments. This is accomplished
through the use of on-chip automatic offset calibration that
takes place immediately following reset. This procedure nulls
out any offsets in the single-to-differential converter, the analog
modulator and the decimation filter. Autocalibration completes
in approximately 8192

� (1/(F

) seconds, and need only be

performed once at power-up in most applications. [In slave
mode, the 8192 cycles required for autocalibration do not start
until after the first rising edge of L

RCK following the first fall-

ing edge of L

RCK.] The autocalibration scheme assumes that

the inputs are ac coupled. DC coupled inputs will work with the
AD1877, but the autocalibration algorithm will yield an incor-
rect offset compensation.

AD1877

REV. A

�8�

The AD1877 also features a power-down mode. It is enabled by
the active LO

RESET Pin 23 (i.e., the AD1877 is in powerdown

mode while

RESET is held LO). The power savings are speci-

fied in the `'Specifications'' section above. The converter is shut
down in the power-down state and will not perform conversions.
The AD1877 will be reset upon leaving the power-down state, and
autocalibration will commence after the

RESET pin goes HI.

Power consumption can be further reduced by slowing down the
master clock input (at the expense of input passband width).
Note that a minimum clock frequency, F

CLKIN

, is specified for

the AD1877.

Tag Overrange Output

The AD1877 includes a TAG serial output (Pin 27) which is
provided to indicate status on the level of the input voltage. The
TAG output is at TTL compatible logic levels. A pair of unsigned
binary bits are output, synchronous with L

RCK (MSB then

LSB), that indicate whether the current signal being converted
is: more than 1 dB under full scale; within 1 dB under full scale;
within 1 dB over full scale; or more than 1 dB over full scale.
The timing for the TAG output is shown in TPCs 7 through 16.
Note that the TAG bits are not "sticky," i.e., they are not peak
reading, but rather change with every sample. Decoding of these
two bits is as follows:

TAG Bits
MSB, LSB

Meaning

More Than 1 dB Under Full Scale

Within 1 dB Under Full Scale

Within 1 dB Over Full Scale

More Than 1 dB Over Full Scale

APPLICATIONS ISSUES
Recommended Input Structure

The AD1877 input structure is single-ended to allow the board
designer to achieve a high level of functional integration. The
very simple recommended input circuit is shown in Figure 2.
Note the 1

�F ac coupling capacitor which allows input level

shifting for 5 V only operation, and for autocalibration to
properly null offsets. The 3 dB point of the single-pole antialias
RC filter is 240 kHz, which results in essentially no attenuation
at 20 kHz. Attenuation at 3 MHz is approximately 22 dB, which
is adequate to suppress F

noise modulation. If the analog inputs

are externally ac coupled, then the 1

�F ac coupling capacitors

shown in Figure 2 are not required.

AD1877

LEFT

INPUT

RIGHT

INPUT

300

2.2nF
NPO

1 F

2.2nF
NPO

300

Figure 2. Recommended Input Structure for Externally
DC Coupled Inputs

Analog Input Voltage Swing

The single-ended input range of the analog inputs is specified in
relative terms in the "Specifications" section of this data sheet.
The input level at which clipping occurs linearly tracks the voltage
reference level, i.e., if the reference is high relative to the typical
2.25 V, the allowable input range without clipping is corre-
spondingly wider; if the reference is low relative to the typical
2.25 V, the allowable input range is correspondingly narrower.

Thus the maximum input voltage swing can be computed using
the following ratio:

2 25

3 1

(

)

(

)

(

)

(

)

al reference voltage

V p p no

al voltage swing

X Volts measured reference voltage

Y Volts

ximum swing without clipping

min

Layout and Decoupling Considerations

Obtaining the best possible performance from the AD1877
requires close attention to board layout. Adhering to the follow-
ing principles will produce typical values of 92 dB dynamic
range and 90 dB S/(THD+N) in target systems. Schematics and
layout artwork of the AD1877 Evaluation Board, which implement
these recommendations, are available from Analog Devices.

The principles and their rationales are listed below. The first
two pertain to bypassing and are illustrated in Figure 3.

ANALOG

DIGITAL

AD1877

CAPL2

CAPL1

CLKIN

AGND

1 DGND1 DGND2 DV

470pF

NPO

AGNDL V

REF

L V

REF

R AGNDR

CAPR2

CAPR1

470pF
NPO

OSCILLATOR

DIGITAL

10nF

470pF
NPO

10nF

1 F

0.1 F

1 F

0.1 F

4.7 F

0.1 F

4.7 F

0.1 F

Figure 3. Recommended Bypassing and Oscillator Circuits

There are two pairs of digital supply pins on opposite sides of
the part (Pins 4 and 5 and Pins 24 and 25). The user should
tie a bypass chip capacitor (10 nF ceramic) in parallel with a
decoupling capacitor (1

�F tantalum) on EACH pair of supply

pins as close to the pins as possible. The traces between these
package pins and the capacitors should be as short and as wide
as possible. This will prevent digital supply current transients
from being inductively transmitted to the inputs of the part.

Use a 0.1

�F chip analog capacitor in parallel with a 1.0 �F

tantalum capacitor from the analog supply (Pin 9) to the analog
ground plane. The trace between this package pin and the
capacitor should be as short and as wide as possible.

The AD1877 should be placed on a split ground plane. The
digital ground plane should be placed under the top end of the
package, and the analog ground plane should be placed under
the bottom end of the package as shown in Figure 4. The split
should be between Pins 8 and 9 and between Pins 20 and 21.

AD1877

REV. A

�9�

The ground planes should be tied together at one spot under-
neath the center of the package with an approximately 3 mm
trace. This ground plane technique also minimizes RF transmis-
sion and reception.

RCK

WCLK

BCLK

DGND1

RDEDGE

384/

256

CAPL1

CAPL2

AGNDL

REF

CLKIN

TAG

SOUT

RESET

MSBDLY

LJUST

AGND

CAPR1

CAPR2

AGNDR

REF

DGND2

DIGITAL GROUND PLANE

ANALOG GROUND PLANE

Figure 4. Recommended Ground Plane

Each reference pin (14 and 15) should be bypassed with a 0.1

�F

ceramic chip capacitor in parallel with a 4.7

�F tantalum capaci-

tor. The 0.1

�F chip cap should be placed as close to the pack-

age pin as possible, and the trace to it from the reference pin
should be as short and as wide as possible. Keep this trace away
from any analog traces (Pins 10, 11, 12, 17, 18, 19)! Coupling
between input and reference traces will cause even order har-
monic distortion. If the reference is needed somewhere else on
the printed circuit board, it should be shielded from any signal
dependent traces to prevent distortion.

Wherever possible, minimize the capacitive load on the digital
outputs of the part. This will reduce the digital spike currents

drawn from the digital supply pins and help keep the IC sub-
strate quiet.

How to Extend SNR

A cost-effective method of improving the dynamic range and
SNR of an analog-to-digital conversion system is to use multiple

AD1877 channels in parallel with a common analog input. This
technique makes use of the fact that the noise in independent
modulator channels is uncorrelated. Thus every doubling of the
number of AD1877 channels used will improve system dynamic
range by 3 dB. The digital outputs from the corresponding deci-
mator channels have to be arithmetically averaged to obtain the
improved results in the correct data format. A microprocessor,
either general-purpose or DSP, can easily perform the averaging
operation.

Shown below in Figure 5 is a circuit for obtaining a 3 dB
improvement in dynamic range by using both channels of a
single AD1877 with a mono input. A stereo implementation
would require using two AD1877s and using the recommended
input structure shown in Figure 2. Note that a single microproces-
sor would likely be able to handle the averaging requirements
for both left and right channels.

AD1877

RECOMMENDED

INPUT BUFFER

SINGLE

CHANNEL

INPUT

DIGITAL

AVERAGER

AD1877

SINGLE
CHANNEL
OUTPUT

Figure 5. Increasing Dynamic Range By Using Two
AD1877 Channels

DIGITAL INTERFACE
Modes of Operation

The AD1877's flexible serial output port produces data in
two's-complement, MSB-first format. The input and output sig-
nals are TTL logic level compatible. Time multiplexed serial
data is output on SOUT (Pin 26), left channel then right chan-
nel, as determined by the left/right clock signal L

RCK (Pin 1).

Note that there is no method for forcing the right channel to
precede the left channel. The port is configured by pin selec-
tions. The AD1877 can operate in either master or slave mode,
with the data in right-justified, I

S-compatible, Word Clock

controlled or left-justified positions.

The various mode options are pin-programmed with the Slave/
Master Pin (7), the Right/Left Justify Pin (21), and the MSB
Delay Pin (22). The function of these pins is summarized as
follows:

AD1877

REV. A

�10�

M RLJUST

MSBDLY

WCLK

BCLK

RCK

Serial Port Operation Mode

Output

Input

Slave Mode. WCLK frames the data. The MSB is output on the
17th BCLK cycle. Provides right-justified data in slave mode
with a 64

� F

BCLK frequency. See Figure 7.

Input

Slave Mode. The MSB is output in the BCLK cycle after
WCLK is detected HI. WCLK is sampled on the BCLK active
edge, with the MSB valid on the next BCLK active edge. Tying
WCLK HI results in I

S-justified data. See Figure 8.

Output

Input

Slave Mode. Data left-justified with WCLK framing the data.
WCLK rises immediately after an L

RCK transition. The MSB is

valid on the first BCLK active edge. See Figure 9.

Output

Input

Slave Mode. Data I

S-justified with WCLK framing the data.

WCLK rises in the second BCLK cycle after an L

RCK transi-

tion. The MSB is valid on the second BCLK active edge. See
Figure 10.

Output

Master Mode. Data right-justified. WCLK frames the data,
going HI in the 17th BCLK cycle. BCLK frequency = 64

� F

See Figure 11.

Output

Master Mode. Data right-justified + 1. WCLK is pulsed in the
17th BCLK cycle, staying HI for only 1 BCLK cycle. BCLK
frequency = 64

� F

. See Figure 12.

Output

Master Mode. Data left-justified. WCLK frames the data.
BCLK frequency = 64

� F

. See Figure 13.

Output

Master Mode. Data I

S-justified. WCLK frames the data.

BCLK frequency = 64

� F

. See Figure 14.

Serial Port Data Timing Sequences
The RDEDGE input (Pin 6) selects the bit clock (BCLK) polarity.
RDEDGE HI causes data to be transmitted on the BCLK falling
edge and valid on the BCLK rising edge; RDEDGE LO causes
data to be transmitted on the BCLK rising edge and valid on
the BCLK falling edge. This is shown in the serial data output
timing diagrams. The term "sampling" is used generically to
denote the BCLK edge (rising or falling) on which the serial
data is valid. The term "transmitting" is used to denote the
other BCLK edge. The S/

M input (Pin 7) selects slave mode (S/

M HI) or master mode (S/M LO). Note that in slave mode,
BCLK may be continuous or gated (i.e., a stream of pulses dur-
ing the data phase followed by periods of inactivity between
channels).

In the master modes, the bit clock (BCLK), the left/right clock
(L

RCK), and the word clock (WCLK) are always outputs, gen-

erated internally in the AD1877 from the master clock (CLKIN)
input. In master mode, a L

RCK cycle defines a 64-bit "frame."

RCK is HI for a 32-bit "field" and LRCK is LO for a 32-bit

"field."

In the slave modes, the bit clock (BCLK), and the left/right clock
(L

RCK) are user-supplied inputs. The word clock (WCLK) is an

internally generated output except when S/

M is HI, RLJUST is

HI, and

MSBDLY is LO, when it is a user-supplied input which

controls the data position. Note that the AD1877 does not sup-
port asynchronous operation in slave mode; the clocks (CLKIN,
L

RCK, BCLK and WCLK) must be externally derived from a

common source. In general, CLKIN should be divided down
externally to create L

RCK, BCLK and WCLK.

In the slave modes, the relationship between L

RCK and BCLK

is not fixed, to the extent that there can be an arbitrary number
of BCLK cycles between the end of the data transmission and
the next L

RCK transition. The slave mode timing diagrams are

therefore simplified as they show precise 32-bit fields and 64-bit
frames.

In two slave modes, it is possible to pack two 16-bit samples in
a single 32-bit frame, as shown in Figures 15 and 16. BCLK,
L

RCK, DATA and TAG operate at one half the frequency

(twice the period) as in the 64-bit frame modes. This 32-bit
frame mode is enabled by pulsing the L

RCK HI for a minimum

of one BCLK period to a maximum of sixteen BCLK periods.
The L

RCK HI for one BCLK period case is shown in Figures

15 and 16. With a one or two BCLK period HI pulse on
L

RCK, note that both the left and right TAG bits are output

immediately, back-to-back. With a three to sixteen BCLK period
HI pulse on L

RCK, the left TAG bits are followed by one to

fourteen "dead" cycles (i.e., zeros) followed by the right TAG
bits. Also note that WCLK stays HI continuously when the
AD1877 is in the 32-bit frame mode. Figure 15 illustrates the
left-justified case, while Figure 16 illustrates the I

S-justified case.

In all modes, the left and right channel data is updated with the
next sample within the last 1/8 of the current conversion cycle (i.e.,
within the last 4 BCLK cycles in 32-bit frame mode, and within
the last 8 BCLK cycles in 64-bit frame mode). The user must
constrain the output timing such that the MSB of the right channel
is read before the final 1/8 of the current conversion period.

AD1877

REV. A

�11�

Two modes deserve special discussion. The first special mode,
"Slave Mode, Data Position Controlled by WCLK Input" (S/

= HI, R

LJUST = HI, MSBDLY = LO), shown in Figure 8, is

the only mode in which WCLK is an input. The 16-bit output
data words can be placed at user-defined locations within 32-bit
fields. The MSB will appear in the BCLK period after WCLK is
detected HI by the BCLK sampling edge. If WCLK is HI dur-
ing the first BCLK of the 32-bit field (if WCLK is tied HI for
example), then the MSB of the output word will be valid on the
sampling edge of the second BCLK. The effect is to delay the
MSB for one bit clock cycle into the field, making the output
data compatible at the data format level with the I

S data for-

mat. Note that the relative placement of the WCLK input can
vary from 32-bit field to 32-bit field, even within the same
64-bit frame. For example, within a single 64-bit frame, the left
word could be right justified (by pulsing WCLK HI on the 16th
BCLK) and the right word could be in an I

S-compatible data

format (by having WCLK HI at the beginning of the second field).

In the second special mode "Master Mode, Right-Justified with
MSB Delay, WCLK Pulsed in 17th Cycle" (S/

M = LO,

LJUST = HI, MSBDLY = LO), shown in Figure 12, WCLK

is an output and is pulsed for one cycle by the AD1877. The
MSB is valid on the 18th BCLK sampling edge, and the LSB
extends into the first BCLK period of the next 32-bit field.

Timing Parameters

For master modes, a BCLK transmitting edge (labeled "XMIT")
will be delayed from a CLKIN rising edge by t

DLYCKB

, as shown

in Figure 17. A L

RCK transition will be delayed from a BCLK

transmitting edge by t

DLYBLR

. A WCLK rising edge will be

delayed from a BCLK transmitting edge by t

DLYBWR

, and a WCLK

falling edge will be delayed from a BCLK transmitting edge by
t

DLYBWF

. The DATA and TAG outputs will be delayed from a

transmitting edge of BCLK by t

DLYDT

For slave modes, an L

RCK transition must be setup to a BCLK

sampling edge (labeled "SAMPLE") by t

SETLRBS.

The DATA

and TAG outputs will be delayed from an L

RCK transition by

DLYLRDT

, and DATA and TAG outputs will be delayed from

BCLK transmitting edge by t

DLYBDT

. For "Slave Mode, Data

Position Controlled by WCLK Input," WCLK must be setup to
a BCLK sampling edge by t

SETWBS

For both master and slave modes, BCLK must have a minimum
LO pulsewidth of t

BPWL

, and a minimum HI pulsewidth of t

BPWH

The AD1877 CLKIN and

RESET timing is shown in Figure

19. CLKIN must have a minimum LO pulsewidth of t

CPWL

, and

a minimum HI pulse width of t

CPWH

. The minimum period of

CLKIN is given by t

CLKIN

RESET must have a minimum LO

pulsewidth of t

RPWL

. Note that there are no setup or hold time

requirements for

RESET.

Synchronizing Multiple AD1877s

Multiple AD1877s can be synchronized by making all the
AD1877s serial port slaves. This option is illustrated in
Figure 6. See the "Reset, Autocalibration and Power Down"
section above for additional information.

#1 AD1877

SLAVE MODE

CLKIN

DATA

BCLK

WCLK

RCK

CLOCK

SOURCE

#2 AD1877

SLAVE MODE

CLKIN

DATA

BCLK

WCLK

RCK

#N AD1877

SLAVE MODE

CLKIN

DATA

BCLK

WCLK

RCK

RESET

Figure 6. Synchronizing Multiple AD1877s

AD1877

REV. A

�13�

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

MSB-14

LSB

PREVIOUS DATA

MSB-1

LEFT DATA

MSB-2

LSB

RIGHT DATA

SOUT

OUTPUT

ZEROS

MSB-1 MSB-2

LSB

ZEROS

WCLK

OUTPUT

TAG

OUTPUT

MSB

LSB

LEFT TAG

MSB

LSB

RIGHT TAG

MSB

LSB

LEFT TAG

RCK

INPUT

MSB

Figure 7. Serial Data Output Timing: Slave Mode, Right-Justified with No MSB Delay,
S/M = Hl, RLJUST = Hl, MSBDLY = Hl

BCLK

RDEDGE= LO

BCLK

RDEDGE = HI

MSB-1

LEFT DATA

MSB-2

LSB

SOUT

OUTPUT

ZEROS

RIGHT DATA

MSB-1 MSB-2

LSB

ZEROS

WCLK
INPUT

TAG

OUTPUT

MSB

LEFT TAG

MSB

RIGHT TAG

ZEROS

INPUT

RCK

INPUT

MSB

LSB

MSB

LSB

Figure 8. Serial Data Output Timing: Slave Mode, Data Position Controlled by WCLK Input,

M = Hl, RLJUST= Hl, MSBDLY = LO

dBFS

�80

1.0

�60

�70

0.1

0.0

�40

�50

�30

�20

�10

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

�110

�90

�100

NORMALIZED F

TPC 7. Digital Filter Signal Transfer Function to F

AD1877

REV. A

�14�

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

WCLK

OUTPUT

TAG

OUTPUT

LSB

LEFT TAG

LSB

RIGHT TAG

MSB-1

LEFT DATA

MSB-2

LSB

MSB

MSB-1

RIGHT DATA

MSB-2

LSB

ZEROS

INPUT

RCK

INPUT

MSB

Figure 9. Serial Data Output Timing: Slave Mode, Left-Justified with No MSB Delay, S/

M = Hl,

LJUST = LO, MSBDLY = Hl

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

WCLK

OUTPUT

TAG

OUTPUT

MSB

LEFT TAG

MSB

RIGHT TAG

MSB-1

LEFT DATA

MSB-2

LSB

MSB-1

RIGHT DATA

MSB-2

LSB

ZEROS

INPUT

RCK

INPUT

MSB

LSB

MSB

LSB

Figure 10. Serial Data Output Timing: Slave Mode, I

S-Justified, S/

M = Hl, RLJUST = LO, MSBDLY = LO

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

MSB-14

LSB

PREVIOUS DATA

MSB-1

LEFT DATA

MSB-2

LSB

RIGHT DATA

SOUT

OUTPUT

ZEROS

MSB-1 MSB-2

LSB

ZEROS

WCLK

OUTPUT

TAG

OUTPUT

MSB

LSB

LEFT TAG

MSB

LSB

RIGHT TAG

MSB

LSB

LEFT TAG

OUTPUT

RCK

OUTPUT

MSB

Figure 11. Serial Data Output Timing: Master Mode, Right-Justified with No MSB Delay, S/

M = LO,

LJUST = Hl, MSBDLY = Hl

AD1877

REV. A

�15�

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

MSB-14

LSB

PREVIOUS DATA

MSB-1

LEFT DATA

MSB-2

LSB

RIGHT DATA

SOUT

OUTPUT

ZEROS

MSB-1 MSB-2

LSB

ZEROS

WCLK

OUTPUT

TAG

OUTPUT

MSB

LSB

LEFT TAG

MSB

LSB

RIGHT TAG

RCK

OUTPUT

MSB

Figure 12. Serial Data Output Timing. Master Mode, Right-Justified with MSB Delay,
WCLK Pulsed in 17th BCLK Cycle, S/

M = LO, RLJUST = Hl, MSBDLY = LO

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

WCLK

OUTPUT

TAG

OUTPUT

LSB

LEFT TAG

LSB

RIGHT TAG

MSB-1

LEFT DATA

MSB-2

LSB

MSB-1

RIGHT DATA

MSB-2

LSB

ZEROS

RCK

OUTPUT

MSB

Figure 13. Serial Data Output Timing: Master Mode, Left-Justified with No MSB Delay,
S/

M = LO, RLJUST = LO, MSBDLY = Hl

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

WCLK

OUTPUT

TAG

OUTPUT

MSB

LEFT TAG

MSB

RIGHT TAG

MSB-1

LEFT DATA

MSB-2

LSB

MSB-1

RIGHT DATA

MSB-2

LSB

ZEROS

OUTPUT

RCK

OUTPUT

MSB

LSB

MSB

LSB

Figure 14. Serial Data Output Timing: Master Mode, I

S-Justified, S/

M = LO, RLJUST = LO,

MSBDLY = LO

AD1877

REV. A

�16�

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

WCLK

OUTPUT

TAG

OUTPUT

INPUT

LSB

LEFT TAG

MSB

LSB

RIGHT TAG

LEFT TAG

LSB

MSB-14

LSB

PREVIOUS DATA

MSB-1 MSB-2 MSB-3

LEFT DATA

MSB-4

MSB-3 MSB-4

LSB

MSB-1 MSB-2

RIGHT DATA

LSB

MSB-1

LEFT DATA

RCK

INPUT

MSB

Figure 15. Serial Data Output Timing: Slave Mode, Left-Justified with No MSB Delay,
32-Bit Frame Mode, S/

M = Hl, RLJUST = LO, MSBDLY = Hl

BCLK

RDEDGE = LO

BCLK

RDEDGE = HI

SOUT

OUTPUT

TAG

OUTPUT

INPUT

WCLK

OUTPUT

MSB

LEFT TAG

MSB

LSB

RIGHT TAG

MSB-14

LSB

PREVIOUS DATA

MSB-1 MSB-2 MSB-3

LEFT DATA

MSB-4

MSB-3 MSB-4

LSB

MSB-1 MSB-2

RIGHT DATA

LSB

MSB-1

LEFT DATA

MSB

LEFT TAG

MSB

RIGHT TAG

RCK

INPUT

MSB

LSB

MSB

LSB

Figure 16. Serial Data Output Timing: Slave Mode, I

S-Justified, 32-Bit Frame Mode,

M = Hl, RLJUST= LO, MSBDLY = LO

BCLK OUTPUT (64 x FS)

RDEDGE = LO

CLKIN

INPUT

BCLK OUTPUT (64 x FS)

RDEDGE = HI

WCLK

OUTPUT

DATA & TAG

OUTPUTS

XMIT

DLYCKB

BPWL

BPWH

BPWL

BPWH

DLYBLR

DLYDT

DLYBWR

DLYBWF

RCK

OUTPUT

Figure 17. Master Mode Clock Timing

Part Number AD1877