AD AD7664AST 16-bit, 570 ksps cmos adc Datasheet

a
FEATURES
Throughput:
570 kSPS (Warp Mode)
500 kSPS (Normal Mode)
INL: ⴞ2.5 LSB Max (ⴞ0.0038% of Full-Scale)
16 Bits Resolution with No Missing Codes
S/(N+D): 90 dB Typ @ 10 kHz
THD: –100 dB Typ @ 10 kHz
Analog Input Voltage Range: 0 V to 2.5 V
Both AC and DC Specifications
No Pipeline Delay
Parallel and Serial 5 V/3 V Interface
Single 5 V Supply Operation
Power Dissipation
97 mW Typical,
21 ␮W @ 100 SPS
Power-Down Mode: 7 ␮W Max
Package: 48-Lead Quad Flat Pack (LQFP)
Pin-to-Pin Compatible Upgrade of the AD7660
16-Bit, 570 kSPS CMOS ADC
AD7664*
FUNCTIONAL BLOCK DIAGRAM
AVDD AGND REF REFGND
DVDD
DGND
OVDD
AD7664
OGND
SERIAL
PORT
IN
SWITCHED
CAP DAC
INGND
16
DATA[15:0]
BUSY
PARALLEL
INTERFACE
CLOCK
PD
RESET
CONTROL LOGIC AND
CALIBRATION CIRCUITRY
RD
CS
SER/PAR
OB/2C
WARP
IMPULSE
CNVST
APPLICATIONS
Data Acquisition
Instrumentation
Digital Signal Processing
Spectrum Analysis
Medical Instruments
Battery-Powered Systems
Process Control
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7664 is a 16-bit, 570 kSPS, charge redistribution SAR,
analog-to-digital converter that operates from a single 5 V power
supply. The part contains a high-speed 16-bit sampling ADC,
an internal conversion clock, error correction circuits, and both
serial and parallel system interface ports.
1. Fast Throughput
The AD7664 is a 570 kSPS, charge redistribution, 16-bit
SAR ADC with internal error correction circuitry.
The AD7664 is hardware factory calibrated and is comprehensively
tested to ensure such ac parameters as signal-to-noise ratio (SNR)
and total harmonic distortion (THD), in addition to the more
traditional dc parameters of gain, offset, and linearity.
It features a very high sampling rate mode (Warp) and, for asynchronous conversion rate applications, a fast mode (Normal)
and, for low power applications, a reduced power mode (Impulse)
where the power is scaled with the throughput.
It is fabricated using Analog Devices’ high-performance, 0.6
micron CMOS process, with correspondingly low cost and is
available in a 48-lead LQFP with operation specified from –40°C
to +85°C.
2. Superior INL
The AD7664 has a maximum integral nonlinearity of 2.5 LSBs
with no missing 16-bit code.
3. Single-Supply Operation
The AD7664 operates from a single 5 V supply and typically
dissipates only 97 mW. In impulse mode, its power dissipation decreases with the throughput to, for instance, only 21 µW
at a 100 SPS throughput. It consumes 7 µW maximum when in
power-down.
4. Serial or Parallel Interface
Versatile parallel or 2-wire serial interface arrangement compatible with both 3 V or 5 V logic.
*Patent pending.
REV. 0
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
AD7664–SPECIFICATIONS (–40ⴗC to +85ⴗC, AVDD = DVDD= 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)
Parameter
Conditions
Min
RESOLUTION
ANALOG INPUT
Voltage Range
Operating Input Voltage
Analog Input CMRR
Input Current
Input Impedance
THROUGHPUT SPEED
Complete Cycle
Throughput Rate
Time Between Conversions
Complete Cycle
Throughput Rate
Complete Cycle
Throughput Rate
DC ACCURACY
Integral Linearity Error
Differential Linearity Error
No Missing Codes
Transition Noise
Full-Scale Error2
Unipolar Zero Error2
Power Supply Sensitivity
AC ACCURACY
Signal-to-Noise
Spurious Free Dynamic Range
Total Harmonic Distortion
Signal-to-(Noise+Distortion)
VIN – VINGND
VIN
VINGND
fIN = 10 kHz
570 kSPS Throughput
In Warp Mode
In Warp Mode
In Warp Mode
In Normal Mode
In Normal Mode
In Impulse Mode
In Impulse Mode
1
0
0
–2.5
–1
16
0.7
±5
±3
AVDD = 5 V ± 5%
fIN = 10 kHz
fIN = 100 kHz
fIN = 10 kHz
fIN = 100 kHz
fIN = 10 kHz
fIN = 100 kHz
fIN = 10 kHz
fIN = 100 kHz
–60 dB Input
Full-Scale Step
REFERENCE
External Reference Voltage Range
External Reference Current Drain
570 kSPS Throughput
VREF
+3
+0.5
V
V
V
dB
µA
1.75
570
1
2
500
2.25
444
µs
kSPS
ms
µs
kSPS
µs
kSPS
+2.5
+1.5
LSB1
LSB
Bits
LSB
% of FSR
LSB
LSB
62
7
See Analog Input Section
REF = 2.5 V
± 0.08
± 15
90
88
100
90
–100
–90
90
85
30
18
dB3
dB
dB
dB
dB
dB
dB
dB
dB
MHz
2
5
ns
ps rms
ns
250
2.3
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
2.5
115
–0.3
+2.0
–1
–1
DIGITAL OUTPUTS
Data Format
Pipeline Delay
ISINK = 1.6 mA
ISOURCE = –500 µA
Unit
Bits
0
–0.1
–0.1
–3 dB Input Bandwidth
POWER SUPPLIES
Specified Performance
AVDD
DVDD
OVDD
Operating Current4
AVDD
DVDD5
OVDD5
Max
16
SAMPLING DYNAMICS
Aperture Delay
Aperture Jitter
Transient Response
VOL
VOH
Typ
2.7
V
µA
+0.8
OVDD + 0.3
+1
+1
V
V
µA
µA
Parallel or Serial 16-Bits
Conversion Results Available Immediately
After Completed Conversion
0.4
OVDD – 0.6
V
V
4.75
4.75
2.7
V
V
V
5
5
5.25
5.25
5.25
570 kSPS Throughput
15.5
3.8
100
–2–
mA
mA
µA
REV. 0
AD7664
Parameter
Conditions
POWER SUPPLIES (Continued)
Power Dissipation7
Min
570 kSPS Throughput4
100 SPS Througput6
In Power-Down Mode7
Typ
Max
Unit
97
21
115
7
mW
µW
µW
+85
°C
8
TEMPERATURE RANGE
Specified Performance
TMIN to TMAX
–40
NOTES
1
LSB means Least Significant Bit. With the 0 V to 2.5 V input range, one LSB is 38.15 µV.
2
See Definition of Specifications section. These specifications do not include the error contribution from the external reference.
3
All specifications in dB are referred to a full-scale input FS. Tested with an input signal at 0.5 dB below full-scale unless otherwise specified.
4
In normal mode.
5
Tested in parallel reading mode.
6
In impulse mode.
7
With all digital inputs forced to OVDD or OGND respect ively.
8
Contact factory for extended temperature range.
Specifications subject to change without notice.
TIMING SPECIFICATIONS (–40ⴗC to +85ⴗC, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)
Refer to Figures 11 and 12
Convert Pulsewidth
Time Between Conversions
(Wrap Mode/Normal Mode/Impulse Mode)
CNVST LOW to BUSY HIGH Delay
BUSY HIGH All Modes Except in
Master Serial Read After Convert Mode
(Warp Mode/Normal Mode/Impulse Mode)
Aperture Delay
End of Conversion to BUSY LOW Delay
Conversion Time
(Warp Mode/Normal Mode/Impulse Mode)
Acquisition Time
RESET Pulsewidth
Symbol
Min
t1
t2
5
1.75/2/2.25
Typ
t3
t4
Refer to Figures 13, 14, and 15 (Parallel Interface Modes)
CNVST LOW to DATA Valid Delay
(Warp Mode/Normal Mode/Impulse Mode)
DATA Valid to BUSY LOW Delay
Bus Access Request to DATA Valid
Bus Relinquish Time
Max
Unit
Note 1
ns
µs
25
1.5/1.75/2
ns
µs
1.5/1.75/2
ns
ns
µs
2
t5
t6
t7
10
t8
t9
250
10
ns
ns
t10
1.5/1.75/2
µs
40
50
ns
ns
ns
45
t11
t12
t13
5
2
Refer to Figures 16 and 17 (Master Serial Interface Modes)
CS LOW to SYNC Valid Delay
CS LOW to Internal SCLK Valid Delay2
CS LOW to SDOUT Delay
CNVST LOW to SYNC Delay
(Warp Mode/Normal Mode/Impulse Mode)
SYNC Asserted to SCLK First Edge Delay
Internal SCLK Period
Internal SCLK HIGH (INVSCLK Low)3
Internal SCLK LOW (INVSCLK Low)3
SDOUT Valid Setup Time
SDOUT Valid Hold Time
SCLK Last Edge to SYNC Delay
CS HIGH to SYNC HI-Z
CS HIGH to Internal SCLK HI-Z
CS HIGH to SDOUT HI-Z
BUSY HIGH in Master Serial Read After Convert
(Warp Mode/Normal Mode/Impulse Mode)
CNVST LOW to SYNC Asserted Delay
(Warp Mode/Normal Mode/Impulse Mode)
SYNC Deasserted to BUSY LOW Delay
REV. 0
t14
t15
t16
t17
10
10
10
25/275/525
t18
t19
t20
t21
t22
t23
t24
t25
t26
t27
t28
4
40
30
9.5
4.5
3
3
75
10
10
10
2.75/3/3.25
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
µs
t29
1/1.25/1.5
µs
t30
50
ns
–3–
AD7664
TIMING SPECIFICATIONS (Continued)
Symbol
Min
t31
t32
t33
t34
t35
t36
t37
5
3
5
5
25
10
10
Typ
Max
Unit
2
Refer to Figures 18 and 20 (Slave Serial Interface Modes)
External SCLK Setup Time
External SCLK Active Edge to SDOUT Delay
SDIN Setup Time
SDIN Hold Time
External SCLK Period
External SCLK HIGH
External SCLK LOW
ns
ns
ns
ns
ns
ns
ns
16
NOTES
1
In warp mode only, the maximum time between conversions is 1 ms, otherwise, there is no required maximum time.
2
In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load C L of 10 pF; otherwise, the load is 60 pF maximum.
3
If the polarity of SCLK is inverted, the timing references of SCLK are also inverted.
Specifications subject to change without notice.
Internal Power Dissipation3 . . . . . . . . . . . . . . . . . . . 700 mW
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature Range
(Soldering 10 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 300°C
ABSOLUTE MAXIMUM RATINGS 1
Analog Inputs
IN2, REF . . . . . . . . . . . . AVDD + 0.3 V to AGND – 0.3 V
INGND, REFGND . . . . . . . . . . . . . . . . . . AGND ± 0.3 V
Ground Voltage Differences
AGND, DGND, OGND . . . . . . . . . . . . . . . . . . . . . ± 0.3 V
Supply Voltages
AVDD, DVDD, OVDD . . . . . . . . . . . . . . . . . . . . . . . . 7 V
AVDD to DVDD, AVDD to OVDD . . . . . . . . . . . . . . ± 7 V
DVDD to OVDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 7 V
Digital Inputs
Except the Data Bus D(7:4) . . . –0.3 V to DVDD + 0.3 V
Data Bus Inputs D(7:4) . . . . . . –0.3 V to OVDD + 0.3 V
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
2
See Analog Input section.
3
Specification is for device in free air:
48-Lead LQFP: θJA = 91°C/W, θJC = 30°C/W.
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
AD7664AST
AD7664ASTRL
EVAL-AD7664CB1
EVAL-CONTROL BOARD2
–40°C to +85°C
–40°C to +85°C
Quad Flatpack (LQFP)
Quad Flatpack (LQFP)
Evaluation Board
Controller Board
ST-48
ST-48
NOTES
1
This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BOARD for evaluation/demonstration purposes.
2
This board allows a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.
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 AD7664 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.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. 0
AD7664
TO OUTPUT
PIN
PIN CONFIGURATION
48-Lead LQFP
(ST-48)
IOL
NC
NC
NC
NC
NC
NC
NC
NC
NC
60pF1
INGND
ⴙ1.4V
CL
REFGND
REF
1.6mA
48 47 46 45 44 43 42 41 40 39 38 37
IOH
AGND 1
AVDD 2
Figure 1. Load Circuit for Digital Interface Timing,
SDOUT, SYNC, SCLK Outputs, CL = 10 pF
NC 3
DGND 4
OB/2C 5
WARP 6
RESET
CS
RD
DGND
29 BUSY
30
D0 9
D1 10
28
2V
0.8V
D2 11
D3 12
26
25
D15
D14
D13
D12
D11/RDERROR
D9/SCLK
D10/SYNC
D8/SDOUT
13 14 15 16 17 18 19 20 21 22 23 24
DVDD
DGND
NC = NO CONNECT
Figure 2. Voltage Reference Levels for Timing
27
OVDD
0.8V
PD
33
t DELAY
D4/EXT/INT
D5/INVSYNC
D6/INVSCLK
2V
34
31
TOP VIEW
(Not to Scale)
SER/PAR 8
0.8V
t DELAY
AGND
CNVST
32
AD7664
IMPULSE 7
2V
35
36
PIN 1
IDENTIFIER
D7/RDC/SDIN
OGND
500␮A
PIN FUNCTION DESCRIPTIONS
Pin
No.
Mnemonic
Type
Description
1
2
3, 40–48
4, 30
5
AGND
AVDD
NC
DGND
OB/2C
P
P
6
WARP
DI
7
IMPULSE
DI
8
SER/PAR
DI
9–12
DATA[0:3]
DO
13
DATA[4]
or EXT/INT
DI/O
14
DATA[5]
or INVSYNC
DI/O
15
DATA[6]
or INVSCLK
DI/O
Analog Power Ground Pin.
Input Analog Power Pins. Nominally 5 V.
No Connect.
Must Be Tied to Analog Ground.
Straight Binary/Binary Two’s Complement. When OB/2C is HIGH, the digital output is
straight binary; when LOW, the MSB is inverted resulting in a two’s complement output
from its internal shift register.
Mode Selection. When HIGH and IMPULSE LOW, this input selects the fastest mode, the
maximum throughput is achievable, and a minimum conversion rate must be applied in order
to guarantee full specified accuracy. When LOW, full accuracy is maintained independent of
the minimum conversion rate.
Mode Selection. When HIGH and WARP LOW, this input selects a reduced power mode. In
this mode, the power dissipation is approximately proportional to the sampling rate.
Serial/Parallel Selection Input. When LOW, the parallel port is selected; when HIGH, the
serial interface mode is selected and some bits of the DATA bus are used as a serial port.
Bit 0 to Bit 3 of the Parallel Port Data Output Bus. These pins are always outputs, regardless
of the state of SER/PAR.
When SER/PAR is LOW, this output is used as Bit 4 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this input, part of the serial port, is used as a digital select input
for choosing the internal or an external data clock. With EXT/INT tied LOW, the internal
clock is selected on SCLK output. With EXT/INT set to a logic HIGH, output data is synchronized to an external clock signal connected to the SCLK input.
When SER/PAR is LOW, this output is used as Bit 5 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this input, part of the serial port, is used to select the active state
of the SYNC signal. It is active in both master and slave mode. When LOW, SYNC is active
HIGH. When HIGH, SYNC is active LOW.
When SER/PAR is LOW, this output is used as Bit 6 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this input, part of the serial port, is used to invert the SCLK signal. It is active in both master and slave mode.
REV. 0
DI
DI
–5–
AD7664
Pin
No.
16
Mnemonic
Type
Description
DATA[7]
or RDC/SDIN
DI/O
When SER/PAR is LOW, this output is used as Bit 7 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this input, part of the serial port, is used as either an external data
input or a read mode selection input depending on the state of EXT/INT.
17
18
OGND
OVDD
P
P
19
20
21
DVDD
DGND
DATA[8]
or SDOUT
P
P
DO
22
DATA[9]
or SCLK
DI/O
23
DATA[10]
or SYNC
DO
24
DATA[11]
or RDERROR
DO
25–28
DATA[12:15]
DO
29
BUSY
DO
30
31
DGND
RD
P
DI
32
CS
DI
33
34
RESET
PD
DI
DI
When EXT/INT is HIGH, RDC/SDIN could be used as a data input to daisy chain the conversion results from two or more ADCs onto a single SDOUT line. The digital data level on SDIN is
output on DATA with a delay of 16 SCLK periods after the initiation of the read sequence.
When EXT/INT is LOW, RDC/SDIN is used to select the read mode. When RDC/SDIN is
HIGH, the data is output on SDOUT during conversion. When RDC/SDIN is LOW, the
data can be output on SDOUT only when the conversion is complete.
Input/Output interface Digital Power Ground.
Input/Output interface Digital Power. Nominally at the same supply than the supply of the
host interface (5 V or 3 V).
Digital Power. Nominally at 5 V.
Digital Power Ground.
When SER/PAR is LOW, this output is used as Bit 8 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this output, part of the serial port, is used as a serial data output
synchronized to SCLK. Conversion results are stored in an on-chip register. The AD7664
provides the conversion result, MSB first, from its internal shift register. The DATA format is
determined by the logic level of OB/2C. In serial mode, when EXT/INT is LOW, SDOUT is
valid on both edges of SCLK.
In serial mode, when EXT/INT is HIGH:
If INVSCLK is LOW, SDOUT is updated on SCLK rising edge and valid on the next
falling edge.
If INVSCLK is HIGH, SDOUT is updated on SCLK falling edge and valid on the next rising
edge.
When SER/PAR is LOW, this output is used as the Bit 9 of the Parallel Port Data
Output Bus.
When SER/PAR is HIGH, this pin, part of the serial port, is used as a serial data clock input
or output, dependent upon the logic state of the EXT/INT pin. The active edge where the
data SDOUT is updated depends upon the logic state of the INVSCLK pin.
When SER/PAR is LOW, this output is used as the Bit 10 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH, this output, part of the serial port, is used as a digital output frame
synchronization for use with the internal data clock (EXT/INT = Logic LOW). When a read
sequence is initiated and INVSYNC is LOW, SYNC is driven HIGH and remains HIGH
while SDOUT output is valid. When a read sequence is initiated and INVSYNC is High,
SYNC is driven LOW and remains LOW while SDOUT output is valid.
When SER/PAR is LOW, this output is used as the Bit 11 of the Parallel Port Data Output Bus.
When SER/PAR is HIGH and EXT/INT is HIGH, this output, part of the serial port, is used
as a incomplete read error flag. In slave mode, when a data read is started and not complete
when the following conversion is complete, the current data is lost and RDERROR is pulsed
high.
Bit 12 to Bit 15 of the Parallel Port Data output bus. These pins are always outputs regardless
of the state of SER/PAR.
Busy Output. Transitions HIGH when a conversion is started, and remains HIGH until the
conversion is complete and the data is latched into the on-chip shift register. The falling edge
of BUSY could be used as a data ready clock signal.
Must Be Tied to Digital Ground.
Read Data. When CS and RD are both LOW, the interface parallel or serial output bus is
enabled. RD and CS are OR’d together internally.
Chip Select. When CS and RD are both LOW, the interface parallel or serial output bus is
enabled. RD and CS are OR’d together internally.
Reset Input. When set to a logic HIGH, reset the AD7664. Current conversion if any is aborted.
Power-Down Input. When set to a logic HIGH, power consumption is reduced and conversions are inhibited after the current one is completed.
–6–
REV. 0
AD7664
Pin
No.
Mnemonic
Type
Description
35
CNVST
DI
36
37
38
39
43
AGND
REF
REFGND
INGND
IN
P
AI
AI
AI
AI
Start Conversion. A falling edge on CNVST puts the internal sample/hold into the hold state and
initiates a conversion. In impulse mode (IMPULSE HIGH and WARP LOW), if CNVST is
held low when the acquisition phase (t8) is complete, the internal sample/hold is put into the
hold state and a conversion is immediately started.
Must Be Tied to Analog Ground.
Reference Input Voltage.
Reference Input Analog Ground.
Analog Input Ground.
Primary Analog Input with a Range of 0 V to VREF.
NOTES
AI = Analog Input
DI = Digital Input
DI/O = Bidirectional Digital
DO = Digital Output
P = Power
DEFINITION OF SPECIFICATIONS
TOTAL HARMONIC DISTORTION (THD)
INTEGRAL NONLINEARITY ERROR (INL)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in decibels.
Linearity error refers to the deviation of each individual code
from a line drawn from “negative full scale” through “positive full
scale.” The point used as “negative full scale” occurs 1/2 LSB
before the first code transition. “Positive full scale” is defined as a
level 1 1/2 LSB beyond the last code transition. The deviation is
measured from the middle of each code to the true straight line.
DIFFERENTIAL NONLINEARITY ERROR (DNL)
In an ideal ADC, code transitions are 1 LSB apart. Differential
nonlinearity is the maximum deviation from this ideal value. It is
often specified in terms of resolution for which no missing codes
are guaranteed.
FULL-SCALE ERROR
The last transition (from 011 . . . 10 to 011 . . . 11 in two’s
complement coding) should occur for an analog voltage 1 1/2 LSB
below the nominal full scale (2.49994278 V for the 0 V–2.5 V
range). The full-scale error is the deviation of the actual level of
the last transition from the ideal level.
UNIPOLAR ZERO ERROR
SIGNAL-TO-NOISE RATIO (SNR)
SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels.
SIGNAL TO (NOISE + DISTORTION) RATIO
(S/[N+D])
S/(N+D) is the ratio of the rms value of the actual input signal
to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc. The
value for S/(N+D) is expressed in decibels.
APERTURE DELAY
Aperture delay is a measure of the acquisition performance and
is measured from the falling edge of the CNVST input to when
the input signal is held for a conversion.
The first transition should occur at a level 1/2 LSB above analog
ground (19.073 µV for the 0 V–2.5 V range). Unipolar zero error is
the deviation of the actual transition from that point.
TRANSIENT RESPONSE
SPURIOUS FREE DYNAMIC RANGE (SFDR)
OVERVOLTAGE RECOVERY
The difference, in decibels (dB), between the rms amplitude of
the input signal and the peak spurious signal.
The time required for the AD7664 to achieve its rated accuracy
after a full-scale step function is applied to its input.
The time required for the ADC to recover to full accuracy after
an analog input signal 150% of full-scale is reduced to 50% of
the full-scale value.
EFFECTIVE NUMBER OF BITS (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to S/(N+D) by the following formula:
ENOB = (S/[N+D]dB – 1.76)/6.02)
and is expressed in bits.
REV. 0
–7–
2.5
1.50
2.0
1.25
1.5
1.00
1.0
0.75
0.5
0.50
DNL – LSB
INL – LSB
AD7664–Typical Performance Characteristics
0
–0.5
0.25
0
–1.0
–0.25
–1.5
–0.50
–2.0
–0.75
–2.5
0
16384
32768
CODE
–1.00
65536
49152
TPC 1. Integral Nonlinearity vs. Code
0
16384
32768
CODE
49152
65536
TPC 4. Differential Nonlinearity vs. Code
8000
10000
7288 7148
9008
9000
7000
8000
6000
7000
COUNTS
COUNTS
5000
4000
6000
5000
4000
3000
3340
3643
3000
2000
753
1000
2000
1173
1000
0
12
10
0
0
0
0
0
7F86 7F87 7F88 7F89 7F8A 7F8B 7F8C 7F8D 7F8E 7F8F
CODE – Hexa
TPC 2. Histogram of 16,384 Conversions of a DC Input
at the Code Transition
257
136
0
0
TPC 5. Histogram of 16,384 Conversions of a DC Input
at the Code Center
0
–96
96
4096 POINT FFT
f S = 570kHz
f IN = 10.39kHz, –0.5dB
SNR = 90.1dB
SINAD = 89.6dB
THD = –98.7dB
SFDR = 100.3dB
–40
–60
–98
93
SNR AND S/(N+D) – dB
–20
AMPLITUDE – dB of Full Scale
0
7FB3 7FB4 7FB5 7FB6 7FB7 7FB8 7FB9 7FBA 7FBB
CODE – Hexa
–80
–100
–120
THD
–100
90
SNR
–102
87
–140
THD – dB
0
–160
84
–55
–180
0
50
100
150
200
FREQUENCY – kHz
250
300
TPC 3. FFT Plot
–35
–15
5
25
45
65
TEMPERATURE – ⴗC
85
105
–104
125
TPC 6. SNR, THD vs. Temperature
–8–
REV. 0
AD7664
100
15.0
110
–60
105
–65
14.5
95
SFDR
–70
100
S/(N+D)
13.0
85
12.5
80
–75
95
–80
90
–85
85
–90
80
–95
2ND HARMONICS
11.5
1
10
100
FREQUENCY – kHz
65
–105
11.0
1000
3RD HARMONICS
10
100
FREQUENCY – kHz
–110
1
TPC 7. SNR, S/(N+D), and ENOB vs. Frequency
60
1k
TPC 10. THD, Harmonics, and SFDR vs. Frequency
92
50
OVDD = 2.7V, 85ⴗC
40
SNR
90
t12 DELAY – ns
SNR (REFERRED TO FULL SCALE) – d
70
–100
75
70
75
THD
12.0
SFDR – dB
13.5
THD, HARMINICS – dB
90
ENOB – Bits
SNR AND S/(N+D) – dB
14.0
ENOB
SNR
S/(N+D)
88
30
OVDD = 2.7V, 25ⴗC
20
OVDD = 5V, 85ⴗC
10
OVDD = 5V, 25ⴗC
86
–60
–50
–40
–20
–30
INPUT LEVEL – dB
0
0
–10
TPC 8. SNR and S/(N+D) vs. Input Level
POWER-DOWN OPERATING CURRENTS – nA
OPERATING CURRENTS – ␮A
10k
DVDD, WARP/NORMAL
AVDD, IMPULSE
100
DVDD, IMPULSE
10
1
OVDD, ALL MODES
0.1
0.01
1
10
100
1k
10k
SAMPLING RATE – SPS
100k
200
150
90
AVDD
80
70
60
50
OVDD
40
30
20
DVDD
10
0
–50
1000k
TPC 9. Operating Currents vs. Sample Rate
REV. 0
100
CL – pF
100
AVDD, WARP/NORMAL
0.001
0.1
50
TPC 11. Typical Delay vs. Load Capacitance CL
100k
1k
0
–25
0
25
50
TEMPERATURE – ⴗC
75
100
TPC 12. Power-Down Operating Currents vs. Temperature
–9–
AD7664
CIRCUIT INFORMATION
Modes of Operation
The AD7664 is a very fast, low power, single supply, precise
16-bit analog-to-digital converter (ADC). The AD7664 features different modes to optimize performances according to
the applications.
The AD7664 features three modes of operations, Warp, Normal,
and Impulse. Each of these modes is more suitable for specific
applications.
The Warp mode allows the fastest conversion rate up to 570 kSPS.
However, in this mode, and this mode only, the full specified accuracy is guaranteed only when the time between conversion does
not exceed 1 ms. If the time between two consecutive conversions
is longer than 1 ms, for instance, after power-up, the first conversion result should be ignored. This mode makes the AD7664
ideal for applications where both high accuracy and fast sample
rate are required.
In warp mode, the AD7664 is capable of converting 570,000
samples per second (570 kSPS).
The AD7664 provides the user with an on-chip track/hold,
successive approximation ADC that does not exhibit any pipeline or latency, making it ideal for multiple multiplexed channel
applications.
The AD7664 can be operated from a single 5 V supply and
be interfaced to either 5 V or 3 V digital logic. It is housed in
a 48-lead LQFP package that saves space and allows flexible configurations as either serial or parallel interface. The AD7664 is a
pin-to-pin compatible upgrade of the AD7660.
The normal mode is the fastest mode (500 kSPS ) without any
limitation about the time between conversions. This mode makes
the AD7664 ideal for asynchronous applications such as data
acquisition systems, where both high accuracy and fast sample
rate are required.
The impulse mode, the lowest power dissipation mode, allows
power saving between conversions. When operating at 100 SPS,
for example, it typically consumes only 21 µW. This feature
makes the AD7664 ideal for battery-powered applications.
CONVERTER OPERATION
The AD7664 is a successive-approximation analog-to-digital
converter based on a charge redistribution DAC. Figure 3 shows
the simplified schematic of the ADC. The capacitive DAC consists
of an array of 16 binary weighted capacitors and an additional
“LSB” capacitor. The comparator’s negative input is connected
to a “dummy” capacitor of the same value as the capacitive
DAC array.
Transfer Functions
Using the OB/2C digital input, the AD7664 offers two output
codings: straight binary and two’s complement. The LSB size is
VREF/65536, which is about 38.15 µV. The ideal transfer characteristic for the AD7664 is shown in Figure 4 and Table I.
During the acquisition phase, the common terminal of the array
tied to the comparator's positive input is connected to AGND
via SWA. All independent switches are connected to the analog
input IN. Thus, the capacitor array is used as a sampling capacitor and acquires the analog signal on IN input. Similarly, the
“dummy” capacitor acquires the analog signal on INGND input.
ADC CODE – Straight Binary
1 LSB = VREF/65536
When the CNVST input goes low, a conversion phase is
initiated. When the conversion phase begins, SWA and SWB
are opened first. The capacitor array and the “dummy” capacitor are then disconnected from the inputs and connected to
the REFGND input. Therefore, the differential voltage between
IN and INGND captured at the end of the acquisition phase is
applied to the comparator inputs, causing the comparator to
become unbalanced. By switching each element of the capacitor
array between REFGND or REF, the comparator input varies by
binary-weighted voltage steps (VREF/2, VREF/4, . . . VREF/65536).
The control logic toggles these switches, starting with the MSB
first, to bring the comparator back into a balanced condition. After
the completion of this process, the control logic generates the
ADC output code and brings BUSY output low.
111...111
111...110
111...101
000...010
000...001
000...000
0V
1 LSB
0.5 LSB
ANALOG INPUT
Figure 4. ADC Ideal Transfer Function
IN
REF
REFGND
LSB
MSB
32,768C 16,384C
4C
2C
VREF –1 LSB
VREF –1.5 LSB
C
SWA
SWITCHES
CONTROL
C
BUSY
COMP
CONTROL
LOGIC
INGND
OUTPUT
CODE
65,536C
SWB
CNVST
Figure 3. ADC Simplified Schematic
–10–
REV. 0
AD7664
Table I. Output Codes and Ideal Input Voltages
TYPICAL CONNECTION DIAGRAM
Description
Analog
Input
Digital Output Code
Hexa
Straight
Two’s
Binary
Complement
FSR –1 LSB
FSR – 2 LSB
Midscale + 1 LSB
Midscale
Midscale – 1 LSB
–FSR + 1 LSB
–FSR
2.499962 V
2.499923 V
1.250038 V
1.25 V
1.249962 V
38 µV
0V
FFFF1
FFFE
8001
8000
7FFF
0001
00002
Figure 5 shows a typical connection diagram for the AD7664.
Analog Input
Figure 6 shows an equivalent circuit of the input structure of
the AD7664.
7FFF1
7FFE
0001
0000
FFFF
8001
80002
AVDD
D1
IN
OR INGND
R1
D2
Figure 6. Equivalent Analog Input Circuit
The two diodes D1 and D2 provide ESD protection for the
analog inputs IN and INGND. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by more
than 0.3 V. This will cause these diodes to become forwardbiased and start conducting current. These diodes can handle
a forward-biased current of 100 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from AVDD. In such case, an input
buffer with a short circuit current limitation can be used to
protect the part.
100⍀
10␮F
100nF
DIGITAL SUPPLY
(3.3V OR 5V)
10␮F
AVDD
C2
AGND
NOTES
1
This is also the code for overrange analog input (V IN – VINGND above
VREF – VREFGND).
2
This is also the code for underrange analog input (V IN below VINGND)
ANALOG
SUPPLY
(5V)
C1
AGND
100nF
DGND
DVDD
100nF
OVDD
OGND
10␮F
SERIAL
PORT
SCLK
2.5V REF1
REF
CREF1
SDOUT
100nF
REFGND
BUSY
␮C/␮P/DSP
AD7664
CNVST
ANALOG INPUT
(0V TO 2.5V)
D3
15⍀
U12
IN
CC
OB/2C
SER/PAR
2.7nF
DVDD
WARP
INGND
PD
IMPULSE
CS
RESET
RD
CLOCK
NOTES:
1
THE AD780 IS RECOMMENDED WITH CREF = 47␮F.
2
THE AD829 IS RECOMMENDED WITH A COMPENSATION CAPACITOR CC = 82 pF, TYPE CERAMIC NPO.
3
OPTIONAL LOW JITTER CNVST.
Figure 5. Typical Connection Diagram
REV. 0
–11–
AD7664
Driver Amplifier Choice
This analog input structure allows the sampling of the differential signal between IN and INGND. Unlike other converters,
the INGND input is sampled at the same time as the IN input.
By using this differential input, small signals common to both
inputs are rejected, as shown in Figure 7, which represents the
typical CMR over frequency. For instance, by using INGND to
sense a remote signal ground, difference of ground potentials
between the sensor and the local ADC ground are eliminated.
Although the AD7664 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
• The driver amplifier and the AD7664 analog input circuit
have to be able together to settle for a full-scale step the
capacitor array at a 16-bit level (0.0015%). For instance,
operation at the maximum throughput of 570 kSPS requires
a minimum gain bandwidth product of 39 MHz.
• The noise generated by the driver amplifier needs to be kept
as low as possible in order to preserve the SNR and transition noise performance of the AD7664. The noise coming
from the driver is filtered by the AD7664 analog input circuit
one-pole low-pass filter made by R1 and C2. For instance, a
driver such as the AD829, with an equivalent input noise of
2 nV/√Hz and configured as a buffer, thus, with a noise gain
of 1, degrades the SNR by only 0.45 dB. A driver amplifier
with an equivalent input noise of 5 nV/√Hz in the same configuration will add 1.9 dB degradation.
70
60
CMRR – dB
50
40
30
20
• To even further reduce the noise filtering done by the AD7664
analog input circuit, an external simple one-pole RC filter
between the amplifier output and the ADC analog input will
slightly improve the ac performances, specially, the SNR and
the transition noise. For example, as shown in Figure 5, a 15 Ω
source resistor with a 2.7 nF good linearity capacitor (NPO or
mica type) limit the bandwidth to 4 MHz.
10
0
1
100
10
FREQUENCY – kHz
1k
Figure 7. Analog Input CMR vs. Frequency
During the acquisition phase, the impedance of the analog input
IN can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 140 Ω and is a lumped component made up of some
serial resistors and the on resistance of the switches. The capacitor
C2 is typically 60 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened, the
input impedance is limited to C1. The R1, C2 makes a one-pole
low-pass filter that reduces undesirable aliasing effect and limits
the noise.
When the source impedance of the driving circuit is low, the
AD7664 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can be
tolerated. The THD degrades in function of the source impedance and the maximum input frequency as shown in Figure 8.
–70
RS = 100⍀
RS = 50⍀
–75
THD – dB
RS = 20⍀
–85
RS = 11⍀
The AD7664 uses an external 2.5 V voltage reference. The voltage
reference input REF of the AD7664 has a dynamic input impedance. Therefore, it should be driven by a low impedance source
with an efficient decoupling between REF and REFGND inputs.
This decoupling depends on the choice of the voltage reference,
but usually consists of a low ESR tantalum capacitor and a 100 nF
ceramic capacitor. Appropriate value for the tantalum capacitor
is 47 µF with the low-cost, low-power ADR291 voltage reference,
or with the low-noise, low-drift AD780 voltage reference. For
applications using multiple AD7664s, it is more effective to buffer
the reference voltage with a low-noise, very stable op amp like
the AD8031.
Power Supply
–90
–95
100
FREQUENCY – kHz
Voltage Reference Input
Care should also be taken with the reference temperature coefficient of the voltage reference which directly affects the full-scale
accuracy if this parameter matters. For instance, a ± 15 ppm/°C
tempco of the reference changes the full scale by ± 1 LSB/°C.
–80
–100
10
• The driver needs to have a THD performance suitable to that
of the AD7664. TPC 10 gives the THD versus frequency
that the driver should preferably exceed. The AD829 meets
these requirements. The AD829 requires an external compensation capacitor of 82 pF. This capacitor should have good
linearity as an NPO ceramic or mica or prolypropylene type.
Moreover, the use of a noninverting 1 gain arrangement is
recommended and helps to obtain the best signal-to-noise ratio.
1k
The AD7664 uses three sets of power supply pins: an analog 5 V
supply AVDD, a digital 5 V core supply DVDD, and a digital
input/output interface supply OVDD. The OVDD supply allows
direct interface with any logic working between 2.7 V and 5.25 V.
To reduce the number of supplies needed, the digital core
(DVDD) can be supplied through a simple RC filter from the
analog supply as shown in Figure 5. The AD7664 is independent
Figure 8. THD vs. Analog Input Frequency and
Source Resistance
–12–
REV. 0
AD7664
of power supply sequencing and thus free from supply voltage
induced latchup. Additionally, it is very insensitive to power supply
variations over a wide frequency range as shown in Figure 9.
t2
t1
CNVST
–50
BUSY
–55
t4
t3
PSRR – dB
–60
t6
t5
MODE
ACQUIRE
CONVERT
ACQUIRE
t7
t8
CONVERT
–65
–70
Figure 11. Basic Conversion Timing
–75
–80
1
100
10
1000
INPUT FREQUENCY – kHz
Figure 9. PSRR vs. Frequency
POWER DISSIPATION VS. THROUGHPUT
Operating currents are very low during the acquisition phase,
which allows a significant power saving when the conversion
rate is reduced as shown in Figure 10. This power saving depends
on the mode used. In impulse mode, the AD7664 automatically
reduces its power consumption at the end of each conversion
phase. This feature makes the AD7664 ideal for very low power
battery applications. It should be noted that the digital interface remains active even during the acquisition phase. To reduce
the operating digital supply currents even further, the digital
inputs need to be driven close to the power supply rails (i.e.,
DVDD or DGND for all inputs except EXT/INT, INVSYNC,
INVSCLK, RDC/SDIN, and OVDD or OGND for these last
four inputs).
In impulse mode, conversions can be automatically initiated. If
CNVST is held low when BUSY is low, the AD7664 controls the
acquisition phase and then automatically initiates a new conversion. By keeping CNVST low, the AD7664 keeps the conversion
process running by itself. It should be noted that the analog
input has to be settled when BUSY goes low. Also, at power-up,
CNVST should be brought low once to initiate the conversion
process. In this mode, the AD7664 could sometimes run slightly
faster then the guaranteed limits in the impulse mode of 444 kSPS.
This feature does not exist in warp or normal modes.
t9
RESET
BUSY
DATA
t8
100k
WARP/NORMAL
CNVST
OPERATING CURRENTS – ␮A
10k
Figure 12. RESET Timing
1k
Although CNVST is a digital signal, it should be designed with
special care with fast, clean edges, and levels with minimum
overshoot and undershoot or ringing.
100
IMPULSE
For applications where the SNR is critical, CNVST signal
should have a very low jitter. Some solutions to achieve that is
to use a dedicated oscillator for CNVST generation or, at least,
to clock it with a high-frequency low-jitter clock as shown in
Figure 5.
10
1
0.1
0.1
1
10
100
1k
10k
SAMPLING RATE – SPS
100k
1M
Figure 10. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
Figure 11 shows the detailed timing diagrams of the conversion
process. The AD7664 is controlled by the signal CNVST which
initiates conversion. Once initiated, it cannot be restarted or
aborted, even by the power-down input PD, until the conversion is complete. The CNVST signal operates independently of
CS and RD signals.
REV. 0
–13–
AD7664
CS = 0
DIGITAL INTERFACE
The AD7664 has a versatile digital interface; it can be interfaced
with the host system by using either a serial or parallel interface.
The serial interface is multiplexed on the parallel data bus. The
AD7664 digital interface also accommodates both 3 V or 5 V logic
by simply connecting the OVDD supply pin of the AD7664 to
the host system interface digital supply. Finally, by using the
OB/2C input pin, both two’s complement or straight binary
coding can be used.
PREVIOUS
CONVERSION
t 13
SERIAL INTERFACE
The AD7664 is configured to use the serial interface when the
SER/PAR is held high. The AD7664 outputs 16 bits of data,
MSB first, on the SDOUT pin. This data is synchronized with
the 16 clock pulses provided on SCLK pin. The output data is
valid on both the rising and falling edge of the data clock.
t1
t 10
MASTER SERIAL INTERFACE
Internal Clock
t4
The AD7664 is configured to generate and provide the serial data
clock SCLK when the EXT/INT pin is held low. The AD7664
also generates a SYNC signal to indicate to the host when the
serial data is valid. The serial clock SCLK and the SYNC signal
can be inverted if desired. Depending on RDC/SDIN input, the
data can be read after each conversion or during the following
conversion. Figure 16 and Figure 17 show the detailed timing
diagrams of these two modes.
t 11
PREVIOUS CONVERSION DATA
t3
Figure 15. Slave Parallel Data Timing for Reading
(Read During Convert)
CNVST
DATA
BUS
t4
t 12
CS = RD = 0
t3
BUSY
DATA
BUS
The two signals CS and RD control the interface. CS and RD
have a similar effect because they are OR’d together internally.
When at least one of these signals is high, the interface outputs
are in high impedance. Usually, CS allows the selection of each
AD7664 in multicircuits applications and is held low in a single
AD7664 design. RD is generally used to enable the conversion
result on the data bus.
BUSY
t1
CNVST,
RD
NEW DATA
Figure 13. Master Parallel Data Timing for Reading
(Continuous Read)
PARALLEL INTERFACE
The AD7664 is configured to use the parallel interface when the
SER/PAR is held low. The data can be read either after each
conversion, which is during the next acquisition phase, or during the following conversion as shown, respectively, in Figure 14
and Figure 15. When the data is read during the conversion,
however, it is recommended that it is read only during the first
half of the conversion phase. That avoids any potential feedthrough between voltage transients on the digital interface and
the most critical analog conversion circuitry.
Usually, because the AD7664 is used with a fast throughput, the
mode master, read during conversion is the most recommended
serial mode when it can be used.
In read-during-conversion mode, the serial clock and data toggle
at appropriate instants which minimize potential feedthrough
between digital activity and the critical conversion decisions.
In read-after-conversion mode, it should be noted that, unlike in
other modes, the signal BUSY returns low after the 16 data bits
are pulsed out and not at the end of the conversion phase which
results in a longer BUSY width.
CS
SLAVE SERIAL INTERFACE
External Clock
RD
BUSY
DATA
BUS
CURRENT
CONVERSION
t 12
The AD7664 is configured to accept an externally supplied
serial data clock on the SCLK pin when the EXT/INT pin is
held high. In this mode, several methods can be used to read the
data. When CS and RD are both low, the data can be read after
each conversion or during the following conversion. The external clock can be either a continuous or discontinuous clock. A
discontinuous clock can be either normally high or normally low
when inactive. Figure 18 and Figure 20 show the detailed timing diagrams of these methods.
t 13
Figure 14. Slave Parallel Data Timing for Reading
(Read After Convert)
–14–
REV. 0
AD7664
EXT/INT = 0
CS, RD
RDC/SDIN = 0
INVSCLK = INVSYNC = 0
t3
CNVST
t 28
BUSY
t 30
t 29
t 25
SYNC
t 14
t 18
t 19
t 20
SCLK
t 24
t 21
1
2
3
D15
D14
14
15
t 26
16
t 15
t 27
SDOUT
X
t 16
D2
D1
D0
t 23
t 22
Figure 16. Master Serial Data Timing for Reading (Read After Convert)
EXT/INT = 0
RDC/SDIN = 1
INVSCLK = INVSYNC = 0
CS, RD
t1
CNVST
t3
BUSY
t 17
t 25
SYNC
t 14
t 19
t 20 t 21
t 15
SCLK
1
t 24
2
3
14
15
t 26
16
t 18
t 27
SDOUT
X
t 16
t 22
D15
D14
D2
D1
D0
t 23
Figure 17. Master Serial Data Timing for Reading (Read Previous Conversion During Convert)
REV. 0
–15–
AD7664
EXT/INT = 1
INVSCLK = 0
CS, RD
BUSY
t 35
t 36 t 37
SCLK
1
2
t 31
3
14
15
16
17
18
t 32
X
SDOUT
t 16
D15
D14
D13
D1
D0
X15
X14
X14
X13
X1
X0
Y15
Y14
t 34
SDIN
X15
t 33
Figure 18. Slave Serial Data Timing for Reading (Read After Convert)
While the AD7664 is performing a bit decision, it is important
that voltage transients not occur on digital input/output pins or
degradation of the conversion result could occur. This is particularly important during the second half of the conversion
phase because the AD7664 provides error correction circuitry
that can correct for an improper bit decision made during the
first half of the conversion phase. For this reason, it is recommended that when an external clock is being provided, it is a
discontinuous clock that is toggling only when BUSY is low or,
more importantly, that is does not transition during the latter
half of BUSY high.
BUSY
OUT
BUSY
BUSY
AD7664
AD7664
#2
(UPSTREAM)
#1
(DOWNSTREAM)
RDC/SDIN
SDOUT
RDC/SDIN
SDOUT
CNVST
CNVST
CS
CS
SCLK
SCLK
DATA
OUT
External Discontinuous Clock Data Read After Conversion
Though the maximum throughput cannot be achieved using this
mode, it is the most recommended of the serial slave modes.
Figure 18 shows the detailed timing diagrams of this method.
After a conversion is complete, indicated by BUSY returning
low, the result of this conversion can be read while both CS and
RD are low. The data is shifted out, MSB first, with 16 clock
pulses and is valid on both rising and falling edge of the clock.
Among the advantages of this method, the conversion performance is not degraded because there are no voltage transients
on the digital interface during the conversion process.
Another advantage is to be able to read the data at any speed up
to 40 MHz which accommodates both slow digital host interface
and the fastest serial reading.
Finally, in this mode only, the AD7664 provides a “daisy chain”
feature using the RDC/SDIN input pin for cascading multiple
converters together. This feature is useful for reducing component
count and wiring connections when desired as, for instance, in
isolated multiconverter applications.
An example of the concatenation of two devices is shown in
Figure 19. Simultaneous sampling is possible by using a common CNVST signal. It should be noted that the RDC/SDIN
input is latched on the opposite edge of SCLK of the one used to
shift out the data on SDOUT. Hence, the MSB of the “upstream”
converter just follows the LSB of the “downstream” converter
on the next SCLK cycle.
SCLK IN
CS IN
CNVST IN
Figure 19. Two AD7664s in a “Daisy Chain” Configuration
External Clock Data Read During Conversion
Figure 20 shows the detailed timing diagrams of this method.
During a conversion, while both CS and RD are both low, the
result of the previous conversion can be read. The data is shifted
out, MSB first, with 16 clock pulses and is valid on both rising and
falling edge of the clock. The 16 bits have to be read before the
current conversion is complete. If that is not done, RDERROR is
pulsed high and can be used to interrupt the host interface to
prevent incomplete data reading. There is no “daisy chain”
feature in this mode and RDC/SDIN input should always be
tied either high or low.
To reduce performance degradation due to digital activity, a fast
discontinuous clock of, at least 18 MHz, when impulse mode is
used, 25 MHz when normal mode is used or 40 MHz when warp
mode is used, is recommended to ensure that all the bits are read
during the first half of the conversion phase. It is also possible
to begin to read the data after conversion and continue to read
the last bits even after a new conversion has been initiated. That
allows the use of a slower clock speed like 14 MHz in impulse
mode, 18 MHz in normal mode and 25 MHz in warp mode.
–16–
REV. 0
AD7664
EXT/INT = 1
INVSCLK = 0
CS, RD
CNVST
BUSY
t3
t 35
t 36 t 37
SCLK
1
2
t 31
14
15
16
t 32
X
SDOUT
3
D15
D14
D1
D13
D0
t 16
Figure 20. Slave Serial Data Timing for Reading (Read Previous Conversion During Convert)
MICROPROCESSOR INTERFACING
ADSP-21065L in Master Serial Interface
The AD7664 is ideally suited for traditional dc measurement
applications supporting a microprocessor, and ac signal processing
applications interfacing to a digital signal processor. The AD7664
is designed to interface either with a parallel 16-bit-wide interface
or with a general-purpose serial port or I/O ports on a microcontroller. A variety of external buffers can be used with the AD7664
to prevent digital noise from coupling into the ADC. The following
sections illustrate the use of the AD7664 with an SPI-equipped
microcontroller, the ADSP-21065L and ADSP-218x signal
processors.
As shown in Figure 22, the AD7664 can be interfaced to the
ADSP-21065L using the serial interface in master mode without
any glue logic required. This mode combines the advantages of
reducing the number of wire connections and being able to read
the data during or after conversion at user convenience.
SPI Interface (MC68HC11)
Figure 21 shows an interface diagram between the AD7664 and
an SPI-equipped microcontroller like the MC68HC11. To accommodate the slower speed of the microcontroller, the AD7664 acts
as a slave device and data must be read after conversion. This
mode allows also the “daisy chain” feature.
The convert command could be initiated in response to an
internal timer interrupt. The reading of output data, one byte
at a time, if necessary, could be initiated in response to the
end-of-conversion signal (BUSY going low) using to an interrupt
line of the microcontroller. The Serial Peripheral Interface
(SPI) on the MC68HC11 is configured for master mode
(MSTR = 1), Clock Polarity Bit (CPOL) = 0, Clock Phase Bit
(CPHA) = 1 and SPI Interrupt Enable (SPIE = 1) by writing to
the SPI Control Register (SPCR). The IRQ is configured for
edge-sensitive-only operation (IRQE = 1 in OPTION register).
The AD7664 is configured for the internal clock mode (EXT/INT
low) and acts, therefore, as the master device. The convert command can be generated by either an external low jitter oscillator
or, as shown, by a FLAG output of the ADSP-21065L or by a
frame output TFS of one serial port of the ADSP-21065L which
can be used as a timer. The serial port on the ADSP-21065L is
configured for external clock (IRFS = 0), rising edge active
(CKRE = 1), external late framed sync signals (IRFS = 0,
LAFS = 1, RFSR = 1) and active high (LRFS = 0). The serial
port of the ADSP-21065L is configured by writing to its receive
control register (SRCTL)—see ADSP-2106x SHARC User’s
Manual. Because the serial port within the ADSP-21065L will
be seeing a discontinuous clock, an initial word reading has to be
done after the ADSP-21065L has been reset to ensure that the
serial port is properly synchronized to this clock during each
following data read operation.
DVDD
OVDD
OR
OGND
AD7664*
ADSP-21065L*
SHARC
SER/PAR
RDC/SDIN
RD
EXT/INT
DVDD
AD7664*
MC68HC11*
CS
OVDD
SER/PAR
SYNC
SDOUT
INVSYNC
SCLK
INVSCLK
CNVST
RFS
DR
RCLK
FLAG OR TFS
EXT/INT
BUSY
CS
SDOUT
RD
SCLK
INVSCLK
CNVST
*ADDITIONAL PINS OMITTED FOR CLARITY
IRQ
Figure 22. Interfacing to the ADSP-21065L Using the
Serial Master Mode
MISO/SDI
SCK
I/O PORT
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 21. Interfacing the AD7664 to SPI Interface
REV. 0
–17–
AD7664
APPLICATION HINTS
Bipolar and Wider Input Ranges
In some applications, it is desired to use a bipolar or wider analog input range like, for instance, ± 10 V, ± 5 V or 0 V to 5 V.
Although the AD7664 has only one unipolar range, by simple
modifications of the input driver circuitry, bipolar and wider
input ranges can be used without any performance degradation.
Figure 23 shows a connection diagram which allows that. Components values required and resulting full-scale ranges are shown in
Table II.
The DVDD supply of the AD7664 can be either a separate supply
or come from the analog supply AVDD or the digital interface
supply OVDD. When the system digital supply is noisy, or fast
switching digital signals are present, it is recommended that if no
separate supply available, connect the DVDD digital supply to
the analog supply, AVDD, through an RC filter as shown in
Figure 5, and connect the system supply to the interface digital
supply, OVDD, and the remaining digital circuitry. When DVDD
is powered from the system supply, it is useful to insert a bead
to further reduce high-frequency spikes.
For applications where accurate gain and offset are desired, they
can be calibrated by acquiring a ground and a voltage reference
using an analog multiplexer, U2, as shown in Figure 23. Also,
CF can be used as a one-pole antialiasing filter.
CF
R1
R2
ANALOG
INPUT
Layout
The AD7664 has very good immunity to noise on the power
supplies as can be seen in Figure 9. However, care should still
be taken with regard to grounding layout.
IN
U1
AD7664
U2
The printed circuit board that houses the AD7664 should be
designed so the analog and digital sections are separated and
confined to certain areas of the board. This facilitates the use of
ground planes that can be easily separated. Digital and analog
ground planes should be joined in only one place, preferably
underneath the AD7664, or, at least, as close as possible to the
AD7664. If the AD7664 is in a system where multiple devices
require analog-to-digital ground connections, the connection
should still be made at one point only, a star ground point,
which should be established as close as possible to the AD7664.
It is recommended to avoid running digital lines under the
device as these will couple noise onto the die. The analog ground
plane should be allowed to run under the AD7664 to avoid noise
coupling. Fast switching signals like CNVST or clocks should be
shielded with digital ground to avoid radiating noise to other
sections of the board, and should never run near analog signal
paths. Crossover of digital and analog signals should be avoided.
Traces on different but close layers of the board should run at right
angles to each other. This will reduce the effect of feedthrough
through the board.
The power supplies lines to the AD7664 should use as large trace
as possible to provide low impedance paths and reduce the effect
of glitches on the power supplies lines. Good decoupling is also
important to lower the supplies impedance presented to the
AD7664 and reduce the magnitude of the supply spikes. Decoupling ceramic capacitors, typically 100 nF, should be placed on
each power supplies pins AVDD, DVDD, and OVDD close to,
and ideally right up against, these pins and their corresponding
ground pins. Additionally, low ESR 10 µF capacitors should be
located in the vicinity of the ADC to further reduce low frequency
ripple.
R3
R4
100nF
INGND
REF
2.5V REF
CREF
100nF
REFGND
Figure 23. Using the AD7664 in 16-Bit Bipolar and/or
Wider Input Ranges
Table II. Component Values and Input Ranges
Input Range
R1
R2
R3
R4
± 10 V
±5 V
0 V to –5 V
250 Ω
500 Ω
1 kΩ
2 kΩ
2 kΩ
1 kΩ
10 kΩ
10 kΩ
None
8 kΩ
6.67 kΩ
0Ω
The AD7664 has five different ground pins: INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the analog input signal. REFGND senses the reference voltage and
should be a low impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most
internal ADC analog signals are referenced. This ground must
be connected with the least resistance to the analog ground
plane. DGND must be tied to the analog or digital ground plane
depending on the configuration. OGND is connected to the
digital system ground.
Evaluating the AD7664 Performance
A recommended layout for the AD7664 is outlined in the
evaluation board for the AD7664. The evaluation board package includes a fully assembled and tested evaluation board,
documentation, and software for controlling the board from a
PC via the Eval-Control Board.
–18–
REV. 0
AD7664
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
0.067 (1.70)
0.059 (1.50)
0.055 (1.40)
0.028 (0.7)
0.020 (0.5)
0.012 (0.3)
0.362 (9.19)
0.354 (9.00) SQ
0.346 (8.79)
36
0.039 (1.00)
REF
25
37
24
SEATING
PLANE
0.280 (7.1)
0.276 (7.0) SQ
0.272 (6.9)
TOP VIEW
(PINS DOWN)
0.006 (0.15)
0.004 (0.10)
0.002 (0.05)
0ⴗ
MIN
48
13
12
1
0.023 (0.58) 0.010 (0.26)
0.020 (0.50) 0.007 (0.18)
0.017 (0.42) 0.006 (0.15)
0.007 (0.177)
0.005 (0.127)
0.004 (0.107)
C02046–7.5–7/00 (rev. 0)
48-Lead Quad Flatpack (LQFP)
(ST-48)
0.057 (1.45)
0.055 (1.40)
0.053 (1.35)
PRINTED IN U.S.A.
7ⴗ
3.5ⴗ
0ⴗ
REV. 0
–19–