AD7984BCPZ [ADI]
18-Bit, 1.33 MSPS PulSAR 10.5 mW ADC in MSOP/QFN; 采用MSOP / QFN封装的18位， 1.33 MSPS的PulSAR 10.5毫瓦ADC
| 型号： | AD7984BCPZ |
| 厂家： | 
ADI |
| 描述： | 18-Bit, 1.33 MSPS PulSAR 10.5 mW ADC in MSOP/QFN |
| 文件： | 总24页 (文件大小：718K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
18-Bit, 1.33 MSPS PulSAR 10.5 mW
ADC in MSOP/QFN
AD7984
APPLICATION DIAGRAM
FEATURES
2.9V TO 5V 2.5V
18-bit resolution with no missing codes
Throughput: 1.33 MSPS
Low power dissipation: 10.5 mW at 1.33 MSPS
INL: 2.25 LSB maximum
Dynamic range: 99.7 dB typical
True differential analog input range: VREF
0 V to VREF with VREF between 2.9 V to 5.0 V
Allows use of any input range
VIO
SDI
1.8V TO 5V
3- OR 4-WIRE
INTERFACE
(SPI, CS
REF VDD
IN+
SCK
SDO
CNV
AD7984
±10V, ±5V, ..
IN–
DAISY CHAIN)
GND
ADA4941
Easy to drive with the ADA4941
Figure 1.
No pipeline delay
Single-supply 2.5 V operation with 1.8 V/2.5 V/3 V/5 V logic
interface
GENERAL DESCRIPTION
The AD7984 is an 18-bit, successive approximation, analog-to-
digital converter (ADC) that operates from a single power
supply, VDD. It contains a low power, high speed, 18-bit
sampling ADC and a versatile serial interface port. On the CNV
rising edge, the AD7984 samples the voltage difference between
the IN+ and IN− pins. The voltages on these pins usually swing
in opposite phases between 0 V and VREF. The reference voltage,
REF, is applied externally and can be set independent of the
supply voltage, VDD.
Serial interface SPI-/QSPI™-/MICROWIRE™-/DSP-compatible
Ability to daisy-chain multiple ADCs and busy indicator
10-lead MSOP (MSOP-8 size) and 10-lead 3 mm × 3 mm QFN
(LFCSP), SOT-23 size
APPLICATIONS
Battery-powered equipment
Data acquisition systems
Medical instruments
The SPI-compatible serial interface also features the ability,
using the SDI input, to daisy-chain several ADCs on a single
3-wire bus and provides an optional busy indicator. It is compatible
with 1.8 V, 2.5 V, 3 V, and 5 V logic, using the separate VIO supply.
Seismic data acquisition systems
The AD7984 is available in a 10-lead MSOP or a 10-lead QFN
(LFCSP) with operation specified from −40°C to +85°C.
Table 1. MSOP, QFN (LFCSP) 14-/16-/18-Bit PulSAR® ADC
Type
14-Bit
16-Bit
100 kSPS
AD7940
AD7680
AD7683
AD7684
250 kSPS
AD79421
AD76851
AD76871
AD7694
AD76911
400 kSPS to 500 kSPS
AD79461
≥1000 kSPS
ADC Driver
AD76861
AD79801
AD79831
ADA4941-x
ADA4841-x
AD76881
AD76931
18-Bit
AD76901
AD79821
AD79841
ADA4941-x
ADA4841-x
1 Pin-for-pin compatible.
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 that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
©2007 Analog Devices, Inc. All rights reserved.
AD7984
TABLE OF CONTENTS
Features .............................................................................................. 1
Driver Amplifier Choice ........................................................... 14
Single-to-Differential Driver .................................................... 15
Voltage Reference Input ............................................................ 15
Power Supply............................................................................... 15
Digital Interface.......................................................................... 16
Applications....................................................................................... 1
Application Diagram........................................................................ 1
General Description......................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Specifications .................................................................. 5
Absolute Maximum Ratings............................................................ 6
ESD Caution.................................................................................. 6
Pin Configurations and Function Descriptions ........................... 7
Typical Performance Characteristics ............................................. 8
Terminology .................................................................................... 11
Theory of Operation ...................................................................... 12
Circuit Information.................................................................... 12
Converter Operation.................................................................. 12
Typical Connection Diagram ................................................... 13
Analog Inputs.............................................................................. 14
CS
CS
CS
CS
Mode, 3-Wire Without Busy Indicator ............................. 17
Mode, 3-Wire with Busy Indicator .................................... 18
Mode, 4-Wire Without Busy Indicator ............................. 19
Mode, 4-Wire with Busy Indicator .................................... 20
Chain Mode Without Busy Indicator ...................................... 21
Chain Mode with Busy Indicator............................................. 22
Application Hints ........................................................................... 23
Layout .......................................................................................... 23
Evaluating the AD7984 Performance...................................... 23
Outline Dimensions....................................................................... 24
Ordering Guide .......................................................................... 24
REVISION HISTORY
11/07—Revision 0: Initial Version
Rev. 0 | Page 2 of 24
AD7984
SPECIFICATIONS
VDD = 2.5 V, VIO = 2.3 V to 5.5 V, REF = 5 V, TA = −40°C to +85°C, unless otherwise noted.
Table 2.
Parameter
Conditions
Min
Typ
Max
Unit
RESOLUTION
18
Bits
ANALOG INPUT
Voltage Range
IN+ − IN−
−VREF
+VREF
V
Absolute Input Voltage
Common-Mode Input Range
Analog Input CMRR
Leakage Current at 25°C
Input Impedance
ACCURACY
IN+, IN−
IN+, IN−
fIN = 450 kHz
Acquisition phase
−0.1
VREF × 0.475
VREF + 0.1
VREF × 0.525
V
V
dB1
nA
VREF × 0.5
67
200
See the Analog Inputs section
No Missing Codes
Differential Linearity Error
Integral Linearity Error
Transition Noise
18
−1
−2.25
Bits
+1.5
LSB2
+2.25
0.95
LSB2
LSB2
% of FS
ppm/°C
μV
ppm/°C
dB1
3
Gain Error, TMIN to TMAX
−0.075
−700
0.022
−0.6
100
0.3
+0.075
Gain Error Temperature Drift
3
Zero Error, TMIN to TMAX
+700
Zero Temperature Drift
Power Supply Sensitivity
90
VDD = 2.5 V ± 5%
THROUGHPUT
Conversion Rate
Transient Response
0
1.33
290
MSPS
ns
Full-scale step
AC ACCURACY
Dynamic Range
Signal-to-Noise, SNR
Spurious-Free Dynamic Range, SFDR
Total Harmonic Distortion4, THD
Signal-to-(Noise + Distortion), SINAD
VREF = 5 V
fIN = 1 kHz, VREF = 5 V, TA = 25°C
fIN = 10 kHz
fIN = 10 kHz
fIN = 10 kHz, VREF = 5 V, TA = 25°C
99.7
98.5
112.5
−110.5
98
dB1
dB1
dB1
dB1
dB1
96.5
1 All specifications expressed in decibels are referred to a full-scale input FSR and tested with an input signal at 0.5 dB below full scale, unless otherwise specified.
2 LSB means least significant bit. With the 5 V input range, one LSB is 38.15 μV.
3 See Terminology section. These specifications include full temperature range variation but not the error contribution from the external reference.
4 Tested fully in production at fIN = 1 kHz.
Rev. 0 | Page 3 of 24
AD7984
VDD = 2.5 V, VIO = 2.3 V to 5.5 V, REF = 5 V, TA = −40°C to +85°C, unless otherwise noted.
Table 3.
Parameter
REFERENCE
Voltage Range
Load Current
SAMPLING DYNAMICS
−3 dB Input Bandwidth
Aperture Delay
DIGITAL INPUTS
Logic Levels
VIL
Conditions
Min
Typ
Max
Unit
2.9
5.1
V
μA
1.33 MSPS
520
10
2
MHz
ns
VIO > 3 V
VIO > 3 V
VIO ≤ 3 V
VIO ≤ 3 V
–0.3
0.7 × VIO
–0.3
0.9 × VIO
−1
−1
+0.3 × VIO
VIO + 0.3
+0.1 × VIO
VIO + 0.3
+1
V
V
V
V
μA
μA
VIH
VIL
VIH
IIL
IIH
+1
DIGITAL OUTPUTS
Data Format
Pipeline Delay
Serial 18 bits, twos complement
Conversion results available immediately
after completed conversion
VOL
VOH
ISINK = +500 μA
ISOURCE = −500 μA
0.4
V
V
VIO − 0.3
POWER SUPPLIES
VDD
VIO
2.375
2.3
2.5
2.625
5.5
V
V
Specified performance
VIO Range
1.8
5.5
V
Standby Current1, 2
Power Dissipation
Energy per Conversion
TEMPERATURE RANGE3
Specified Performance
VDD and VIO = 2.5 V
1.33 MSPS throughput
1.1
10.5
7.9
mA
mW
nJ/sample
14
TMIN to TMAX
−40
+85
°C
1 With all digital inputs forced to VIO or GND as required.
2 During acquisition phase.
3 Contact an Analog Devices, Inc., sales representative for the extended temperature range.
Rev. 0 | Page 4 of 24
AD7984
TIMING SPECIFICATIONS
TA = −40°C to +85°C, VDD = 2.37 V to 2.63 V, VIO = 2.3 V to 5.5 V, unless otherwise noted.1
Table 4.
Parameter
Symbol
Min
300
250
750
10
Typ
Max
Unit
ns
ns
ns
ns
Conversion Time: CNV Rising Edge to Data Available
Acquisition Time
Time Between Conversions
CNV Pulse Width (CS Mode)
SCK Period (CS Mode)
tCONV
tACQ
tCYC
tCNVH
tSCK
500
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
10.5
12
13
ns
ns
ns
ns
15
SCK Period (Chain Mode)
tSCK
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
SCK Low Time
SCK High Time
SCK Falling Edge to Data Remains Valid
SCK Falling Edge to Data Valid Delay
VIO Above 4.5 V
11.5
13
14
16
4.5
4.5
3
ns
ns
ns
ns
ns
ns
ns
tSCKL
tSCKH
tHSDO
tDSDO
9.5
11
12
14
ns
ns
ns
ns
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV or SDI Low to SDO D15 MSB Valid (CS Mode)
VIO Above 3 V
tEN
10
15
20
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
VIO Above 2.3 V
CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
SDI Valid Setup Time from CNV Rising Edge
SDI Valid Hold Time from CNV Rising Edge (CS Mode)
SDI Valid Hold Time from CNV Rising Edge (Chain Mode)
SCK Valid Setup Time from CNV Rising Edge (Chain Mode)
SCK Valid Hold Time from CNV Rising Edge (Chain Mode)
SDI Valid Setup Time from SCK Falling Edge (Chain Mode)
SDI Valid Hold Time from SCK Falling Edge (Chain Mode)
SDI High to SDO High (Chain Mode with Busy Indicator)
tDIS
tSSDICNV
tHSDICNV
tHSDICNV
tSSCKCNV
tHSCKCNV
tSSDISCK
tHSDISCK
tDSDOSDI
5
2
0
5
5
2
3
15
1 See Figure 2 and Figure 3 for load conditions.
1
Y% VIO
500µA
I
OL
1
X% VIO
tDELAY
tDELAY
2
2
V
V
V
IH
IH
1.4V
TO SDO
2
2
V
IL
IL
C
L
20pF
1
2
FOR VIO ≤ 3.0V, X = 90, AND Y = 10; FOR VIO > 3.0V, X = 70, AND Y = 30.
MINIMUM V AND MAXIMUM V USED. SEE DIGITAL INPUTS
500µA
I
OH
IH
IL
SPECIFICATIONS IN TABLE 3.
Figure 2. Load Circuit for Digital Interface Timing
Figure 3. Voltage Levels for Timing
Rev. 0 | Page 5 of 24
AD7984
ABSOLUTE MAXIMUM RATINGS
Table 5.
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.
Parameter
Rating
Analog Inputs
IN+, IN− to GND1
−0.3 V to VREF + 0.3 V
or 130 mA
Supply Voltage
REF, VIO to GND
VDD to GND
VDD to VIO
−0.3 V to +6.0 V
−0.3 V to +3.0 V
+3 V to −6 V
Digital Inputs to GND
Digital Outputs to GND
Storage Temperature Range
Junction Temperature
θJA Thermal Impedance
10-Lead MSOP
10-Lead QFN (LFCSP)
θJC Thermal Impedance
10-Lead MSOP
−0.3 V to VIO + 0.3 V
−0.3 V to VIO + 0.3 V
−65°C to +150°C
150°C
ESD CAUTION
200°C/W
48.7°C/W
44°C/W
10-Lead QFN (LFCSP)
Lead Temperatures
Vapor Phase (60 sec)
Infrared (15 sec)
2.96°C/W
215°C
220°C
1 See the Analog Inputs section for an explanation of IN+ and IN−.
Rev. 0 | Page 6 of 24
AD7984
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
REF 1
VDD 2
IN+ 3
10 VIO
REF
VDD
IN+
1
2
3
4
5
10 VIO
9
8
7
6
SDI
9
8
7
6
SDI
AD7984
AD7984
TOP VIEW
(Not to Scale)
SCK
SDO
CNV
TOP VIEW
SCK
SDO
CNV
(Not to Scale)
IN– 4
IN–
GND 5
GND
Figure 4. 10-Lead MSOP Pin Configuration
Figure 5. 10-Lead QFN (LFCSP) Pin Configuration
Table 6. Pin Function Descriptions
Pin No. Mnemonic
Type1 Description
1
REF
AI
Reference Input Voltage. The REF range is 2.9 V to 5.1 V. This pin is referred to the GND pin and
should be decoupled closely to the GND pin with a 10 μF capacitor.
2
3
4
5
6
VDD
IN+
IN−
GND
CNV
P
Power Supply.
AI
AI
P
Differential Positive Analog Input.
Differential Negative Analog Input.
Power Supply Ground.
Convert Input. This input has multiple functions. On its rising edge, it initiates the conversions
and selects the interface mode of the part: chain mode or CS mode. In CS mode, the SDO pin is
enabled when CNV is low. In chain mode, the data should be read when CNV is high.
DI
7
8
9
SDO
SCK
SDI
DO
DI
DI
Serial Data Output. The conversion result is output on this pin. It is synchronized to SCK.
Serial Data Clock Input. When the part is selected, the conversion result is shifted out by this clock.
Serial Data Input. This input provides multiple features. It selects the interface mode of the ADC as
follows:
Chain mode is selected if SDI is low during the CNV rising edge. In this mode, SDI is used as a
data input to daisy-chain the conversion results of two or more ADCs onto a single SDO line. The
digital data level on SDI is output on SDO with a delay of 18 SCK cycles.
CS mode is selected if SDI is high during the CNV rising edge. In this mode, either SDI or CNV can
enable the serial output signals when low. If SDI or CNV is low when the conversion is complete,
the busy indicator feature is enabled.
10
VIO
P
Input/Output Interface Digital Power. Nominally at the same supply as the host interface
(1.8 V, 2.5 V, 3 V, or 5 V).
1 AI = analog input, DI = digital input, DO = digital output, and P = power.
Rev. 0 | Page 7 of 24
AD7984
TYPICAL PERFORMANCE CHARACTERISTICS
VDD = 2.5 V, REF = 5.0 V, VIO = 3.3 V.
2.0
2.0
1.5
POSITIVE INL: +1.07LSB
POSITIVE DNL: +0.63LSB
NEGATIVE DNL: –0.34LSB
NEGATIVE INL: –0.73LSB
1.5
1.0
0.5
1.0
0.5
0
0
–0.5
–1.0
–1.5
–2.0
–0.5
–1.0
–1.5
–2.0
0
65536
131072
CODE
196608
262144
0
65536
131072
CODE
196608
262144
Figure 6. Integral Nonlinearity vs. Code
Figure 9. Differential Nonlinearity vs. Code
60k
50k
40k
30k
20k
10k
0
60k
50k
40k
30k
20k
10k
0
55354
48273 48266
32350
31003
16593
14653
5992
5708
1801
1378
326
326
0
0
7
6
0
0
0
0
2
69
37
0
0
1C 1D 1E 1F 20 21 22 23 24 25 26 27 28
CODE IN HEX
1D 1E 1F 20 21 22 23 24 25 26 27 28 29
CODE IN HEX
Figure 7. Histogram of a DC Input at the Code Center
Figure 10. Histogram of a DC Input at the Code Transition
0
–20
100
fS = 1.33MSPS
fIN = 10kHz
SNR = 98.2dB
THD = –110.6dB
SFDR = 112.5dB
SINAD = 98.0dB
99
98
97
96
95
94
93
92
91
90
–40
–60
–80
–100
–120
–140
–160
–180
0
100
200
300
400
500
600
–10
–9
–8
–7
–6
–5
–4
–3
–2
–1
0
FREQUENCY (kHz)
INPUT LEVEL (dB of Full Scale)
Figure 8. FFT Plot
Figure 11. SNR vs. Input Level
Rev. 0 | Page 8 of 24
AD7984
100
95
90
85
80
18
17
16
15
14
–100
–105
–110
–115
–120
SNR
SINAD
ENOB
2.5
3.0
3.5
4.0
4.5
5.0
5.5
2.5
3.0
3.5
4.0
4.5
5.0
5.5
REFERENCE VOLTAGE (V)
REFERENCE VOLTAGE (V)
Figure 12. SNR, SINAD, and ENOB vs. Reference Voltage
Figure 15. THD vs. Reference Voltage
100
–100
–105
–110
–115
–120
98
96
94
92
90
–55
–35
–15
5
25
45
65
85
105
–55
–35
–15
5
25
45
65
85
105
125
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 13. SNR vs. Temperature
Figure 16. THD vs. Temperature
100
95
90
85
80
–80
–85
–90
–95
–100
–105
–110
–115
1
10
100
1000
1
10
100
1000
FREQUENCY (kHz)
FREQUENCY (kHz)
Figure 14. SINAD vs. Frequency
Figure 17. THD vs. Frequency
Rev. 0 | Page 9 of 24
AD7984
2.5
2.0
1.5
1.0
0.5
2.5
2.0
1.5
1.0
0.5
0
I
VDD
I
VDD
I
I
REF
REF
I
I
VIO
VIO
25
0
2.375
2.425
2.475
2.525
2.575
2.625
–55
–35
–15
5
45
65
85
105
125
V
VOLTAGE (V)
TEMPERATURE (°C)
DD
Figure 20. Operating Currents vs. Temperature
Figure 18. Operating Currents vs. Supply
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
I
+ I
VIO
VDD
0.5
–55
–35
–15
5
25
45
65
85
105
125
TEMPERATURE (°C)
Figure 19. Standby Currents vs. Temperature
Rev. 0 | Page 10 of 24
AD7984
TERMINOLOGY
Integral Nonlinearity Error (INL)
Noise-Free Code Resolution
INL 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 ½ LSB before the first
code transition. Positive full scale is defined as a level 1½ LSB
beyond the last code transition. The deviation is measured from
the middle of each code to the true straight line (see Figure 22).
Noise-free code resolution is the number of bits beyond which it is
impossible to distinctly resolve individual codes. It is calculated as
Noise-Free Code Resolution = log2(2N/Peak-to-Peak Noise)
and is expressed in bits.
Effective Resolution
Effective resolution is calculated as
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. DNL is the
maximum deviation from this ideal value. It is often specified in
terms of resolution for which no missing codes are guaranteed.
Effective Resolution = log2(2N/RMS Input Noise)
and is expressed in bits.
Total Harmonic Distortion (THD)
Zero Error
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.
Zero error is the difference between the ideal midscale voltage,
that is, 0 V, from the actual voltage producing the midscale
output code, that is, 0 LSB.
Dynamic Range
Gain Error
Dynamic range is the ratio of the rms value of the full scale to
the total rms noise measured with the inputs shorted together.
The value for dynamic range is expressed in decibels. It is
measured with a signal at −60 dBF so that it includes all noise
sources and DNL artifacts.
The first transition (from 100 ... 00 to 100 ... 01) should occur at
a level ½ LSB above nominal negative full scale (−4.999981 V
for the 5 V range). The last transition (from 011 … 10 to
011 … 11) should occur for an analog voltage 1½ LSB below
the nominal full scale (+4.999943 V for the 5 V range). The
gain error is the deviation of the difference between the actual
level of the last transition and the actual level of the first
transition from the difference between the ideal levels.
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.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms
amplitude of the input signal and the peak spurious signal.
Signal-to-(Noise + Distortion) Ratio (SINAD)
SINAD is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components that are less than
the Nyquist frequency, including harmonics but excluding dc.
The value of SINAD is expressed in decibels.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to SINAD as follows:
ENOB = (SINADdB − 1.76)/6.02
Aperture Delay
and is expressed in bits.
Aperture delay is the measurement of the acquisition
performance and is the time between the rising edge of the
CNV input and when the input signal is held for a conversion.
Transient Response
Transient response is the time required for the ADC to accurately
acquire its input after a full-scale step function is applied.
Rev. 0 | Page 11 of 24
AD7984
THEORY OF OPERATION
IN+
SWITCHES CONTROL
CONTROL
SW+
SW–
MSB
LSB
LSB
131,072C 65,536C
4C
4C
2C
2C
C
C
C
C
BUSY
REF
COMP
LOGIC
GND
131,072C 65,536C
MSB
OUTPUT CODE
CNV
IN–
Figure 21. ADC Simplified Schematic
During the acquisition phase, terminals of the array tied to the
input of the comparator are connected to GND via SW+ and
SW−. All independent switches are connected to the analog
inputs. Therefore, the capacitor arrays are used as sampling
capacitors and acquire the analog signal on the IN+ and IN−
inputs. When the acquisition phase is complete and the CNV
input goes high, a conversion phase is initiated. When the
conversion phase begins, SW+ and SW− are opened first. The
two capacitor arrays are then disconnected from the inputs and
connected to the GND input. Therefore, the differential voltage
between the inputs IN+ and IN− 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 GND and REF, the
comparator input varies by binary-weighted voltage steps
(VREF/2, VREF/4 ... VREF/262,144). The control logic toggles these
switches, starting with the MSB, to bring the comparator back
into a balanced condition. After the completion of this process,
the part returns to the acquisition phase, and the control logic
generates the ADC output code and a busy signal indicator.
CIRCUIT INFORMATION
The AD7984 is a fast, low power, single-supply, precise, 18-bit
ADC using a successive approximation architecture and is
capable of converting 1,330,000 samples per second (1.33 MSPS).
The AD7984 provides the user with an on-chip track-and-hold
and does not exhibit any pipeline delay or latency, making it
ideal for multiple multiplexed channel applications.
The AD7984 can be interfaced to any 1.8 V to 5 V digital logic
family. It is available in a 10-lead MSOP or a tiny 10-lead QFN
(LFCSP) that allows space savings and flexible configurations.
It is pin-for-pin-compatible with the 18-bit AD7982.
CONVERTER OPERATION
The AD7984 is a successive approximation ADC based on a
charge redistribution DAC. Figure 21 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 18 binary-weighted capacitors, which are
connected to the two comparator inputs.
Because the AD7984 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
Rev. 0 | Page 12 of 24
AD7984
Transfer Functions
Table 7. Output Codes and Ideal Input Voltages
The ideal transfer characteristic for the AD7984 is shown in
Figure 22 and Table 7.
Analog Input
VREF = 5 V
Digital Output
Code (Hex)
Description
FSR − 1 LSB
Midscale + 1 LSB
Midscale
+4.999962 V
+38.15 μV
0 V
0x1FFFF1
0x00001
0x00000
0x3FFFF
0x20001
0x200002
011 ... 111
011 ... 110
011 ... 101
Midscale − 1 LSB
−FSR + 1 LSB
−FSR
−38.15 μV
−4.999962 V
−5 V
1 This is also the code for an overranged analog input (VIN+ − VIN− above VREF − VGND).
2 This is also the code for an underranged analog input (VIN+ − VIN− below VGND).
100 ... 010
100 ... 001
100 ... 000
TYPICAL CONNECTION DIAGRAM
Figure 23 shows an example of the recommended connection
diagram for the AD7984 when multiple supplies are available.
–FSR
–FSR + 1 LSB
+FSR – 1 LSB
+FSR – 1.5 LSB
ANALOG INPUT
–FSR + 0.5 LSB
Figure 22. ADC Ideal Transfer Function
1
2.5V
REF
V+
V+
2
100nF
10µF
1.8V TO 5V
100nF
15Ω
0 TO VREF
REF
VDD
VIO
2.7nF
SDI
IN+
IN–
V–
V+
SCK
SDO
CNV
4
AD7984
3-WIRE INTERFACE
15Ω
GND
VREF TO 0
2.7nF
ADA48412, 3
V–
4
NOTES
1
SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.
IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
2
C
REF
SEE RECOMMENDED LAYOUT IN FIGURE 40 AND FIGURE 41.
SEE DRIVER AMPLIFIER CHOICE SECTION.
OPTIONAL FILTER. SEE ANALOG INPUTS SECTION.
3
4
Figure 23. Typical Application Diagram with Multiple Supplies
Rev. 0 | Page 13 of 24
AD7984
When the source impedance of the driving circuit is low, the
AD7984 can be driven directly. Large source impedances
significantly affect the ac performance, especially THD. The
dc performances are less sensitive to the input impedance. The
maximum source impedance depends on the amount of THD
that can be tolerated. The THD degrades as a function of the
source impedance and the maximum input frequency.
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7984.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs, IN+ and IN−. Care must be taken to ensure that
the analog input signal does not exceed the reference input
voltage (REF) by more than 0.3 V. If the analog input signal
exceeds this level, the diodes become forward-biased and start
conducting current. These diodes can handle a forward-biased
current of 130 mA maximum. However, if the supplies of the
input buffer (for example, the supplies of the ADA4841 in
Figure 23) are different from those of REF, the analog input
signal may eventually exceed the supply rails by more than
0.3 V. In such a case (for example, an input buffer with a short-
circuit), the current limitation can be used to protect the part.
DRIVER AMPLIFIER CHOICE
Although the AD7984 is easy to drive, the driver amplifier must
meet the following requirements:
•
The noise generated by the driver amplifier must be kept as
low as possible to preserve the SNR and transition noise
performance of the AD7984. The noise from the driver is
filtered by the AD7984 analog input circuit’s 1-pole, low-
pass filter made by RIN and CIN or by the external filter, if
one is used. Because the typical noise of the AD7984 is
36.24 μV rms, the SNR degradation due to the amplifier is
REF
D1
D2
C
IN
R
IN
IN+ OR IN–
GND
⎛
⎜
⎞
⎟
C
PIN
36.24
⎜
⎜
⎜
⎝
⎟
⎟
⎟
⎠
SNRLOSS = 20 log
π
36.242 + f−3dB (NeN )2
2
Figure 24. Equivalent Analog Input Circuit
where:
–3dB is the input bandwidth, in megahertz, of the AD7984
(10 MHz) or the cutoff frequency of the input filter, if
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. By using these
differential inputs, signals common to both inputs are rejected.
f
one is used.
90
85
80
75
70
N is the noise gain of the amplifier (for example, 1 in buffer
configuration).
eN is the equivalent input noise voltage of the op amp, in
nV/√Hz.
•
•
For ac applications, the driver should have a THD perfor-
mance commensurate with the AD7984.
For multichannel multiplexed applications, the driver
amplifier and the AD7984 analog input circuit must settle
for a full-scale step onto the capacitor array at an 18-bit level
(0.0004%, 4 ppm). In the data sheet of the amplifier,
settling at 0.1% to 0.01% is more commonly specified. This
may differ significantly from the settling time at an 18-bit
level and should be verified prior to driver selection.
65
60
1
10
100
1000
10000
FREQUENCY (kHz)
Figure 25. Analog Input CMRR vs. Frequency
Table 8. Recommended Driver Amplifiers
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, CPIN, and the network formed by the series connection
of RIN and CIN. CPIN is primarily the pin capacitance. RIN is typically
400 Ω and is a lumped component composed of serial resistors
and the on resistance of the switches. CIN is typically 30 pF and
is mainly the ADC sampling capacitor.
Amplifier
ADA4941-x
ADA4841-x
AD8021
Typical Application
Very low noise, low power single-to-differential
Very low noise, small, and low power
Very low noise and high frequency
Low noise and high frequency
AD8022
OP184
AD8655
Low power, low noise, and low frequency
5 V single supply, low noise
During the sampling phase, where the switches are closed, the
input impedance is limited to CPIN. RIN and CIN make a 1-pole,
low-pass filter that reduces undesirable aliasing effects and
limits noise.
AD8605, AD8615 5 V single supply, low power
Rev. 0 | Page 14 of 24
AD7984
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR43x reference.
SINGLE-TO-DIFFERENTIAL DRIVER
For applications using a single-ended analog signal, either
bipolar or unipolar, the ADA4941-x single-ended-to-
differential driver allows for a differential input into the part. The
schematic is shown in Figure 26.
If desired, a reference-decoupling capacitor with values as small
as 2.2 μF can be used with a minimal impact on performance,
especially DNL.
R1 and R2 set the attenuation ratio between the input range and
the ADC range (VREF). R1, R2, and CF are chosen depending on
the desired input resistance, signal bandwidth, antialiasing, and
noise contribution. For example, for the 10 V range with a 4 kΩ
impedance, R2 = 1 kΩ and R1 = 4 kΩ.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor (for example, 100 nF) between the
REF and GND pins.
R3 and R4 set the common mode on the IN− input, and R5 and R6
set the common mode on the IN+ input of the ADC. The common
mode should be close to VREF/2. For example, for the 10 V range
with a single supply, R3 = 8.45 kΩ, R4 = 11.8 kΩ, R5 = 10.5 kΩ,
and R6 = 9.76 kΩ.
POWER SUPPLY
The AD7984 uses two power supply pins: a core supply (VDD) and
a digital input/output interface supply (VIO). VIO allows direct
interface with any logic between 1.8 V and 5.5 V. To reduce the
number of supplies needed, VIO and VDD can be tied together.
The AD7984 is independent of power supply sequencing between
VIO and VDD. Additionally, it is very insensitive to power supply
variations over a wide frequency range, as shown in Figure 27.
R5
R6
R3
R4
+5V REF
+2.5V
10µF
+5.2V
100nF
100nF
95
90
85
80
75
70
65
60
REF
15Ω
15Ω
OUTN
OUTP
REF
VDD
IN+
IN–
2.7nF
2.7nF
AD7984
IN
GND
FB
ADA4941
–0.2V
R2
R1
±10V,
±5V, ..
C
F
Figure 26. Single-Ended-to-Differential Driver Circuit
VOLTAGE REFERENCE INPUT
1
10
100
FREQUENCY (kHz)
1000
The AD7984 voltage reference input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
Figure 27. PSRR vs. Frequency
To ensure optimum performance, VDD should be roughly half
of REF, the voltage reference input. For example, if REF is 5.0 V,
VDD should be set to 2.5 V ( 5%).
When REF is driven by a very low impedance source (for
example, a reference buffer using the AD8031 or the AD8605),
a 10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate
for optimum performance.
Rev. 0 | Page 15 of 24
AD7984
The mode in which the part operates depends on the SDI level
CS
SDI is high, and the chain mode is selected if SDI is low. The
SDI hold time is such that when SDI and CNV are connected,
the chain mode is always selected.
DIGITAL INTERFACE
when the CNV rising edge occurs. The
mode is selected if
Although the AD7984 has a reduced number of pins, it offers
flexibility in its serial interface modes.
CS
When in
mode, the AD7984 is compatible with SPI, QSPI,
digital hosts, and DSPs. In this mode, the AD7984 can use
either a 3-wire or 4-wire interface. A 3-wire interface using the
CNV, SCK, and SDO signals minimizes wiring connections
useful, for instance, in isolated applications. A 4-wire interface
using the SDI, CNV, SCK, and SDO signals allows CNV, which
initiates the conversions, to be independent of the readback
timing (SDI). This is useful in low jitter sampling or
simultaneous sampling applications.
In either mode, the AD7984 offers the option of forcing a start
bit in front of the data bits. This start bit can be used as a busy
signal indicator to interrupt the digital host and trigger the data
reading. Otherwise, without a busy indicator, the user must timeout
the maximum conversion time prior to readback.
The busy indicator feature is enabled
CS
mode if CNV or SDI is low when the ADC
•
•
In
When in chain mode, the AD7984 provides a daisy-chain feature
using the SDI input for cascading multiple ADCs on a single
data line similar to a shift register.
conversion ends (see Figure 31 and Figure 35).
In chain mode if SCK is high during the CNV rising edge
(see Figure 39).
Rev. 0 | Page 16 of 24
AD7984
high for the maximum possible conversion time to avoid the
CS MODE, 3-WIRE WITHOUT BUSY INDICATOR
generation of the busy signal indicator. When the conversion is
complete, the AD7984 enters the acquisition phase and goes
into standby mode. When CNV goes low, the MSB is output
onto SDO. The remaining data bits are clocked by subsequent
SCK falling edges. The data is valid on both SCK edges. Although
the rising edge can be used to capture the data, a digital host
using the SCK falling edge allows a faster reading rate, provided
it has an acceptable hold time. After the 18th SCK falling edge or
when CNV goes high (whichever occurs first), SDO returns to
high impedance.
This mode is usually used when a single AD7984 is connected
to an SPI-compatible digital host. The connection diagram is
shown in Figure 28, and the corresponding timing is given in
Figure 29.
With SDI tied to VIO, a rising edge on CNV initiates a
CS
conversion, selects the
mode, and forces SDO to high
impedance. When a conversion is initiated, it continues until
completion irrespective of the state of CNV. This can be useful,
for example, to bring CNV low to select other SPI devices, such
as analog multiplexers; however, CNV must be returned high
before the minimum conversion time elapses and then held
CONVERT
DIGITAL HOST
CNV
VIO
SDI
SDO
DATA IN
AD7984
SCK
CLK
CS
Figure 28. Mode, 3-Wire Without Busy Indicator Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
tCONV
tACQ
ACQUISITION
CONVERSION
ACQUISITION
tSCK
tSCKL
SCK
1
2
3
16
17
18
tHSDO
tSCKH
tDSDO
D15
tEN
tDIS
SDO
D17
D16
D1
D0
CS
Figure 29. Mode, 3-Wire Without Busy Indicator Serial Interface Timing (SDI High)
Rev. 0 | Page 17 of 24
AD7984
When the conversion is complete, SDO goes from high
impedance to low impedance. With a pull-up on the SDO line,
this transition can be used as an interrupt signal to initiate the
data reading controlled by the digital host. The AD7984 then
enters the acquisition phase and goes into standby mode. The
data bits are then clocked out, MSB first, by subsequent SCK
falling edges. The data is valid on both SCK edges. Although the
rising edge can be used to capture the data, a digital host using
the SCK falling edge allows a faster reading rate, provided it has
an acceptable hold time. After the optional 19th SCK falling edge
or when CNV goes high (whichever occurs first), SDO returns
to high impedance.
CS MODE, 3-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7984 is connected
to an SPI-compatible digital host having an interrupt input.
The connection diagram is shown in Figure 30, and the
corresponding timing is given in Figure 31.
With SDI tied to VIO, a rising edge on CNV initiates a
CS
conversion, selects the
mode, and forces SDO to high
impedance. SDO is maintained in high impedance until the
completion of the conversion irrespective of the state of CNV.
Prior to the minimum conversion time, CNV can be used to
select other SPI devices, such as analog multiplexers, but CNV
must be returned low before the minimum conversion time
elapses and then held low for the maximum possible conversion
time to guarantee the generation of the busy signal indicator.
If multiple AD7984s are selected at the same time, the SDO
output pin handles this contention without damage or induced
latch-up. Meanwhile, it is recommended to keep this contention
as short as possible to limit extra power dissipation.
CONVERT
VIO
47kΩ
DIGITAL HOST
CNV
VIO
SDI
SDO
DATA IN
AD7984
IRQ
SCK
CLK
CS
Figure 30. Mode, 3-Wire with Busy Indicator Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
tCONV
tACQ
ACQUISITION
CONVERSION
ACQUISITION
tSCK
tSCKL
SCK
1
2
3
17
18
19
tHSDO
tSCKH
tDSDO
tDIS
SDO
D17
D16
D1
D0
CS
Figure 31. Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High)
Rev. 0 | Page 18 of 24
AD7984
but SDI must be returned high before the minimum conversion
time elapses and then held high for the maximum possible
conversion time to avoid the generation of the busy signal
indicator. When the conversion is complete, the AD7984
enters the acquisition phase and goes into standby mode. Each
ADC result can be read by bringing its SDI input low, which
consequently outputs the MSB onto SDO. The remaining data
bits are then clocked by subsequent SCK falling edges. The data
is valid on both SCK edges. Although the rising edge can be
used to capture the data, a digital host using the SCK falling
edge allows a faster reading rate, provided it has an acceptable
hold time. After the 18th SCK falling edge or when SDI goes
high (whichever occurs first), SDO returns to high impedance
and another AD7984 can be read.
CS MODE, 4-WIRE WITHOUT BUSY INDICATOR
This mode is usually used when multiple AD7984s are connected
to an SPI-compatible digital host.
A connection diagram example using two AD7984s is shown in
Figure 32, and the corresponding timing is given in Figure 33.
With SDI high, a rising edge on CNV initiates a conversion,
CS
selects the
mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback (if SDI and CNV are low, SDO is
driven low). Prior to the minimum conversion time, SDI can be
used to select other SPI devices, such as analog multiplexers,
CS2
CS1
CONVERT
CNV
CNV
DIGITAL HOST
SDI
SDO
AD7984
SDI
SDO
AD7984
SCK
SCK
DATA IN
CLK
CS
Figure 32. Mode, 4-Wire Without Busy Indicator Connection Diagram
tCYC
CNV
tACQ
tCONV
ACQUISITION
tSSDICNV
CONVERSION
ACQUISITION
SDI(CS1)
tHSDICNV
SDI(CS2)
SCK
tSCK
tSCKL
1
2
3
16
17
18
19
20
34
35
36
tHSDO
tSCKH
tDSDO
D15
tDIS
tEN
SDO
D17
D16
D1
D0
D17
D16
D1
D0
CS
Figure 33. Mode, 4-Wire Without Busy Indicator Serial Interface Timing
Rev. 0 | Page 19 of 24
AD7984
used to select other SPI devices, such as analog multiplexers,
but SDI must be returned low before the minimum conversion
time elapses and then held low for the maximum possible
conversion time to guarantee the generation of the busy signal
indicator. When the conversion is complete, SDO goes from
high impedance to low impedance. With a pull-up on the SDO
line, this transition can be used as an interrupt signal to initiate
the data readback controlled by the digital host. The AD7984
then enters the acquisition phase and goes into standby mode.
The data bits are then clocked out, MSB first, by subsequent
SCK falling edges. The data is valid on both SCK edges. Although
the rising edge can be used to capture the data, a digital host
using the SCK falling edge allows a faster reading rate, provided
it has an acceptable hold time. After the optional 19th SCK
falling edge or SDI going high (whichever occurs first), SDO
returns to high impedance.
CS MODE, 4-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7984 is connected
to an SPI-compatible digital host with an interrupt input and
when it is desired to keep CNV, which is used to sample the
analog input, independent of the signal used to select the data
reading. This independence is particularly important in
applications where low jitter on CNV is desired.
The connection diagram is shown in Figure 34, and the
corresponding timing is given in Figure 35.
With SDI high, a rising edge on CNV initiates a conversion,
CS
selects the
mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback. (If SDI and CNV are low, SDO is
driven low.) Prior to the minimum conversion time, SDI can be
CS1
CONVERT
VIO
47kΩ
DIGITAL HOST
CNV
SDI
SDO
DATA IN
AD7984
IRQ
SCK
CLK
CS
Figure 34. Mode, 4-Wire with Busy Indicator Connection Diagram
tCYC
CNV
tACQ
tCONV
ACQUISITION
CONVERSION
ACQUISITION
tSSDICNV
SDI
tSCK
tHSDICNV
tSCKL
SCK
SDO
1
2
3
17
tSCKH
18
19
tHSDO
tDSDO
tDIS
tEN
D17
D16
D1
D0
CS
Figure 35. Mode, 4-Wire with Busy Indicator Serial Interface Timing
Rev. 0 | Page 20 of 24
AD7984
readback. When the conversion is complete, the MSB is output
onto SDO and the AD7984 enters the acquisition phase and
goes into standby mode. The remaining data bits stored in the
internal shift register are clocked by subsequent SCK falling
edges. For each ADC, SDI feeds the input of the internal shift
register and is clocked by the SCK falling edge. Each ADC in
the chain outputs its data MSB first, and 18 × N clocks are
required to read back the N ADCs. The data is valid on both
SCK edges. Although the rising edge can be used to capture the
data, a digital host using the SCK falling edge will allow a faster
reading rate and consequently more AD7984s in the chain,
provided the digital host has an acceptable hold time. The
maximum conversion rate may be reduced due to the total
readback time.
CHAIN MODE WITHOUT BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7984s on
a 3-wire serial interface. This feature is useful for reducing
component count and wiring connections, for example, in
isolated multiconverter applications or for systems with a
limited interfacing capacity. Data readback is analogous to
clocking a shift register.
A connection diagram example using two AD7984s is shown in
Figure 36, and the corresponding timing is given in Figure 37.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion, selects the chain
mode, and disables the busy indicator. In this mode, CNV is
held high during the conversion phase and the subsequent data
CONVERT
CNV
CNV
DIGITAL HOST
DATA IN
SDI
SDO
AD7984
SDI
SDO
AD7984
A
B
SCK
SCK
CLK
Figure 36. Chain Mode Without Busy Indicator Connection Diagram
SDI = 0
A
tCYC
CNV
tACQ
tCONV
ACQUISITION
CONVERSION
ACQUISITION
tSCK
tSCKL
tSSCKCNV
SCK
1
A
B
2
3
A
B
16
17
18
19
20
34
35
36
tHSCKCNV
tSSDISCK
tSCKH
tHSDISCK
tEN
D
D
17
D
16
D
D
15
15
D
1
1
D
0
SDO = SDI
A
A
A
A
B
tHSDO
tDSDO
17
D
16
D
B
D
0
D
17
D
16
D
1
D 0
A
SDO
B
B
A
A
A
B
Figure 37. Chain Mode Without Busy Indicator Serial Interface Timing
Rev. 0 | Page 21 of 24
AD7984
subsequent data readback. When all ADCs in the chain have
completed their conversions, the SDO pin of the ADC closest to
the digital host (see the AD7984 ADC labeled C in Figure 38) is
driven high. This transition on SDO can be used as a busy indicator
to trigger the data readback controlled by the digital host. The
AD7984 then enters the acquisition phase and goes into standby
mode. The data bits stored in the internal shift register are clocked
out, MSB first, by subsequent SCK falling edges. For each ADC,
SDI feeds the input of the internal shift register and is clocked
by the SCK falling edge. Each ADC in the chain outputs its data
MSB first, and 18 × N + 1 clocks are required to read back the N
ADCs. Although the rising edge can be used to capture the data, a
digital host using the SCK falling edge allows a faster reading
rate and, consequently, more AD7984s in the chain, provided
the digital host has an acceptable hold time.
CHAIN MODE WITH BUSY INDICATOR
This mode can also be used to daisy-chain multiple AD7984s
on a 3-wire serial interface while providing a busy indicator.
This feature is useful for reducing component count and wiring
connections, for example, in isolated multiconverter applications or
for systems with a limited interfacing capacity. Data readback is
analogous to clocking a shift register.
A connection diagram example using three AD7984s is shown
in Figure 38, and the corresponding timing is given in Figure 39.
When SDI and CNV are low, SDO is driven low. With SCK
high, a rising edge on CNV initiates a conversion, selects the
chain mode, and enables the busy indicator feature. In this
mode, CNV is held high during the conversion phase and the
CONVERT
CNV
CNV
CNV
DIGITAL HOST
SDI
SDO
AD7984
SDI
SDO
SDI
SDO
AD7984
AD7984
DATA IN
IRQ
A
B
C
SCK
SCK
SCK
CLK
Figure 38. Chain Mode with Busy Indicator Connection Diagram
tCYC
CNV = SDI
A
tCONV
tACQ
ACQUISITION
CONVERSION
ACQUISITION
tSCK
tSSCKCNV
tSCKH
SCK
1
2
3
4
17
18
19
20
21
35
36
37
38
39
53
54
55
tHSCKCNV
tSSDISCK
tSCKL
tDSDOSDI
tHSDISCK
tEN
SDO = SDI
D
17
D
16
D
15
D
A
1
D 0
A
A
B
A
A
A
tHSDO
tDSDO
tDSDOSDI
tDSDOSDI
tDSDOSDI
SDO = SDI
B
D
17
D
D
16
D
D
15
B
D
D
1
D
0
D
17
D
16
D 1
A
D 0
A
C
B
B
B
B
A
A
tDSDOSDI
SDO
D
17
16
15
1
D
0
D
17
D
16
D
1
D
0
D
17
D
16
D
1
D 0
A
C
C
C
C
C
C
B
B
B
B
A
A
A
Figure 39. Chain Mode with Busy Indicator Serial Interface Timing
Rev. 0 | Page 22 of 24
AD7984
APPLICATION HINTS
LAYOUT
AD7984
The printed circuit board (PCB) that houses the AD7984
should be designed so that the analog and digital sections are
separated and confined to certain areas of the board. The
pinout of the AD7984, with its analog signals on the left side
and its digital signals on the right side, eases this task.
Avoid running digital lines under the device because these
couple noise onto the die, unless a ground plane under the
AD7984 is used as a shield. Fast switching signals, such as CNV
or clocks, should not run near analog signal paths. Crossover of
digital and analog signals should be avoided.
At least one ground plane should be used. It can be common or
split between the digital and analog sections. In the latter case,
the planes should be joined underneath the AD7984.
Figure 40. Example Layout of the AD7984 (Top Layer)
The AD7984 voltage reference input REF has a dynamic input
impedance and should be decoupled with minimal parasitic
inductances. This is done by placing the reference decoupling
ceramic capacitor close to, ideally right up against, the REF and
GND pins and connecting them with wide, low impedance traces.
Finally, the power supplies VDD and VIO of the AD7984
should be decoupled with ceramic capacitors, typically 100 nF,
placed close to the AD7984 and connected using short, wide
traces to provide low impedance paths and to reduce the effect
of glitches on the power supply lines.
An example of layout following these rules is shown in
Figure 40 and Figure 41.
EVALUATING THE AD7984 PERFORMANCE
Figure 41. Example Layout of the AD7984 (Bottom Layer)
Other recommended layouts for the AD7984 are outlined
in the documentation of the evaluation board for the AD7984
(EVAL-AD7984CBZ). 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 BRD3Z.
Rev. 0 | Page 23 of 24
AD7984
OUTLINE DIMENSIONS
3.10
3.00
2.90
6
10
5.15
4.90
4.65
3.10
3.00
2.90
1
5
PIN 1
0.50 BSC
0.95
0.85
0.75
1.10 MAX
0.80
0.60
0.40
8°
0°
0.15
0.05
0.33
0.17
SEATING
PLANE
0.23
0.08
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 42. 10-Lead Mini Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
0.30
0.23
0.18
3.00
BSC SQ
0.50 BSC
10
6
5
PIN 1 INDEX
1.74
1.64
1.49
*
EXPOSED
PAD
AREA
(BOTTOM VIEW)
0.50
0.40
0.30
1
PIN 1
INDICATOR
(R 0.19)
TOP VIEW
2.48
2.38
2.23
0.80 MAX
0.55 NOM
0.80
0.75
0.70
0.05 MAX
0.02 NOM
*
PADDLE CONNECTED TO GND.
THIS CONNECTION IS NOT
REQUIRED TO MEET THE
SEATING
PLANE
ELECTRICAL PERFORMANCES.
0.20 REF
Figure 43. 10-Lead Lead Frame Chip Scale Package [QFN (LFCSP_WD)]
3 mm × 3 mm Body, Very Very Thin, Dual Lead (CP-10-9)
Dimensions shown in millimeters
ORDERING GUIDE
Model
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
10-Lead MSOP
10-Lead MSOP
10-Lead QFN (LFCSP_WD)
10-Lead QFN (LFCSP_WD)
10-Lead QFN (LFCSP_WD)
Evaluation Board
Package Option
RM-10
RM-10
CP-10-9
CP-10-9
Ordering Quantity
Tube, 50
Reel, 1,000
Tube, 75
Reel, 1,000
Reel, 5,000
Branding
C60
C60
C60
C60
AD7984BRMZ1
AD7984BRMZ-RL71
AD7984BCPZ1
AD7984BCPZ-RL71
AD7984BCPZ-RL1
EVAL-AD7984CBZ1, 2
EVAL-CONTROL BRD3Z1, 3
CP-10-9
C60
Evaluation Board
1 Z = RoHS compliant part.
2 This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BRD3Z for evaluation/demonstration purposes.
3 This board allows a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designator.
©2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D06973-0-11/07(0)
Rev. 0 | Page 24 of 24
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