AD AD7401 Isolated sigma-delta modulator Datasheet

Isolated Sigma-Delta Modulator
AD7401
FEATURES
GENERAL DESCRIPTION
20 MHz maximum external clock rate
Second-order modulator
16 bits no missing codes
±2 LSB INL typical at 16 bits
3.5 μV/°C maximum offset drift
On-board digital isolator
On-board reference
Low power operation: 20 mA maximum at 5.25 V
−40°C to +105°C operating range
16-lead SOIC package
AD7400, internal clock version
Safety and regulatory approvals
UL recognition
3750 V rms for 1 minute per UL 1577
CSA Component Acceptance Notice #5A
VDE Certificate of Conformity
DIN EN 60747-5-2 (VDE 0884 Part 2): 2003-01
DIN EN 60950 (VDE 0805): 2001-12; EN 60950: 2000
VIORM = 891 V peak
The AD7401 1 is a second-order, Σ-Δ modulator that converts
an analog input signal into a high speed, 1-bit data stream with
on-chip digital isolation based on Analog Devices, Inc.
iCoupler® technology. The AD7401 operates from a 5 V power
supply and accepts a differential input signal of ±200 mV
(±320 mV full scale). The analog input is continuously sampled
by the analog modulator, eliminating the need for external
sample-and-hold circuitry. The input information is contained
in the output stream as a density of ones with a data rate up to
20 MHz. The original information can be reconstructed with an
appropriate digital filter. The serial I/O can use a 5 V or a 3 V
supply (VDD2).
APPLICATIONS
1
The serial interface is digitally isolated. High speed CMOS,
combined with monolithic air core transformer technology,
means the on-chip isolation provides outstanding performance
characteristics, superior to alternatives such as optocoupler
devices. The part contains an on-chip reference. The AD7401
is offered in a 16-lead SOIC and has an operating temperature
range of −40°C to +105°C.
AC motor controls
Data acquisition systems
A/D + opto-isolator replacements
Protected by U.S. Patents 5,952,849; 6,873,065; and 7,075,329. Other patents
pending.
FUNCTIONAL BLOCK DIAGRAM
VDD1
VDD2
AD7401
VIN+
T/H
Σ-Δ ADC
UPDATE
ENCODE
BUF
REF
WATCHDOG
CONTROL LOGIC
WATCHDOG
DECODE
DECODE
MDAT
UPDATE
MCLKIN
ENCODE
GND2
GND1
05851-001
VIN–
Figure 1.
Rev. A
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 registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2006 Analog Devices, Inc. All rights reserved.
AD7401
TABLE OF CONTENTS
Features .............................................................................................. 1
Typical Performance Characteristics ..............................................9
Applications....................................................................................... 1
Terminology .................................................................................... 12
General Description ......................................................................... 1
Theory of Operation ...................................................................... 13
Functional Block Diagram .............................................................. 1
Circuit Information.................................................................... 13
Revision History ............................................................................... 2
Analog Input ............................................................................... 13
Specifications..................................................................................... 3
Differential Inputs ...................................................................... 14
Timing Specifications .................................................................. 4
Digital Filter ................................................................................ 15
Insulation and Safety Related Specifications ............................ 5
Application Information................................................................ 17
Regulatory Information............................................................... 5
Grounding and Layout .............................................................. 17
DIN EN 60747-5-2 (VDE 0884 Part 2) Insulation
Characteristics .............................................................................. 6
Evaluating the AD7401 Performance ...................................... 17
Absolute Maximum Ratings............................................................ 7
Outline Dimensions ....................................................................... 18
ESD Caution.................................................................................. 7
Ordering Guide .......................................................................... 18
Insulation Lifetime ..................................................................... 17
Pin Configuration and Function Descriptions............................. 8
REVISION HISTORY
12/06—Rev. 0 to Rev. A
Changes to Features and General Description ............................. 1
Changes to Table 1............................................................................ 3
Changes to Table 2............................................................................ 4
Changes to Table 6............................................................................ 7
Changes to Table 8............................................................................ 8
Changes to Circuit Information Section ..................................... 13
Changes to Figure 27...................................................................... 15
1/06—Revision 0: Initial Version
Rev. A | Page 2 of 20
AD7401
SPECIFICATIONS
VDD1 = 4.5 V to 5.25 V, VDD2 = 3 V to 5.5 V, VIN+ = −200 mV to +200 mV, and VIN− = 0 V (single-ended); TA = TMIN to TMAX,
fMCLK = 16 MHz maximum, tested with Sinc3 filter, 256 decimation rate, as defined by Verilog code, unless otherwise noted.
Table 1.
Parameter
STATIC PERFORMANCE
Resolution
Integral Nonlinearity 3
Differential Nonlinearity3
Offset Error3
Offset Drift vs. Temperature3
Offset Drift vs. VDD13
Gain Error3
Gain Error Drift vs. Temperature3
Gain Error Drift vs. VDD13
ANALOG INPUT
Input Voltage Range
Dynamic Input Current
DC Leakage Current
Input Capacitance
DYNAMIC SPECIFICATIONS
Signal-to-(Noise + Distortion) Ratio (SINAD)3
Signal-to-Noise Ratio (SNR)3
Total Harmonic Distortion (THD)3
Peak Harmonic or Spurious Noise (SFDR)3
Effective Number of Bits (ENOB)3
Isolation Transient Immunity3
LOGIC INPUTS
Input High Voltage, VIH
Input Low Voltage, VIL
Input Current, IIN
Input Capacitance, CIN 5
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
Y Version 1, 2
Unit
Test Conditions/Comments
16
±15
±25
±55
±0.9
Bits min
LSB max
LSB max
LSB max
LSB max
±0.6
±50
3.5
1
120
±1.6
±2
±1
23
110
mV max
μV typ
μV/°C max
μV/°C typ
μV/V typ
mV max
mV max
mV typ
μV/°C typ
μV/V typ
Filter output truncated to 16 bits
−40°C to +85°C; ±2 LSB typical; fMCLK = 20 MHz maximum 4
>85°C to 105°C
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
Guaranteed no missed codes to 16 bits;
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
TA = 25°C
−40°C to +105°C
±200
±9
±0.5
10
mV min/mV max
μA max
μA max
pF typ
70
68
65
65
81
80
80
−92
−92
11.5
25
30
dB min
dB min
dB min
dB min
dB typ
dB min
dB min
dB typ
dB typ
Bits
kV/μs min
kV/μs typ
0.8 × VDD2
0.2 × VDD2
±0.5
10
V min
V max
μA max
pF max
VDD2 − 0.1
0.4
V min
V max
Rev. A | Page 3 of 20
−40°C to +85°C
>85°C to 105°C
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
−40°C to +105°C
For specified performance; full range ±320 mV
VIN+ = 400 mV, VIN− = 0 V
VIN+ = 5 kHz, 400 mV p-p sine
−40°C to +85°C; fMCLK = 9 MHz to 20 MHz4
−40°C to +85°C; fMCLK = 5 MHz to <9 MHz
>85°C to 105°C
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
−40°C to +105°C; 82 dB typ
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
fMCLK = 20 MHz maximum4; VIN+ = −250 mV to +250 mV
IO = −200 μA
IO = +200 μA
AD7401
Parameter
POWER REQUIREMENTS
VDD1
VDD2
IDD1 6
IDD2 7
Y Version 1, 2
Unit
Test Conditions/Comments
4.5/5.25
3/5.5
12
8
4
V min/V max
V min/V max
mA max
mA max
mA max
VDD1 = 5.25 V
VDD2 = 5.5 V
VDD2 = 3.3 V
1
Temperature range is −40°C to +85°C.
All voltages are relative to their respective ground.
See the Terminology section.
4
For fMCLK > 16 MHz to 20 MHz, mark space ratio is 48/52 to 52/48, VDD1 = VDD2 = 5 V ± 5% and TA = −40°C to +85°C.
5
Sample tested during initial release to ensure compliance.
6
See Figure 15.
7
See Figure 17.
2
3
TIMING SPECIFICATIONS 1
VDD1 = 4.5 V to 5.25 V, VDD2 = 3 V to 5.5 V, TA = TMAX to TMIN, unless otherwise noted.
Table 2.
Parameter
fMCLKIN 2,3
t1 4
t24
t3
t4
Limit at TMIN, TMAX
20
5
25
15
0.4 × tMCLKIN
0.4 × tMCLKIN
Unit
MHz max
MHz min
ns max
ns min
ns min
ns min
Description
Master clock input frequency
Master clock input frequency
Data access time after MCLK rising edge
Data hold time after MCLK rising edge
Master clock low time
Master clock high time
1
Sample tested during initial release to ensure compliance.
Mark space ratio for clock input is 40/60 to 60/40 for fMCLKIN to 16 MHz and 48/52 to 52/48 for fMCLKIN > 16 MHz to 20 MHz.
3
VDD1 = VDD2 = 5 V ± 5% for fMCLKIN > 16 MHz to 20 MHz.
4
Measured with the load circuit of Figure 2 and defined as the time required for the output to cross 0.8 V or 2.0 V.
2
200µA
1.6V
CL
25pF
200µA
05851-002
TO OUTPUT
PIN
IOL
IOH
Figure 2. Load Circuit for Digital Output Timing Specifications
t4
t1
t2
MDAT
Figure 3. Data Timing
Rev. A | Page 4 of 20
t3
05851-003
MCLKIN
AD7401
INSULATION AND SAFETY-RELATED SPECIFICATIONS
Table 3.
Parameter
Input-to-Output Withstand Momentary Withstand Voltage
Minimum External Air Gap (Clearance)
Symbol
VISO
L(I01)
Value
3750 min
7.46 min
Unit
V
mm
Minimum External Tracking (Creepage)
L(I02)
8.1 min
mm
Minimum Internal Gap (Internal Clearance)
Tracking Resistance (Comparative Tracking Index)
Isolation Group
CTI
0.017 min
>175
IIIa
mm
V
Conditions
1-minute duration
Measured from input terminals to output
terminals, shortest distance through air
Measured from input terminals to output
terminals, shortest distance path along body
Insulation distance through insulation
DIN IEC 112/VDE 0303 Part 1
Material Group (DIN VDE 0110, 1/89, Table I)
REGULATORY INFORMATION
Table 4.
UL 1
Recognized under 1577
Component Recognition Program1
3750 V rms Isolation Voltage
File E214100
1
2
CSA
Approved under CSA Component
Acceptance Notice #5A
Reinforced insulation per
CSA 60950-1-03 and IEC 60950-1,
630 V rms maximum working
voltage
File 205078
VDE 2
Certified according to DIN EN 60747-5-2
(VDE 0884 Part 2): 2003-012
Basic insulation, 891 V peak
Complies with DIN EN 60747-5-2 (VDE 0884 Part 2): 2003-01,
DIN EN 60950 (VDE 0805): 2001-12; EN 60950: 2000
Reinforced insulation, 891 V peak
File 2471900-4880-0001
In accordance with UL 1577, each AD7401 is proof tested by applying an insulation test voltage ≥ 4500 V rms for 1 second (current leakage detection limit = 7.5 μA).
In accordance with DIN EN 60747-5-2, each AD7401 is proof tested by applying an insulation test voltage ≥ 1671 V peak for 1 second (partial discharge detection
limit = 5 pC).
Rev. A | Page 5 of 20
AD7401
DIN EN 60747-5-2 (VDE 0884 PART 2) INSULATION CHARACTERISTICS
This isolator is suitable for basic electrical isolation only within the safety limit data. Maintenance of the safety data is ensured by means
of protective circuits.
Table 5.
Description
INSTALLATION CLASSIFICATION PER DIN VDE 0110
For Rated Mains Voltage ≤ 300 V rms
For Rated Mains Voltage ≤ 450 V rms
For Rated Mains Voltage ≤ 600 V rms
CLIMATIC CLASSIFICATION
POLLUTION DEGREE (DIN VDE 0110, TABLE I)
MAXIMUM WORKING INSULATION VOLTAGE
INPUT-TO-OUTPUT TEST VOLTAGE, METHOD B1
VIORM × 1.875 = VPR, 100% Production Test, tm = 1 sec, Partial Discharge < 5 pC
INPUT-TO-OUTPUT TEST VOLTAGE, METHOD A
After Environmental Test Subgroup 1
VIORM × 1.6 = VPR, tm = 60 sec, Partial Discharge < 5 pC
After Input and/or Safety Test Subgroup 2/3
VIORM × 1.2 = VPR, tm = 60 sec, Partial Discharge < 5 pC
HIGHEST ALLOWABLE OVERVOLTAGE (TRANSIENT OVERVOLTAGE, tTR = 10 sec)
SAFETY-LIMITING VALUES (MAXIMUM VALUE ALLOWED IN THE EVENT OF A FAILURE, ALSO SEE Figure 4)
Case Temperature
Side 1 Current
Side 2 Current
INSULATION RESISTANCE AT TS, VIO = 500 V
250
SIDE #2
200
150
SIDE #1
100
50
50
100
150
CASE TEMPERATURE (°C)
200
05851-004
SAFETY-LIMITING CURRENT (mA)
300
0
Figure 4. Thermal Derating Curve, Dependence of Safety-Limiting Values
with Case Temperature per DIN EN 60747-5-2
Rev. A | Page 6 of 20
Characteristic
Unit
VIORM
I–IV
I–II
I–II
40/105/21
2
891
V peak
1671
V peak
1426
V peak
1069
V peak
VTR
6000
V peak
TS
IS1
IS2
RS
150
265
335
>109
°C
mA
mA
Ω
VPR
VPR
350
0
Symbol
AD7401
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted. All voltages are relative to
their respective ground.
Table 6.
Parameter
VDD1 to GND1
VDD2 to GND2
Analog Input Voltage to GND1
Digital Input Voltage to GND2
Output Voltage to GND2
Input Current to Any Pin Except
Supplies 1
Operating Temperature Range
Storage Temperature Range
Junction Temperature
SOIC Package
θJA Thermal Impedance
θJC Thermal Impedance
Resistance (Input to Output), RI-O
Capacitance (Input to Output), CI-O 3
Pb-Free Temperature , Soldering
Reflow
ESD
Rating
−0.3 V to +6.5 V
−0.3 V to +6.5 V
−0.3 V to VDD1 + 0.3 V
−0.3 V to VDD1 + 0.5 V
−0.3 V to VDD2 + 0.3 V
±10 mA
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.
Table 7. Maximum Continuous Working Voltage1
−40°C to +105°C
−65°C to +150°C
150°C
113°C (UL) 2
89.2°C/W
55.6°C/W
1012 Ω
1.7 pF typ
260 (+0)°C
1.5 kV
Parameter
AC Voltage,
Bipolar Waveform
AC Voltage,
Unipolar Waveform
DC Voltage
1
Max
565
Unit
VPK
Constraint
50-year minimum lifetime
891
VPK
891
V
Maximum CSA/VDE
approved working voltage
Maximum CSA/VDE
approved working voltage
Refers to continuous voltage magnitude imposed across the isolation
barrier. See the Insulation Lifetime section for more details.
ESD CAUTION
1
Transient currents of up to 100 mA do not cause SCR to latch up.
UL certification applies up to 113°C only.
3
f = 1 MHz.
2
Rev. A | Page 7 of 20
AD7401
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
16
GND2
VIN+ 2
15
NC
VIN–
3
14
VDD2
NC
4
NC
5
NC
6
11
MDAT
VDD1
7
10
NC
GND1 8
9
GND2
AD7401
MCLKIN
TOP VIEW
(Not to Scale) 12 NC
13
NC = NO CONNECT
05851-005
VDD1
Figure 5. Pin Configuration
Table 8. Pin Function Descriptions
Pin No.
1, 7
2
3
4 to 6, 10,
12, 15
8
9, 16
11
Mnemonic
VDD1
VIN+
VIN−
NC
Description
Supply Voltage, 4.5 V to 5.25 V. This is the supply voltage for the isolated side of the AD7401 and is relative to GND1.
Positive Analog Input. Specified range of ±200 mV.
Negative Analog Input. Normally connected to GND1.
No Connect.
GND1
GND2
MDAT
13
14
MCLKIN
VDD2
Ground 1. This is the ground reference point for all circuitry on the isolated side.
Ground 2. This is the ground reference point for all circuitry on the nonisolated side.
Serial Data Output. The single bit modulator output is supplied to this pin as a serial data stream.
The bits are clocked out on the rising edge of the MCLKIN input and valid on the following MCLKIN rising edge.
Master Clock Logic Input. 20 MHz maximum. The bit stream from the modulator is valid on the rising edge of MCLKIN.
Supply Voltage. 3 V to 5.5 V. This is the supply voltage for the nonisolated side and is relative to GND2.
Rev. A | Page 8 of 20
AD7401
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, using 25 kHz brick-wall filter, unless otherwise noted.
100
–90
90
VDD1 = VDD2 = 5V
–85
80
MCLKIN = 10MHz
MCLKIN = 16MHz
–80
60
50
MCLKIN = 5MHz
SINAD (dB)
PSRR (dB)
70
MCLKIN = 10MHz
40
MCLKIN = 16MHz
–75
–70
–65
30
–60
600
700
800
900
1000
SUPPLY RIPPLE FREQUENCY (kHz)
–50
0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 0.31 0.32 0.33
± INPUT AMPLITUDE (V)
Figure 6. PSRR vs. Supply Ripple Frequency Without Supply Decoupling
–90
Figure 9. SINAD vs. VIN
0.4
=V
=5V
VDD1 V= V
DD2 = 5V
DD1
DD2
0.3
–85
MCLKIN = 16MHz
0.2
0.1
–75
DNL ERROR
MCLKIN = 10MHz
–70
–65
MCLKIN = 5MHz
0
–0.1
–0.2
–60
1k
2k
3k
4k
5k
6k
7k
8k
9k
10k
INPUT FREQUENCY (Hz)
05851-027
0
–0.4 V + = –200mV TO +200mV
IN
VIN– = 0V
–0.5
0
10k
20k
30k
–20
60k
0.8
VIN+ = –200mV TO +200mV
VIN– = 0V
0.6
INL ERROR (LSB)
–40
–60
–80
–100
–120
–140
0.4
0.2
0
–0.2
–160
–180
0
5
10
15
20
FREQUENCY (kHz)
25
30
05851-042
(dB)
60k
Figure 10. Typical DNL (±200 mV Range)
4096 POINT FFT
fIN = 5kHz
SINAD = 81.984dB
THD = –96.311dB
DECIMATION BY 256
0
50k
CODE
Figure 7. SINAD vs. Analog Input Frequency
20
40k
05851-043
–0.3
–55
05851-044
SINAD (dB)
–80
–50
05851-028
–55
05851-006
20 200mV p-p SINE WAVE ON V
DD1
NO DECOUPLING
10 V
=
V
=
5V
DD1
DD2
1MHz CUTOFF FILTER
0
0
100 200 300 400 500
–0.4
0
10k
20k
30k
40k
50k
CODE
Figure 11. Typical INL (±200 mV Range)
Figure 8. Typical FFT (±200 mV Range)
Rev. A | Page 9 of 20
AD7401
0.0105
250
150
0.0100
0.0095
IDD1 (A)
0
–50
–100
–150
–200
VDD1 = VDD2 = 5.25V
MCLKIN = 16MHz
5
15 25 35 45 55 65 75 85 95 105
0.0085
0.0080
0.0065
MCLKIN = 16MHz
TA = +105°C
MCLKIN = 10MHz
TA = +105°C
MCLKIN = 10MHz
TA = +85°C
MCLKIN = 5MHz
TA = +85°C
MCLKIN = 5MHz
TA = –40°C
MCLKIN = 5MHz
TA = +105°C
0.0060
–0.33 –0.28 –0.23 –0.18 –0.13 –0.08 –0.03 0.03 0.08 0.13 0.18 0.23 0.28 0.33
VIN DC INPUT VOLTAGE (V)
Figure 15. IDD1 vs. VIN at Various Temperatures
Figure 12. Offset Drift vs. Temperature for Various Supply Voltages
0.0070
200.4
VDD1 = VDD2 = 4.5V
MCLKIN = 16MHz
VDD1 = VDD2 = 4.5V
MCLKIN = 10MHz
0.0065
200.3
VDD1 = VDD2 = 4.5V
MCLKIN = 5MHz
VDD1 = VDD2 = 5V
MCLKIN = 5MHz
0.0060
200.2
VDD1 = VDD2 = 5V
MCLKIN = 16MHz
VDD1 = VDD2 = 5.25V
MCLKIN = 10MHz
0.0055
200.1
VDD1 = VDD2 = 5.25V
MCLKIN = 16MHz
VDD1 = VDD2 = 5.25V
MCLKIN = 5MHz
0.0050
200.0
VDD1 = VDD2 = 5V
MCLKIN = 10MHz
IDD2 (A)
GAIN (mV)
MCLKIN = 16MHz
TA = +85°C
MCLKIN = 10MHz
TA = –40°C
0.0070
VDD1 = VDD2 = 5.25V
MCLKIN = 5MHz
TEMPERATURE (°C)
200.5
MCLKIN = 16MHz
TA = –40°C
0.0075
VDD1 = VDD2 = 5.25V
MCLKIN = 10MHz
VDD1 = VDD2 = 5V
MCLKIN = 10MHz
–250
–45 –35 –25 –15 –5
VDD1 = VDD2 = 5V
0.0090
50
05851-029
OFFSET (µV)
100
VDD1 = VDD2 = 4.5V
MCLKIN = 10MHz
VDD1 = VDD2 = 5V
MCLKIN = 5MHz
05851-034
200
VDD1 = VDD2 = 4.5V
MCLKIN = 16MHz
VDD1 = VDD2 = 4.5V
MCLKIN = 5MHz
VDD1 = VDD2 = 5V
MCLKIN = 16MHz
VDD1 = VDD2 = 5V
TA = 25°C
MCLKIN = 16MHz
MCLKIN = 10MHz
0.0045
199.9
0.0040
199.8
0.0035
199.7
0.0030
199.6
0.0025
MCLKIN = 5MHz
TEMPERATURE (°C)
0.0020
–0.225
–0.125
–0.025
0.075
0.175
0.275
–0.325
–0.275
–0.175
–0.075
0.025
0.125
0.225
0.325
VIN DC INPUT VOLTAGE (V)
Figure 13. Gain Error Drift vs. Temperature for Various Supply Voltages
Figure 16. IDD2 vs. VIN DC Input Voltage
0.0070
0.0105
VDD1 = VDD2 = 5V
TA = 25°C
0.0100
0.0095
0.0060
0.0055
MCLKIN = 16MHz
0.0050
IDD2 (A)
MCLKIN = 10MHz
TA = –40°C
MCLKIN = 16MHz
TA = +85°C
MCLKIN = 10MHz
TA = +105°C
MCLKIN = 5MHz
TA = –40°C
MCLKIN = 10MHz
TA = +85°C
0.0030
MCLKIN = 5MHz
0.0025
0.0070
0.0020
0.0065
VIN DC INPUT VOLTAGE (V)
MCLKIN = 16MHz
TA = +105°C
0.0045
0.0035
–0.33 –0.28 –0.23 –0.18 –0.13 –0.08 –0.03 0.03 0.08 0.13 0.18 0.23 0.28 0.33
MCLKIN = 16MHz
TA = –40°C
0.0040
05851-033
IDD1 (A)
MCLKIN = 10MHz
0.0080
0.0075
VDD1 = VDD2 = 5V
0.0065
0.0090
0.0085
05851-037
15 25 35 45 55 65 75 85 95 105
Figure 14. IDD1 vs. VIN DC Input Voltage
Rev. A | Page 10 of 20
MCLKIN = 5MHz
TA = +85°C
MCLKIN = 5MHz
TA = +105°C
–0.225
–0.125
–0.025
0.075
0.175
0.275
–0.325
–0.275
–0.175
–0.075
0.025
0.125
0.225
0.325
VIN DC INPUT VOLTAGE (V)
Figure 17. IDD2 vs. VIN at Various Temperatures
05851-038
5
05851-036
199.5
–45 –35 –25 –15 –5
AD7401
8
1.0
VDD1 = VDD2 = 4.5V TO 5.25V
MCLKIN = 16MHz
6
0.8
MCLKIN = 10MHz
4
NOISE (mV)
IIN (µA)
2
MCLKIN = 5MHz
0
VDD1 = VDD2 = 5V
50kHz BRICK WALL FILTER
–2
0.6
0.4
MCLKIN = 5MHz
–4
0.2
0
=V =5V
VDD1 = VVDD2
= 5V
DD1
DD2
–20
–40
MCLKIN = 5MHz
–60
MCLKIN = 10MHz
–80
–120
0.1
1
10
100
RIPPLE FREQUENCY (kHz)
1000
05851-031
MCLKIN = 16MHz
–100
Figure 19. CMRR vs. Common-Mode Ripple Frequency
Rev. A | Page 11 of 20
0.30
0.25
0.20
0.15
0.05
0
–0.05
–0.10
–0.15
–0.20
–0.30
–0.25
VIN DC INPUT (V)
Figure 20. RMS Noise Voltage vs. VIN DC Input
Figure 18. IIN vs. VIN− DC Input
05851-032
VIN– DC INPUT (V)
0
05851-030
0.30
0.35
0.25
0.20
0.15
0.10
0
0.05
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
–0.35
–8
CMRR (dB)
MCLKIN = 16MHz
MCLKIN = 10MHz
0.10
–6
AD7401
TERMINOLOGY
Differential Nonlinearity
Differential nonlinearity is the difference between the measured
and the ideal 1 LSB change between any two adjacent codes
in the ADC.
Integral Nonlinearity
Integral nonlinearity is the maximum deviation from a straight
line passing through the endpoints of the ADC transfer function.
The endpoints of the transfer function are specified negative
full-scale, −200 mV (VIN+ − VIN−), Code 12,288 for the 16-bit
level, and specified positive full-scale, +200 mV (VIN+ − VIN−),
Code 53,248 for the 16-bit level.
Offset Error
Offset is the deviation of the midscale code (Code 32,768 for
the 16-bit level) from the ideal VIN+ − VIN− (that is, 0 V).
Gain Error
This includes both positive full-scale gain error and negative
full-scale gain error. Positive full-scale gain error is the deviation of
the specified positive full-scale code (53,248 for the 16-bit level)
from the ideal VIN+ − VIN− (+200 mV) after the offset error is
adjusted out. Negative full-scale gain error is the deviation of
the specified negative full-scale code (12,288 for the 16-bit
level) from the ideal VIN+ − VIN− (−200 mV) after the offset
error is adjusted out. Gain error includes reference error.
Signal-to-(Noise + Distortion) Ratio
This ratio is the measured ratio of signal-to-(noise + distortion)
at the output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals up
to half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on the number of quantization levels in the digitization
process; the more levels, the smaller the quantization noise. The
theoretical signal-to-(noise + distortion) ratio for an ideal N-bit
converter with a sine wave input is given by
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
Therefore, for a 12-bit converter, this is 74 dB.
Effective Number of Bits (ENOB)
The ENOB is defined by
ENOB = (SINAD − 1.76)/6.02
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7401, it is defined as
THD (dB ) = 20 log
V 2 2 + V 3 2 + V 4 2 + V5 2 + V 6 2
V1
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5, and V6 are the rms amplitudes of the second
through the sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to fS/2, excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
ADCs where the harmonics are buried in the noise floor, it is a
noise peak.
Common-Mode Rejection Ratio (CMRR)
CMRR is defined as the ratio of the power in the ADC output at
±200 mV frequency, f, to the power of a 200 mV p-p sine wave
applied to the common-mode voltage of VIN+ and VIN− of
frequency fS as
CMRR (dB) = 10log(Pf/PfS)
where:
Pf is the power at frequency f in the ADC output.
PfS is the power at frequency fS in the ADC output.
Power Supply Rejection Ratio (PSRR)
Variations in power supply affect the full-scale transition but
not the converter’s linearity. PSRR is the maximum change in
the specified full-scale (±200 mV) transition point due to a
change in power supply voltage from the nominal value (see
Figure 6).
Isolation Transient Immunity
The isolation transient immunity specifies the rate of rise/fall of
a transient pulse applied across the isolation boundary beyond
which clock or data is corrupted. (It was tested using a transient
pulse frequency of 100 kHz.)
Rev. A | Page 12 of 20
AD7401
THEORY OF OPERATION
A differential input of 320 mV results in a stream of ideally all
1s. This is the absolute full-scale range of the AD7401, while
200 mV is the specified full-scale range, as shown in Table 9.
CIRCUIT INFORMATION
The AD7401 isolated Σ-Δ modulator converts an analog input
signal into a high speed (20 MHz maximum), single-bit data
stream; the time average of the modulator’s single-bit data is
directly proportional to the input signal. Figure 23 shows a
typical application circuit where the AD7401 is used to provide
isolation between the analog input, a current sensing resistor,
and the digital output, which is then processed by a digital filter
to provide an N-bit word.
Table 9. Analog Input Range
Analog Input
Full-Scale Range
Positive Full-Scale
Positive Specified Input Range
Zero
Negative Specified Input Range
Negative Full-Scale
ANALOG INPUT
The differential analog input of the AD7401 is implemented
with a switched capacitor circuit. This circuit implements a
second-order modulator stage that digitizes the input signal
into a 1-bit output stream. The sample clock (MCLKIN)
provides the clock signal for the conversion process as well as
the output data-framing clock. This clock source is external
on the AD7401. The analog input signal is continuously
sampled by the modulator and compared to an internal
voltage reference. A digital stream that accurately represents
the analog input over time appears at the output of the
converter (see Figure 21).
Voltage Input
+640 mV
+320 mV
+200 mV
0 mV
−200 mV
−320 mV
To reconstruct the original information, this output needs to be
digitally filtered and decimated. A Sinc3 filter is recommended
because this is one order higher than that of the AD7401
modulator. If a 256 decimation rate is used, the resulting
16-bit word rate is 62.5 kHz, assuming a 16 MHz external clock
frequency. Figure 22 shows the transfer function of the AD7401
relative to the 16-bit output.
65535
MODULATOR OUTPUT
53248
05851-020
12288
Figure 21. Analog Input vs. Modulator Output
A differential signal of 0 V results (ideally) in a stream of 1s and
0s at the MDAT output pin. This output is high 50% of the time
and low 50% of the time. A differential input of 200 mV
produces a stream of 1s and 0s that are high 81.25% of the time.
A differential input of −200 mV produces a stream of 1s and 0s
that are high 18.75% of the time.
ISOLATED
5V
+
INPUT
CURRENT
0
–320mV
–200mV
+200mV +320mV
ANALOG INPUT
Figure 22. Filtered and Decimated 16-Bit Transfer Characteristic
NONISOLATED
5V/3V
VDD1
AD7401
VIN+
Σ-Δ
MOD/
ENCODER
VDD2
VDD
SINC3 FILTER
DECODER
VIN–
MDAT
MDAT
MCLKIN
MCLK
CS
SCLK
SDAT
RSHUNT
GND1
ENCODER
GND2
GND
05851-019
DECODER
05851-021
–FS ANALOG INPUT
ANALOG INPUT
SPECIFIED RANGE
ADC CODE
+FS ANALOG INPUT
Figure 23. Typical Application Circuit
Rev. A | Page 13 of 20
AD7401
DIFFERENTIAL INPUTS
The analog input to the modulator is a switched capacitor
design. The analog signal is converted into charge by highly
linear sampling capacitors. A simplified equivalent circuit
diagram of the analog input is shown in Figure 24. A signal
source driving the analog input must be able to provide the
charge onto the sampling capacitors every half MCLKOUT cycle
and settle to the required accuracy within the next half cycle.
φA
MCLKIN
2pF
φA
2pF
The recommended circuit configuration for driving
the differential inputs to achieve best performance is shown in
Figure 25. A capacitor between the two input pins sources or
sinks charge to allow most of the charge that is needed by one
input to be effectively supplied by the other input. The series
resistor again isolates any op amp from the current spikes
created during the sampling process. Recommended values for
the resistors and capacitor are 22 Ω and 47 pF, respectively.
φB
φA φB φA φB
Figure 24. Analog Input Equivalent Circuit
Because the AD7401 samples the differential voltage across its
analog inputs, low noise performance is attained with an input
circuit that provides low common-mode noise at each input.
The amplifiers used to drive the analog inputs play a critical role
in attaining the high performance available from the AD7401.
Rev. A | Page 14 of 20
VIN+
R
C
VIN–
R
AD7401
05851-023
VIN–
1kΩ
φB
05851-022
VIN+
1kΩ
When a capacitive load is switched onto the output of an op
amp, the amplitude momentarily drops. The op amp tries to
correct the situation and, in the process, hits its slew rate limit.
This nonlinear response, which can cause excessive ringing, can
lead to distortion. To remedy the situation, a low-pass RC filter
can be connected between the amplifier and the input to the
AD7401. The external capacitor at each input aids in supplying
the current spikes created during the sampling process, and the
resistor isolates the op amp from the transient nature of the load.
Figure 25. Differential Input RC Network
AD7401
DIGITAL FILTER
A Sinc3 filter is recommended for use with the AD7401. This
filter can be implemented on an FPGA or possibly a DSP. The
following Verilog code provides an example of a Sinc3 filter
implementation on a Xylinx® Spartan-II 2.5 V FPGA. This code
can possibly be compiled for another FPGA, such as an Altera®
device. Note that the data is read on the negative clock edge in
this case; although, it can be read on the positive edge, if
preferred. Figure 29 shows the effect of using different
decimation rates with various filter types.
/*`Data is read on negative clk edge*/
module DEC256SINC24B(mdata1, mclk1, reset,
DATA);
/*used to clk filter*/
/*used to reset filter*/
/*ip data to be
output [15:0] DATA;
/*filtered op*/
ip_data1;
reg [23:0]
acc1;
reg [23:0]
acc2;
reg [23:0]
acc3;
reg [23:0]
acc3_d1;
reg [23:0]
acc3_d2;
reg [23:0]
diff1;
reg [23:0]
diff2;
reg [23:0]
diff3;
reg [23:0]
diff1_d;
reg [23:0]
diff2_d;
reg [15:0]
DATA;
reg [7:0]
word_count;
/*DECIMATION STAGE (MCLKOUT/ WORD_CLK)
*/
always @ (negedge mclk1 or posedge reset)
if (reset)
word_count <= 0;
else
word_count <= word_count + 1;
integer location;
integer info_file;
reg [23:0]
always @ (posedge mclk1 or posedge reset)
if (reset)
begin
/*initialize acc registers on reset*/
acc1 <= 0;
acc2 <= 0;
acc3 <= 0;
end
else
begin
/*perform accumulation process*/
acc1 <= acc1 + ip_data1;
acc2 <= acc2 + acc1;
acc3 <= acc3 + acc2;
end
always @ (word_count)
word_clk <= word_count[7];
/*DIFFERENTIATOR ( including decimation
stage)
Perform the differentiation stage (FIR) at a
lower speed.
Z–1
+
ACC2+
Z
+
Z
+
ACC3+
05851-024
MCLKIN
Figure 26. Accumulator
Rev. A | Page 15 of 20
+
–
Z–1
Z = one sample delay
WORD_CLK = output word rate
*/
/* change from a 0
ACC1+
DIFF2
Figure 27. Differentiator
/*ACCUMULATOR (INTEGRATOR)
Perform the accumulation (IIR) at the speed
of the modulator.
Z
+
WORD_CLK
/*Perform the Sinc ACTION*/
IP_DATA1
DIFF1
–
reg word_clk;
reg init;
always @ (mdata1)
if(mdata1==0)
ip_data1 <= 0;
to a -1 for 2's comp */
else
ip_data1 <= 1;
+
ACC3
DIFF3
–
Z–1
05851-025
input mclk1;
input reset;
input mdata1;
filtered*/
Z = one sample delay
MCLKOUT = modulators conversion bit rate
*/
AD7401
always @ (posedge word_clk or posedge reset)
if(reset)
begin
acc3_d2 <= 0;
diff1_d <= 0;
diff2_d <= 0;
diff1 <= 0;
diff2 <= 0;
diff3 <= 0;
end
DATA[9]
DATA[8]
DATA[7]
DATA[6]
DATA[5]
DATA[4]
DATA[3]
DATA[2]
DATA[1]
DATA[0]
<=
<=
<=
<=
<=
<=
<=
<=
<=
<=
diff3[17];
diff3[16];
diff3[15];
diff3[14];
diff3[13];
diff3[12];
diff3[11];
diff3[10];
diff3[9];
diff3[8];
end
else
endmodule
begin
diff1 <= acc3 - acc3_d2;
diff2 <= diff1 - diff1_d;
diff3 <= diff2 - diff2_d;
acc3_d2 <= acc3;
diff1_d <= diff1;
diff2_d <= diff2;
end
90
70
SNR (dB)
diff3[23];
diff3[22];
diff3[21];
diff3[20];
diff3[19];
diff3[18];
1
10
100
1k
DECIMATION RATE
05851-035
0
WORD_CLK = output word rate
*/
<=
<=
<=
<=
<=
<=
SINC1
10
Figure 28. Clocking Sinc Output into an Output Register
DATA[15]
DATA[14]
DATA[13]
DATA[12]
DATA[11]
DATA[10]
40
20
05851-026
DATA
50
30
WORD_CLK
always @ (posedge word_clk)
begin
SINC2
60
/* Clock the Sinc output into an output
register
DIFF3
SINC3
80
Figure 29. SNR vs. Decimation Rate for Different Filter Types
Figure 29 shows a plot of SNR performance vs. decimation rate
with different filter types. Note that for a given bandwidth
requirement, a higher MCLKIN frequency can allow for higher
decimation rates to be used resulting in higher SNR
performance.
Rev. A | Page 16 of 20
AD7401
APPLICATION INFORMATION
GROUNDING AND LAYOUT
Supply decoupling with a value of 100 nF is strongly recommended on both VDD1 and VDD2. Decoupling on one or both
VDD1 pins does not affect performance significantly. In applications
involving high common-mode transients, care should be taken
to ensure that board coupling across the isolation barrier is
minimized. Furthermore, the board layout should be designed
so that any coupling that occurs equally affects all pins on a
given component side. Failure to ensure this could cause voltage
differentials between pins to exceed the device’s absolute
maximum ratings, thereby leading to latch-up or permanent
damage. Any decoupling used should be placed as close to the
supply pins as possible.
Series resistance in the analog inputs should be minimized to
avoid any distortion effects, especially at high temperatures. If
possible, equalize the source impedance on each analog input to
minimize offset. Beware of mismatch and thermocouple effects
on the analog input PCB tracks to reduce offset drift.
These tests subjected populations of devices to continuous
cross-isolation voltages. To accelerate the occurrence of failures,
the selected test voltages were values exceeding those of normal
use. The time to failure values of these units were recorded and
used to calculate acceleration factors. These factors were then
used to calculate the time to failure under normal operating
conditions. The values shown in Table 7 are the lesser of the
following two values:
•
The value that ensures at least a 50-year lifetime of
continuous use.
•
The maximum CSA/VDE approved working voltage.
It should also be noted that the lifetime of the AD7401 varies
according to the waveform type imposed across the isolation
barrier. The iCoupler insulation structure is stressed differently
depending on whether the waveform is bipolar ac, unipolar ac,
or dc. Figure 30, Figure 31, and Figure 32 illustrate the different
isolation voltage waveforms.
RATED PEAK VOLTAGE
0V
Figure 30. Bipolar AC Waveform
RATED PEAK VOLTAGE
05851-040
A simple standalone AD7401 evaluation board is available with
split ground planes and a board split beneath the AD7401
package to ensure isolation. This board allows access to each
pin on the device for evaluation purposes. External supplies and
all other circuitry (such as a digital filter) must be provided by
the user.
05851-039
EVALUATING THE AD7401 PERFORMANCE
0V
INSULATION LIFETIME
Figure 31. Unipolar AC Waveform
Rev. A | Page 17 of 20
RATED PEAK VOLTAGE
05851-041
All insulation structures, subjected to sufficient time and/or
voltage, are vulnerable to breakdown. In addition to the testing
performed by the regulatory agencies, Analog Devices has
carried out an extensive set of evaluations to determine the
lifetime of the insulation structure within the AD7401.
0V
Figure 32. DC Waveform
AD7401
OUTLINE DIMENSIONS
10.50 (0.4134)
10.10 (0.3976)
9
16
7.60 (0.2992)
7.40 (0.2913)
8
1.27 (0.0500)
BSC
0.75 (0.0295)
0.25 (0.0098)
2.65 (0.1043)
2.35 (0.0925)
0.30 (0.0118)
0.10 (0.0039)
COPLANARITY
0.10
10.65 (0.4193)
10.00 (0.3937)
0.51 (0.0201)
0.31 (0.0122)
SEATING
PLANE
45°
8°
0°
0.33 (0.0130)
0.20 (0.0079)
1.27 (0.0500)
0.40 (0.0157)
COMPLIANT TO JEDEC STANDARDS MS-013- AA
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
112906-B
1
Figure 33. 16-Lead Standard Small Outline Package [SOIC_W]
Wide Body
(RW-16)
Dimensions shown in millimeters and (inches)
ORDERING GUIDE
Model
AD7401YRWZ 1
AD7401YRWZ-REEL1
AD7401YRWZ-REEL71
EVAL-AD7401EB
1
Temperature Range
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
Package Description
16-Lead Standard Small Outline Package (SOIC_W)
16-Lead Standard Small Outline Package (SOIC_W)
16-Lead Standard Small Outline Package (SOIC_W)
Standalone Evaluation Board
Z = Pb-free part.
Rev. A | Page 18 of 20
Package Option
RW-16
RW-16
RW-16
AD7401
NOTES
Rev. A | Page 19 of 20
AD7401
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05851-0-12/06(A)
Rev. A | Page 20 of 20
Similar pages