3 mW, 100 kSPS,
16-Bit ADC in 6-Lead SOT-23
AD7680
FUNCTIONAL BLOCK DIAGRAM
FEATURES
APPLICATIONS
Battery-powered systems:
Personal digital assistants
Medical instruments
Mobile communications
Instrumentation and control systems
Remote data acquisition systems
High speed modems
Optical sensors
VDD
VIN
T/H
16-BIT SUCCESSIVE
APPROXIMATION
ADC
AD7680
CONTROL
LOGIC
SCLK
SDATA
CS
03643-0-001
Fast throughput rate: 100 kSPS
Specified for VDD of 2.5 V to 5.5 V
Low power
3 mW typ at 100 kSPS with 2.5 V supply
3.9 mW typ at 100 kSPS with 3 V supply
16.7 mW typ at 100 kSPS with 5 V supply
Wide input bandwidth
86 dB SNR at 10 kHz input frequency
Flexible power/serial clock speed management
No pipeline delays
High speed serial interface
SPI®/QSPI™/μWire/DSP compatible
Standby mode: 0.5 μA max
6-Lead SOT-23 and 8-Lead MSOP packages
GND
Figure 1.
Table 1. MSOP/SOT-23 16-Bit PulSAR ADC
Type/kSPS
True Differential
Pseudo Differential
Unipolar
100 kSPS
AD7684
AD7683
AD7680
250 kSPS
AD7687
AD7685
500 kSPS
AD7688
AD7686
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7680 is a 16-bit, fast, low power, successive
approximation ADC. The part operates from a single 2.5 V to
5.5 V power supply and features throughput rates up to 100 kSPS.
The part contains a low noise, wide bandwidth track-and-hold
amplifier that can handle input frequencies in excess of 7 MHz.
1.
First 16-bit ADC in a SOT-23 package.
2.
High throughput with low power consumption.
3.
Flexible power/serial clock speed management. The
conversion rate is determined by the serial clock, allowing
the conversion time to be reduced through the serial clock
speed increase. This allows the average power consumption
to be reduced when a power-down mode is used while not
converting. The part also features a shutdown mode to
maximize power efficiency at lower throughput rates.
Power consumption is 0.5 μA max when in shutdown.
4.
Reference derived from the power supply.
5.
No pipeline delays.
The conversion process and data acquisition are controlled
using CS and the serial clock, allowing the devices to interface
with microprocessors or DSPs. The input signal is sampled on
the falling edge of CS and the conversion is also initiated at this
point. There are no pipeline delays associated with the part.
The AD7680 uses advanced design techniques to achieve very
low power dissipation at fast throughput rates. The reference for
the part is taken internally from VDD, which allows the widest
dynamic input range to the ADC. Thus, the analog input range
for this part is 0 V to VDD. The conversion rate is determined by
the SCLK frequency.
This part features a standard successive approximation ADC
with accurate control of the sampling instant via a CS input and
once-off conversion control.
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: 781.326.8703© 2004-2011 Analog Devices, Inc. All rights reserved.
AD7680
TABLE OF CONTENTS
Specifications..................................................................................... 2
Typical Connection Diagram ................................................... 13
Specifications..................................................................................... 4
Digital Inputs .......................................................................... 13
Timing Specifications....................................................................... 6
Modes of Operation ....................................................................... 14
Absolute Maximum Ratings............................................................ 7
Normal Mode.............................................................................. 14
ESD Caution .................................................................................. 7
Power-Down Mode .................................................................... 15
Pin Configurations and Function Descriptions ........................... 8
Power vs. Throughput Rate ........................................................... 16
Terminology ...................................................................................... 9
Serial Interface ................................................................................ 17
Typical Performance Characteristics ........................................... 10
AD7680 to ADSP-218x .............................................................. 18
Circuit Information ........................................................................ 12
Application Hints ........................................................................... 19
Converter Operation .................................................................. 12
Grounding and Layout .............................................................. 19
Analog Input ............................................................................... 12
Outline Dimensions ....................................................................... 20
ADC Transfer Function ................................................................. 13
Ordering Guide .......................................................................... 21
REVISION HISTORY
5/11—Rev. 0 to Rev. A
Deleted the Evaluating the AD7680 Performance Section ...... 19
Changes to Ordering Guide .......................................................... 21
1/04—Revision 0: Initial Version
Rev. A | Page 2 of 24
AD7680
SPECIFICATIONS 1
Table 2. VDD = 4.5 V to 5.5 V, fSCLK = 2.5 MHz, fSAMPLE = 100 kSPS, unless otherwise noted; TA = TMIN to TMAX, unless otherwise noted
Parameter
DYNAMIC PERFORMANCE
Signal-to-Noise + Distortion (SINAD) 2
Signal-to-Noise Ratio (SNR)2
Total Harmonic Distortion (THD)2
Peak Harmonic or Spurious Noise (SFDR)2
Intermodulation Distortion (IMD)2
Second-Order Terms
Third-Order Terms
Aperture Delay
Aperture Jitter
Full Power Bandwidth
DC ACCURACY
No Missing Codes
Integral Nonlinearity2
Offset Error2
Gain Error2
ANALOG INPUT
Input Voltage Ranges
DC Leakage Current
Input Capacitance
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN2, 3
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output Capacitance2, 3
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time
Throughput Rate
POWER REQUIREMENTS
VDD
IDD
Normal Mode (Static)
Normal Mode (Operational)
Full Power-Down Mode
Power Dissipation 4
Normal Mode (Operational)
Full Power-Down
A, B Versions1
Unit
83
85
84
86
−97
−95
dB min
dB typ
dB min
dB typ
dB typ
dB typ
−94
−100
20
30
8
2.2
dB typ
dB typ
ns max
ps typ
MHz typ
MHz typ
15
±4
±1.68
±0.038
Bits typ
LSB typ
mV max
% FS max
0 to VDD
±0.3
30
V
μA max
pF typ
2.8
0.4
±0.3
10
V min
V max
μA max
pF max
Test Conditions/Comments
fIN = 10 kHz sine wave
@ −3 dB
@ −0.1 dB
Typically 10 nA, VIN = 0 V or VDD
VDD − 0.2
V min
0.4
V max
±0.3
μA max
10
pF max
Straight (Natural) Binary
ISOURCE = 200 μA
ISINK = 200 μA
8
9.6
1.5
400
100
μs max
μs max
μs max
ns max
kSPS
20 SCLK cycles with SCLK at 2.5 MHz
24 SCLK cycles with SCLK at 2.5 MHz
4.5/5.5
V min/V max
5.2
4.8
0.5
mA max
mA max
μA max
26.4
2.75
mW max
μW max
1
Temperature range as follows: B Version: −40°C to +85°C.
See the Terminology section.
Sample tested during initial release to ensure compliance.
4
See the Power vs. Throughput Rate section.
2
3
Rev. A | Page 3 of 24
Sine wave input ≤ 10 kHz
See the Serial Interface section
Digital I/PS = 0 V or VDD
SCLK on or off. VDD = 5.5 V
fSAMPLE = 100 kSPS. VDD = 5.5 V; 3.3 mA typ
SCLK on or off. VDD = 5.5 V
VDD = 5.5 V
fSAMPLE = 100 kSPS
AD7680
SPECIFICATIONS 1
Table 3. VDD = 2.5 V to 4.096 V, fSCLK = 2.5 MHz, fSAMPLE = 100 kSPS, unless otherwise noted; TA = TMIN to TMAX, unless otherwise noted.
Parameter
DYNAMIC PERFORMANCE
Signal-to-Noise + Distortion (SINAD) 2
Signal-to-Noise Ratio (SNR)2
Total Harmonic Distortion (THD) 2
Peak Harmonic or Spurious Noise (SFDR)2
Intermodulation Distortion (IMD)2
Second-Order Terms
Third-Order Terms
Aperture Delay
Aperture Jitter
Full Power Bandwidth
DC ACCURACY
No Missing Codes
Integral Nonlinearity2
Offset Error2
Gain Error2
ANALOG INPUT
Input Voltage Ranges
DC Leakage Current
Input Capacitance
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN2, 3
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output Capacitance2, 3
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time
Throughput Rate
A Version1
B Version1
Unit
83
82
86
84
83
86
−98
−95
83
82
86
84
83
86
−98
−99
dB min
dB min
dB typ
dB min
dB min
dB typ
dB typ
dB typ
−94
−100
20
30
7
5
2
1.6
−94
−100
10
30
7
5
2
1.6
dB typ
dB typ
ns max
ps typ
MHz typ
MHz typ
MHz typ
MHz typ
14
±3.5
±3
±1.25
±1.098
±0.038
15
±3.5
±3
±1.25
±1.098
±0.038
Bits min
LSB max
LSB max
mV max
mV max
% FS max
0 to VDD
±0.3
30
0 to VDD
±0.3
30
V
μA max
pF typ
2.4
0.4
±0.3
10
2.4
0.4
±0.3
10
V min
V max
μA max
pF max
Test Conditions/Comments
fIN = 10 kHz sine wave
VDD = 4.096 V
VDD = 2.5 V to 3.6 V
VDD = 4.096 V
VDD = 2.5 V to 3.6 V
@ −3 dB; VDD = 4.096 V
@ −3 dB; VDD = 2.5 V to 3.6 V
@ −0.1 dB; VDD = 4.096 V
@ −0.1 dB; VDD = 2.5 V to 3.6 V
VDD = 4.096 V
VDD = 2.5 V to 3.6 V
VDD = 4.096 V
VDD = 2.5 V to 3.6 V
Typically 10 nA, VIN = 0 V or VDD
VDD − 0.2
VDD − 0.2
0.4
0.4
±0.3
±0.3
10
10
Straight (Natural) Binary
V min
V max
μA max
pF max
ISOURCE = 200 μA
ISINK = 200 μA
8
9.6
1.5
400
100
μs max
μs max
μs max
ns max
kSPS
20 SCLK cycles with SCLK at 2.5 MHz
24 SCLK cycles with SCLK at 2.5 MHz
Full-scale step input
Sine wave input ≤ 10 kHz
See the Serial Interface section
8
9.6
1.5
400
100
Rev. A | Page 4 of 24
AD7680
Parameter
POWER REQUIREMENTS
VDD
IDD
Normal Mode (Static)
Normal Mode (Operational)
Full Power-Down Mode
Power Dissipation 4
Normal Mode (Operational)
Full Power-Down
A Version1
B Version1
Unit
2.5/4.096
2.5/4.096
V min/max
2.8
2
2.6
1.9
0.3
2.8
2
2.6
1.9
0.3
mA max
mA max
mA max
mA max
μA max
Digital I/Ps = 0 V or VDD
SCLK on or off; VDD = 4.096 V
SCLK on or off; VDD = 3.6 V
fSAMPLE = 100 kSPS; VDD = 4.096 V; 1.75 mA typ
fSAMPLE = 100 kSPS; VDD = 3.6 V; 1.29 mA typ
SCLK on or off
10.65
6.84
3
1.23
1.08
10.65
6.84
3
1.23
1.08
mW max
mW max
mW typ
μW max
μW max
fSAMPLE = 100 kSPS; VDD = 4.096 V
fSAMPLE = 100 kSPS; VDD = 3.6 V
VDD = 2.5 V
VDD = 4.096V
VDD = 3.6 V
1
Temperature range as follows: A, B Versions: −40°C to +85°C.
See the Terminology section.
3
Sample tested during initial release to ensure compliance.
4
See the Power vs. Throughput Rate section.
2
Rev. A | Page 5 of 24
Test Conditions/Comments
AD7680
TIMING SPECIFICATIONS 1
Table 4. VDD = 2.5 V to 5.5 V; TA = TMIN to TMAX, unless otherwise noted.
Parameter
fSCLK 2
tCONVERT
tQUIET
t1
t2
t3 3
t43
t5
t6
t7
t8 4
tPOWER-UP 5
Limit at TMIN, TMAX
3V
5V
250
250
2.5
2.5
20 × tSCLK 20 × tSCLK
100
100
10
10
10
10
48
35
120
80
0.4 tSCLK
0.4 tSCLK
0.4 tSCLK
0.4 tSCLK
10
10
45
35
1
1
Unit
kHz min
MHz max
min
ns min
ns min
ns min
ns max
ns max
ns min
ns min
ns min
ns max
μs typ
Description
Minimum quiet time required between bus relinquish and start of next conversion
Minimum CS pulse width
CS to SCLK setup time
Delay from CS until SDATA three-state disabled
Data access time after SCLK falling edge
SCLK low pulse width
SCLK high pulse width
SCLK to data valid hold time
SCLK falling edge to SDATA high impedance
Power up time from full power-down
1
Sample tested during initial release to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.
Mark/space ratio for the SCLK input is 40/60 to 60/40.
3
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.
4
t8 is derived form the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated
back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t8, quoted in the timing characteristics is the true bus relinquish
time of the part and is independent of the bus loading.
5
See Power vs. Throughput Rate section.
2
200μA
1.6V
CL
50pF
200μA
IOH
03643-0-002
TO OUTPUT
PIN
IOL
Figure 2. Load Circuit for Digital Output Timing Specification
Rev. A | Page 6 of 24
AD7680
ABSOLUTE MAXIMUM RATINGS
Table 5. TA = 25°C, unless otherwise noted.
Parameter
VDD to GND
Analog Input Voltage to GND
Digital Input Voltage to GND
Digital Output Voltage to GND
Input Current to Any Pin Except Supplies1
Operating Temperature Range
Commercial (B Version)
Storage Temperature Range
Junction Temperature
SOT-23 Package, Power Dissipation
θJA Thermal Impedance
θJC Thermal Impedance
MSOP Package, Power Dissipation
θJA Thermal Impedance
θJC Thermal Impedance
Lead Temperature, Soldering
Vapor Phase (60 secs)
Infared (15 secs)
ESD
Rating
−0.3 V to +7 V
−0.3 V to VDD + 0.3 V
−0.3 V to +7 V
−0.3 V to VDD + 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 listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
−40°C to +85°C
−65°C to +150°C
150°C
450 mW
229.6°C/W
91.99°C/W
450 mW
205.9°C/W
43.74°C/W
215°C
220°C
2 kV
1
Transient currents of up to 100 mA do not cause SCR latch-up.
ESD 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 this product 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.
Rev. A | Page 7 of 24
AD7680
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
SOT-23
6
CS
5
SDATA
TOP VIEW
VIN 3 (Not to Scale) 4 SCLK
VDD 1
8
AD7680
CS
SDATA
TOP VIEW
GND 3 (Not to Scale) 6 NC
VIN 4
5 SCLK
GND 2
Figure 3. SOT-23 Pin Configuration
7
NC = NO CONNECT
03643-0-022
AD7680
03643-0-003
VDD 1
GND 2
MSOP
Figure 4. MSOP Pin Configuration
Table 6. Pin Function Descriptions
Pin No.
SOT-23
1
2
Pin No.
MSOP
1
2, 3
Mnemonic
VDD
GND
3
4
4
5
VIN
SCLK
5
7
SDATA
6
8
CS
N/A
6
NC
Function
Power Supply Input. The VDD range for the AD7680 is from 2.5 V to 5.5 V.
Analog Ground. Ground reference point for all circuitry on the AD7680. All analog input signals should
be referred to this GND voltage.
Analog Input. Single-ended analog input channel. The input range is 0 V to VDD.
Serial Clock. Logic input. SCLK provides the serial clock for accessing data from this part. This clock
input is also used as the clock source for the AD7680's conversion process.
Data Out. Logic output. The conversion result from the AD7680 is provided on this output as a serial
data stream. The bits are clocked out on the falling edge of the SCLK input. The data stream from the
AD7680 consists of four leading zeros followed by 16 bits of conversion data that are provided MSB
first. This will be followed by four trailing zeroes if CS is held low for a total of 24 SCLK cycles. See the
Serial Interface section.
Chip Select. Active low logic input. This input provides the dual function of initiating conversions on
the AD7680 and framing the serial data transfer.
No Connect. This pin should be left unconnected.
Rev. A | Page 8 of 24
AD7680
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale, a point
1/2 LSB below the first code transition, and full scale, a point
1/2 LSB above the last code transition.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7680, it is defined as
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5, and V6 are the rms amplitudes of the second through the
sixth harmonics.
Offset Error
This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal, i.e., AGND + 1 LSB.
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.
THD (dB) = 20 log
Gain Error
This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (i.e., VREF − 1 LSB) after the offset
error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of conversion. The track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±1 LSB, after the end of the conversion.
See the Serial Interface section for more details.
Signal-to-(Noise + Distortion) Ratio
This 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
depends 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
Thus, for a 16-bit converter, this is 98 dB.
V2 2 + V3 2 + V4 2 + V5 2 + V6 2
V1
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and fb,
any active device with nonlinearities creates distortion products
at the sum and difference frequencies of mfa ± nfb where m, n =
0, 1, 2, 3. Intermodulation distortion terms are those for which
neither m nor n are equal to zero. For example, the second-order
terms include (fa + fb) and (fa − fb), while the third-order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa −2fb).
The AD7680 is tested using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second-order terms are usually distanced in
frequency from the original sine waves, while the third-order
terms are usually at a frequency close to the input frequencies.
As a result, the second- and third-order terms are specified
separately. The calculation of the intermodulation distortion is
as per the THD specification where it is the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals expressed in dBs.
Rev. A | Page 9 of 24
AD7680
TYPICAL PERFORMANCE CHARACTERISTICS
Figure 5 shows a typical FFT plot for the AD7680 at 100 kSPS
sample rate and 10 kHz input frequency. Figure 6 shows the
signal-to-(noise + distortion) ratio performance versus the
input frequency for various supply voltages while sampling at
100 kSPS with an SCLK of 2.5 MHz.
Figure 7 shows a graph of the total harmonic distortion versus
the analog input frequency for various supply voltages, while
Figure 8 shows a graph of the total harmonic distortion versus
the analog input frequency for various source impedances (see
the Analog Input section). Figure 9 and Figure 10 show the
typical INL and DNL plots for the AD7680.
110
0
VDD = 5V
FSAMPLE = 100kSPS
FIN = 10kHz
SNR = 88.28dB
SINAD = 87.82dB
THD = –97.76dB
SFDR = –98.25dB
–20
–40
FSAMPLE = 100kSPS
TA = 25°C
105
VDD = 4.3V
VDD = 4.75V
THD (dB)
dB
–60
–80
VDD = 3.6V
100
VDD = 5.25V
VDD = 3.0V
–100
VDD = 2.7V
95
–140
–160
0
10k
20k
30k
40k
VDD = 2.5V
03643-0-017
03643-0-021
–120
90
10
50k
100
INPUT FREQUENCY (kHz)
FREQUENCY (kHz)
Figure 5. AD7680 Dynamic Performance at 100 kSPS
Figure 7. AD7680 THD vs. Analog Input Frequency
for Various Supply Voltages at 100 kSPS
110
95
FSAMPLE = 100kSPS
TA = 25°C
RIN = 10Ω
105
100
RIN = 50Ω
VDD = 5.25V
THD (dB)
SINAD (dB)
90
95
RIN = 100Ω
90
VDD = 4.75V
VDD = 4.3V
85
VDD = 3.6V
VDD = 2.5V
VDD = 2.7V
80
10
80
03643-0-016
VDD = 3.0V
100
FSAMPLE = 100kSPS
TA = 25°C
VDD = 4.75V
RIN = 1000Ω
75
10
100
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (kHz)
Figure 8. AD7680 THD vs. Analog Input Frequency
for Various Source Impedances
Figure 6. AD7680 SINAD vs. Analog Input Frequency
for Various Supply Voltages at 100 kSPS
Rev. A | Page 10 of 24
03643-0-018
85
AD7680
2.5
1.5
VDD = 3.0V
TEMP = 25°C
VDD = 3.0V
TEMP = 25°C
2.0
1.0
DNL ERROR (LSB)
1.0
0.5
0.5
0
–0.5
0
–0.5
–1.0
0
10000
20000
30000
40000
50000
60000
70000
–1.0
03643-0-020
03643-0-019
INL ERROR (LSB)
1.5
–1.5
0
10000
20000
30000
40000
50000
CODE
CODE
Figure 9. AD7680 Typical INL
Figure 10. AD7680 Typical DNL
Rev. A | Page 11 of 24
60000
70000
AD7680
CIRCUIT INFORMATION
The AD7680 provides the user with an on-chip track-and-hold
ADC and a serial interface housed in a tiny 6-lead SOT-23
package or in an 8-lead MSOP package, which offer the user
considerable space-saving advantages over alternative solutions.
The serial clock input accesses data from the part and also
provides the clock source for the successive approximation
ADC. The analog input range for the AD7680 is 0 V to VDD. An
external reference is not required for the ADC nor is there a
reference on-chip. The reference for the AD7680 is derived from
the power supply and thus gives the widest dynamic input range.
The AD7680 also features a power-down option to save power
between conversions. The power-down feature is implemented
across the standard serial interface as described in the Modes of
Operation section.
CONVERTER OPERATION
The AD7680 is a 16-bit, successive approximation ADC based
around a capacitive DAC. The AD7680 can convert analog
input signals in the 0 V to VDD range. Figure 11 and Figure 12
show simplified schematics of the ADC. The ADC comprises
control logic, SAR, and a capacitive DAC. Figure 11 shows the
ADC during its acquisition phase. SW2 is closed and SW1 is in
Position A. The comparator is held in a balanced condition and
the sampling capacitor acquires the signal on the selected VIN
channel.
CAPACITIVE
DAC
SAMPLING
CAPACITOR
CONTROL
LOGIC
B
ACQUISITION SW2
PHASE
VDD/2
COMPARATOR
SAMPLING
CAPACITOR
A
VIN
SW1
CONTROL
LOGIC
B
CONVERSION SW2
PHASE
COMPARATOR
VDD/2
Figure 12. ADC Conversion Phase
ANALOG INPUT
Figure 13 shows an equivalent circuit of the analog input
structure of the AD7680. The two diodes, D1 and D2, provide
ESD protection for the analog inputs. Care must be taken to
ensure that the analog input signal never exceeds the supply
rails by more than 300 mV. This causes these diodes to become
forward-biased and to start conducting current into the
substrate. The maximum current these diodes can conduct
without causing irreversible damage to the part is 10 mA.
Capacitor C1 in Figure 13 is typically about 5 pF and can be
attributed primarily to pin capacitance. Resistor R1 is a lumped
component made up of the on resistance of a track-and-hold
switch. This resistor is typically about 25 Ω. Capacitor C2 is the
ADC sampling capacitor and has a capacitance of 25 pF
typically. For ac applications, removing high frequency
components from the analog input signal is recommended by
use of an RC low-pass filter on the relevant analog input pin. In
applications where harmonic distortion and signal-to-noise
ratio are critical, the analog input should be driven from a low
impedance source. Large source impedances significantly affect
the ac performance of the ADC. This may necessitate the use of
an input buffer amplifier. The choice of the op amp is a function
of the particular application. When no amplifier is used to drive
the analog input, the source impedance should be limited to low
values. The maximum source impedance depends on the
amount of total harmonic distortion (THD) that can be
tolerated. The THD increases as the source impedance
increases, and performance degrades (see Figure 8).
Figure 11. ADC Acquisition Phase
VDD
When the ADC starts a conversion, SW2 opens and SW1 moves
to Position B, causing the comparator to become unbalanced
(Figure 12). The control logic and the capacitive DAC are used
to add and subtract fixed amounts of charge from the sampling
capacitor to bring the comparator back into a balanced
condition. When the comparator is rebalanced, the conversion
is complete. The control logic generates the ADC output code
(see the ADC Transfer Function section).
Rev. A | Page 12 of 24
D1
R1
C2
25pF
VIN
C1
5pF
D2
CONVERSION PHASE - SWITCH OPEN
TRACK PHASE - SWITCH CLOSED
Figure 13. Equivalent Analog Input Circuit
03643-0-006
SW1
03643-0-004
A
VIN
CAPACITIVE
DAC
03643-0-005
The AD7680 is a fast, low power, 16-bit, single-supply ADC. The
part can be operated from a 2.5 V to 5.5 V supply and is capable of
throughput rates of 100 kSPS when provided with a 2.5 MHz clock.
AD7680
ADC TRANSFER FUNCTION
The output coding of the AD7680 is straight binary. The
designed code transitions occur at successive integer LSB
values, i.e., 1 LSB, 2 LSBs. The LSB size is VDD/65536. The ideal
transfer characteristic for the AD7680 is shown in Figure 14.
111...111
111...110
111...000
1 LSB = VDD/65536
011...111
In fact, because the supply current required by the AD7680 is so
low, a precision reference can be used as the supply source to
the AD7680. For example, a REF19x voltage reference (REF195
for 5 V or REF193 for 3 V) or an AD780 can be used to supply
the required voltage to the ADC (see Figure 15). This
configuration is especially useful if the power supply available is
quite noisy, or if the system supply voltages are at some value
other than the required operating voltage of the AD7680, e.g.,
15 V. The REF19x or AD780 outputs a steady voltage to the
AD7680. Recommended decoupling capacitors are a 100 nF low
ESR ceramic (Farnell 335-1816) and a 10 μF low ESR tantalum
(Farnell 197-130).
3V
10F
TANT
000...010
000...000
0V
1 LSB
+VDD–1 LSB
ANALOG INPUT
03643-0-007
000...001
0V TO VDD
INPUT
Figure 14. AD7680 Transfer Characteristic
VIN
10F
0.1F
VDD
5V
SUPPLY
REF193
0.1F
SCLK
AD7680
SDATA
C/P
CS
Figure 15 shows a typical connection diagram for the AD7680.
VREF is taken internally from VDD and as such should be well
decoupled. This provides an analog input range of 0 V to VDD.
The conversion result is output in a 24-bit word, or alternatively,
all 16 bits of the conversion result may be accessed using a
minimum of 20 SCLKs. This 20-/24-bit data stream consists of
a four leading zeros, followed by the 16 bits of conversion data,
followed by four trailing zeros in the case of the 24 SCLK
transfer. For applications where power consumption is of
concern, the power-down mode should be used between
conversions or bursts of several conversions to improve power
performance (see the Modes of Operation section).
SERIAL
INTERFACE
03643-0-008
GND
TYPICAL CONNECTION DIAGRAM
Figure 15. Typical Connection Diagram
Digital Inputs
The digital inputs applied to the AD7680 are not limited by the
maximum ratings that limit the analog inputs. Instead, the
digital inputs applied can go to 7 V and are not restricted by the
VDD + 0.3 V limit as on the analog inputs. For example, if the
AD7680 were operated with a VDD of 3 V, 5 V logic levels could
be used on the digital inputs. However, it is important to note
that the data output on SDATA still has 3 V logic levels when
VDD = 3 V.
Another advantage of SCLK and CS not being restricted by the
VDD + 0.3 V limit is that power supply sequencing issues are
avoided. If one of these digital inputs is applied before VDD, then
there is no risk of latch-up as there would be on the analog
inputs if a signal greater than 0.3 V were applied prior to VDD.
Rev. A | Page 13 of 24
AD7680
MODES OF OPERATION
The mode of operation of the AD7680 is selected by controlling
the (logic) state of the CS signal during a conversion. There are
two possible modes of operation, normal and power-down. The
point at which CS is pulled high after the conversion has been
initiated determines whether or not the AD7680 enters powerdown mode. Similarly, if the AD7680 is already in power-down,
CS can control whether the device returns to normal operation
or remains in power-down. These modes of operation are
designed to provide flexible power management options. These
options can optimize the power dissipation/throughput rate
ratio for differing application requirements.
NORMAL MODE
This mode provides the fastest throughput rate performance,
because the user does not have to worry about the power-up
times with the AD7680 remaining fully powered all the time.
Figure 16 shows the general diagram of the operation of the
AD7680 in this mode.
The conversion is initiated on the falling edge of CS as described
in the Serial Interface section. To ensure that the part remains
fully powered up at all times, CS must remain low until at least
10 SCLK falling edges have elapsed after the falling edge of CS.
If CS is brought high any time after the 10th SCLK falling edge,
but before the 20th SCLK falling edge, the part remains
powered up, but the conversion is terminated and SDATA goes
back into three-state. At least 20 serial clock cycles are required
to complete the conversion and access the complete conversion
result. In addition, a total of 24 SCLK cycles accesses four
trailing zeros. CS may idle high until the next conversion or
may idle low until CS returns high sometime prior to the next
conversion, effectively idling CS low.
Once a data transfer is complete (SDATA has returned to threestate), another conversion can be initiated after the quiet time,
tQUIET, has elapsed by bringing CS low again.
CS
1
10
20
SDATA
4 LEADING ZEROS + CONVERSION RESULT
Figure 16. Normal Mode Operation
Rev. A | Page 14 of 24
03643-0-009
SCLK
AD7680
POWER-DOWN MODE
This mode is intended for use in applications where slower
throughput rates are required. Either the ADC is powered
down between each conversion, or a series of conversions may
be performed at a high throughput rate, and then the ADC is
powered down for a relatively long duration between these
bursts of several conversions. When the AD7680 is in
power-down, all analog circuitry is powered down.
To enter power-down, the conversion process must be
interrupted by bringing CS high anywhere after the second
falling edge of SCLK and before the 10th falling edge of SCLK
as shown in Figure 17. Once CS has been brought high in this
window of SCLKs, the part enters power-down, the conversion
that was initiated by the falling edge of CS is terminated, and
SDATA goes back into three-state. If CS is brought high before
the second SCLK falling edge, the part remains in normal mode
and will not power down. This avoids accidental power-down
due to glitches on the CS line.
In order to exit this mode of operation and power up the
AD7680 again, a dummy conversion is performed. On the
falling edge of CS, the device begins to power up and continues
to power up as long as CS is held low until after the falling edge
of the 10th SCLK. The device is fully powered up once at least
16 SCLKs (or approximately 6 μs) have elapsed and valid data
results from the next conversion as shown in Figure 18. If CS is
brought high before the 10th falling edge of SCLK, regardless of
the SCLK frequency, the AD7680 goes back into power-down
again. This avoids accidental power-up due to glitches on the
CS line or an inadvertent burst of 8 SCLK cycles while CS is low.
So although the device may begin to power-up on the falling
edge of CS, it powers down again on the rising edge of CS as
long as it occurs before the 10th SCLK falling edge.
CS
1
2
10
20
THREE-STATE
SDATA
03643-0-010
SCLK
Figure 17. Entering Power-Down Mode
THE PART IS FULLY POWERED
UP WITH VIN FULLY ACQUIRED
THE PART BEGINS
TO POWER UP
tPOWER UP
CS
1
10
20
1
20
SDATA
INVALID DATA
VALID DATA
Figure 18. Exiting Power-Down Mode
Rev. A | Page 15 of 24
03643-0-011
SCLK
AD7680
POWER VS. THROUGHPUT RATE
Figure 19 shows the power dissipation versus the throughput
rate when using the power-down mode with 3.6 V supplies, a
2.5 MHz SCLK, and a 20 SCLK serial transfer.
(9/100) × (6.84 mW) + (91/100) × (1.08 μW) = 0.62 mW
Rev. A | Page 16 of 24
VDD = 3.6V
FSCLK = 2.5MHz
1
0.1
03643-0-012
For example, if the AD7680 is operated in a continuous
sampling mode, with a throughput rate of 10 kSPS and an SCLK
of 2.5 MHz (VDD = 3.6 V), and the device is placed in powerdown mode between conversions, the power consumption is
calculated as follows. The maximum power dissipation during
normal operation is 6.84 mW (VDD = 3.6 V). If the power-up
time from power-down is 1 μs, and the remaining conversion
time is 8 μs, (using a 20 SCLK transfer), then the AD7680 can
be said to dissipate 6.84 mW for 9 μs during each conversion
cycle. With a throughput rate of 10 kSPS, the cycle time is 100
μs.
For the remainder of the conversion cycle, 91 μs, the part
remains in power-down mode. The AD7680 can be said to
dissipate 1.08 μW for the remaining 91 μs of the conversion
cycle. Therefore, with a throughput rate of 10 kSPS, the average
power dissipated during each cycle is
10
POWER (mW)
By using the power-down mode on the AD7680 when not
converting, the average power consumption of the ADC
decreases at lower throughput rates. Figure 19 shows how as the
throughput rate is reduced, the part remains in its shut-down
state longer, and the average power consumption over time
drops accordingly.
0.01
0
5
10
15
20
25
30
35
40
45
THROUGHPUT (kSPS)
Figure 19. Power vs. Throughput Using
Power-Down Mode with 20 SCLK Transfer at 3.6 V
50
AD7680
SERIAL INTERFACE
Figure 20 shows the detailed timing diagram for serial
interfacing to the AD7680. The serial clock provides the
conversion clock and also controls the transfer of information
from the AD7680 during conversion.
A minimum of 20 serial clock cycles are required to perform
the conversion process and to access data from the AD7680.
CS going low provides the first leading zero to be read in by the
microcontroller or DSP. The remaining data is then clocked out
by subsequent SCLK falling edges beginning with the second
leading zero; thus the first falling clock edge on the serial clock
has the first leading zero provided and also clocks out the
second leading zero. If a 24 SCLK transfer is used as in Figure 20,
the data transfer consists of four leading zeros followed by the
16 bits of data, followed by four trailing zeros. The final bit
(fourth trailing zero) in the data transfer is valid on the 24th
falling edge, having been clocked out on the previous (23rd)
falling edge. If a 20 SCLK transfer is used as shown in Figure 21,
the data output stream consists of only four leading zeros
followed by 16 bits of data with the final bit valid on the 20th
SCLK falling edge. A 20 SCLK transfer allows for a shorter cycle
time and therefore a faster throughput rate is achieved.
The CS signal initiates the data transfer and conversion process.
The falling edge of CS puts the track-and-hold into hold mode,
takes the bus out of three-state, and samples the analog input.
The conversion is also initiated at this point and requires at least
20 SCLK cycles to complete. Once 17 SCLK falling edges have
elapsed, the track-and-hold goes back into track mode on the
next SCLK rising edge. Figure 20 shows a 24 SCLK transfer that
allows a 100 kSPS throughput rate. On the 24th SCLK falling
edge, the SDATA line goes back into three-state. If the rising
edge of CS occurs before 24 SCLKs have elapsed, the conversion
terminates and the SDATA line goes back into three-state;
otherwise SDATA returns to three-state on the 24th SCLK
falling edge as shown in Figure 20.
t1
CS
tCONVERT
t2
t6
2
3
4
t3
0
SDATA
5
t4
ZERO
ZERO
ZERO
18
19
t5
20
21
22
23
24
t8
t7
DB15
DB1
tQUIET
DB0
ZERO
ZERO
ZERO
ZERO
3-STATE
3-STATE
4 LEADING ZEROS
4 TRAILING ZEROS
Figure 20. AD7680 Serial Interface Timing Diagram—24 SCLK Transfer
t1
CS
tCONVERT
t6
1
SCLK
2
3
4
5
18
t5
t3
SDATA
3-STATE
0
t4
ZERO
ZERO
ZERO
DB15
19
20
t8
tQUIET
t7
DB1
DB0
0
4 LEADING ZEROS
Figure 21. AD7680 Serial Interface Timing Diagram—20 SCLK Transfer
Rev. A | Page 17 of 24
3-STATE
03643-0-014
t2
03643-0-013
1
SCLK
AD7680
an input. The DSP operates in alternate framing mode and the
SPORT control register is set up as described. Transmit and
receive autobuffering is used in order to get a 24 SCLK transfer.
Each buffer contains three 8-bit words. The frame synchronization signal generated on the TFS is tied to CS, and as with all
signal processing applications, equidistant sampling is necessary.
In this example, the timer interrupt is used to control the
sampling rate of the ADC.
AD7680*
SCLK
SDATA
AD7680 TO ADSP-218x
CS
The ADSP-218x family of DSPs can be interfaced directly to the
AD7680 without any glue logic required. The SPORT control
register should be set up as follows:
ADSP-218x*
SCLK
DR
RFS
TFS
03643-0-015
It is also possible to take valid data on each SCLK rising edge
rather than falling edge, since the SCLK cycle time is long
enough to ensure the data is ready on the rising edge of SCLK.
However, the first leading zero is still driven by the CS falling
edge, and so it can be taken on only the first SCLK falling edge.
It may be ignored and the first rising edge of SCLK after the CS
falling edge would have the second leading zero provided and
the 23rd rising SCLK edge would have the final trailing zero
provided. This method may not work with most
microcontrollers/DSPs but could possibly be used with FPGAs
and ASICs.
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 22. Interfacing to the ADSP-218x
TFSW = RFSW = 1, Alternate Framing
INVRFS = INVTFS = 1, Active Low Frame Signal
DTYPE = 00, Right Justify Data
SLEN = 0111, 8-Bit Data-Words
ISCLK = 1, Internal Serial Clock
TFSR = RFSR = 0, Frame First Word
IRFS = 0
ITFS = 1
To implement the power-down mode, SLEN should be set to
0111 to issue an 8-bit SCLK burst. The connection diagram is
shown in Figure 22. The ADSP-218x has the TFS and RFS of the
SPORT tied together, with TFS set as an output and RFS set as
The timer register is loaded with a value that provides an
interrupt at the required sample interval. When an interrupt is
received, the values in the transmit autobuffer start to be
transmitted and TFS is generated. The TFS is used to control
the RFS and therefore the reading of data. The data is stored in
the receive autobuffer for processing or to be shifted later. The
frequency of the serial clock is set in the SCLKDIV register.
When the instruction to transmit with TFS is given, i.e.,
TX0 = AX0, the state of the SCLK is checked. The DSP waits
until the SCLK has gone high, low, and high again before
transmission starts. If the timer and SCLK values are chosen
such that the instruction to transmit occurs on or near the
rising edge of SCLK, the data may be transmitted or it may wait
until the next clock edge.
Rev. A | Page 18 of 24
AD7680
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7680 should be
designed such that 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 separated easily. A minimum
etch technique is generally best for ground planes, because it
gives the best shielding. Digital and analog ground planes
should be joined at only one place. If the AD7680 is in a system
where multiple devices require an AGND to DGND
connection, the connection should still be made at one point
only, a star ground point that should be established as close as
possible to the AD7680.
Avoid running digital lines under the device because these
couple noise onto the die. The analog ground plane should be
allowed to run under the AD7680 to avoid noise coupling. The
power supply lines to the AD7680 should use as large a trace as
possible to provide low impedance paths and reduce the effects
of glitches on the power supply line. Fast switching signals, such
as clocks, should be shielded with digital ground to avoid
radiating noise to other sections of the board, and clock signals
should never be run near the analog inputs. Avoid crossover of
digital and analog signals. Traces on opposite sides of the board
should run at right angles to each other, which reduces the
effects of feedthrough on the board. A microstrip technique is
by far the best but is not always possible with a double-sided
board. In this technique, the component side of the board is
dedicated to ground planes while the signals are placed on the
solder side.
Good decoupling is also very important. All analog supplies
should be decoupled with 10 μF tantalum in parallel with
0.1 μF capacitors to AGND, as discussed in the Typical
Connection Diagram section. To achieve the best performance
from these decoupling components, the user should attempt to
keep the distance between the decoupling capacitors and the
VDD and GND pins to a minimum, with short track lengths
connecting the respective pins.
Rev. A | Page 19 of 24
AD7680
OUTLINE DIMENSIONS
3.00
2.90
2.80
1.70
1.60
1.50
6
5
4
1
2
3
3.00
2.80
2.60
PIN 1
INDICATOR
0.95 BSC
1.90
BSC
1.30
1.15
0.90
0.20 MAX
0.08 MIN
0.15 MAX
0.05 MIN
10°
4°
0°
SEATING
PLANE
0.50 MAX
0.30 MIN
0.60
BSC
0.55
0.45
0.35
12-16-2008-A
1.45 MAX
0.95 MIN
COMPLIANT TO JEDEC STANDARDS MO-178-AB
Figure 23. 6-Lead Small Outline Transistor Package [SOT-23] (RJ-6) Dimensions shown in millimeters
3.20
3.00
2.80
8
3.20
3.00
2.80
1
5.15
4.90
4.65
5
4
PIN 1
IDENTIFIER
0.65 BSC
0.95
0.85
0.75
15° MAX
1.10 MAX
0.40
0.25
6°
0°
0.23
0.09
COMPLIANT TO JEDEC STANDARDS MO-187-AA
0.80
0.55
0.40
10-07-2009-B
0.15
0.05
COPLANARITY
0.10
Figure 24. 8-Lead Micro Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
Rev. A | Page 20 of 24
AD7680
ORDERING GUIDE
Model1
AD7680ARJZ-REEL7
AD7680ARM
AD7680ARM-REEL
AD7680ARM-REEL7
AD7680ARMZ
AD7680BRJZ-R2
AD7680BRJZ-REEL7
AD7680BRM
AD7680BRM-REEL
AD7680BRM-REEL7
AD7680BRMZ
AD7680BRMZ-REEL
AD7680BRMZ-REEL7
1
2
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
−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
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Linearity
Error (LSB)2
14 Bits Min
14 Bits Min
14 Bits Min
14 Bits Min
14 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
15 Bits Min
Package Description
6-Lead Small Outline Transistor Package (SOT-23)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
6-Lead Small Outline Transistor Package (SOT-23)
6-Lead Small Outline Transistor Package (SOT-23)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
8-Lead Mini Small Outline Package (MSOP)
Z = RoHS Compliant Part.
Linearity error here refers to no missing codes.
Rev. A | Page 21 of 24
Package
Option
RJ-6
RM-8
RM-8
RM-8
RM-8
RJ-6
RJ-6
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
Branding
C40
CQA
CQA
CQA
C40
C3H
C3H
CQB
CQB
CQB
C3H
C3H
C3H
AD7680
NOTES
Rev. A | Page 22 of 24
AD7680
NOTES
Rev. A | Page 23 of 24
AD7680
NOTES
© 2004-2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D03643-0-5/11(A)
Rev. A | Page 24 of 24
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