TI1 ADS7822-Q1 12-bit 200-khz micropower sampling analog-to-digital converter Datasheet

ADS7822-Q1
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12-BIT 200-kHz MICROPOWER SAMPLING ANALOG-TO-DIGITAL CONVERTER
FEATURES
APPLICATIONS
• Qualified for Automotive Applications
• 200-kHz Sampling Rate
• Micropower:
1.6 mW at 200 kHz
0.54 mW at 75 kHz
0.06 mW at 7.5 kHz
• Power-Down Current: 3 µA Max
• MSOP-8 Package
• Pseudo-Differential Input
• Serial Interface
•
•
•
•
1
2
Battery-Operated Systems
Remote Data Acquisition
Isolated Data Acquisition
Simultaneous Sampling, Multichannel Systems
DESCRIPTION
The ADS7822 is a 12-bit sampling analog-to-digital (A/D) converter with ensured specifications over a 2.7-V to
5.25-V supply range. It requires very little power even when operating at the full 200-kHz rate. At lower
conversion rates, the high speed of the device enables it to spend most of its time in the power-down mode—the
power dissipation is less than 60 µW at 7.5 kHz.
The ADS7822 also features operation from 2.0 V to 5 V, a synchronous serial interface, and a pseudo-differential
input. The reference voltage can be set to any level within the range of 50 mV to VCC.
Ultra low power and small size make the ADS7822 ideal for battery-operated systems. It is also a perfect fit for
remote data-acquisition modules, simultaneous multichannel systems, and isolated data acquisition. The
ADS7822 is available in an MSOP-8 package.
Control
SAR
VREF
DOUT
+In
Serial
Interface
CDAC
-In
S/H Amp
Comparator
DCLOCK
CS/SHDN
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2009, Texas Instruments Incorporated
ADS7822-Q1
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This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
ORDERING INFORMATION (1)
PACKAGE (2)
TA
–40°C to 85°C
(1)
(2)
MSOP – DGK
Reel of 2500
ORDERABLE PART NUMBER
ADS7822IDGKRQ1
TOP-SIDE MARKING
OCV
For the most current package and ordering information, see the Package Option Addendum at the end of this document, or see the TI
web site at www.ti.com.
Package drawings, thermal data, and symbolization are available at www.ti.com/packaging.
ABSOLUTE MAXIMUM RATINGS (1)
over operating free-air temperature range (unless otherwise noted)
VCC
Supply voltage
6V
Analog inputs
–0.3 V to VCC + 0.3 V
VIN
Input voltage
TC
Case temperature
100°C
TJ
Junction temperature
150°C
TSTG
Storage temperature
125°C
VREF
External reference voltage
5.5 V
(1)
2
Logic inputs
–0.3 V to 6 V
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating
Conditions is not implied. Exposure to absolute-maximum rated conditions for extended periods may affect device reliability.
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ELECTRICAL CHARACTERISTICS: +VCC = 2.7 V
At –40°C to 85°C, +VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, and fCLK = 16 × fSAMPLE (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUT
Full-scale input span
Absolute input range
+In – (–In)
0
VREF
V
+In – GND
–0.2
VCC + 0.2
V
–In – GND
–0.2
+1.0
V
Capacitance
25
pF
Leakage current
±1
µA
12
Bits
SYSTEM PERFORMANCE
Resolution
No missing codes
11
Integral linearity error
–2
±0.5
+2
LSB (1)
Bits
Differential linearity error
–2
±0.5
+2
LSB
Offset error
–3
+3
LSB
Gain error
–3
+3
LSB
Noise
33
µVrms
Power-supply rejection
82
dB
12
Clk Cycles
SAMPLING DYNAMICS
Conversion time
Acquisition time (2)
1.5
Clk Cycles
Throughput rate
75
kHz
DYNAMIC CHARACTERISTICS
Total harmonic distortion
VIN = 2.5 VPP at 1 kHz
–82
dB
SINAD
VIN = 2.5 VPP at 1 kHz
71
dB
Spurious-free dynamic range
VIN = 2.5 VPP at 1 kHz
86
dB
REFERENCE OUTPUT
Voltage range
0.05
Resistance
Current drain
VCC
CS = GND, fSAMPLE = 0 Hz
5
CS = VCC
5
At code 710h
8
fSAMPLE = 7.5 kHz
GΩ
GΩ
40
0.001
µA
µA
0.8
CS = VCC
V
3
µA
DIGITAL INPUT/OUTPUT
Logic family
Logic levels
CMOS
VIH
IIH = +5 µA
2.0
5.5
V
VIL
IIL = +5 µA
–0.3
0.8
V
VOH
IOH = –250 µA
VOL
IOL = 250 µA
Data format
(1)
(2)
2.1
V
0.4
V
Straight Binary
LSB means least significant bit. With VREF equal to 2.5 V, one LSB is 0.61 mV.
Not production tested
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ELECTRICAL CHARACTERISTICS: +VCC = 2.7 V (continued)
At –40°C to 85°C, +VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, and fCLK = 16 × fSAMPLE (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
POWER-SUPPLY REQUIREMENTS
Specified performance
VCC
See Notes
See Note
Quiescent current
Power down current
(3)
and
(4)
(4)
2.7
3.6
V
2.0
2.7
V
2.7
3.6
fSAMPLE = 7.5 kHz (5) (6)
fSAMPLE = 75 kHz (6)
200
CS = VCC
V
µA
20
325
µA
10
µA
85
°C
TEMPERATURE RANGE
Specified performance
(3)
(4)
(5)
(6)
4
–40
The maximum clock rate of the ADS7822 is less than 1.2 MHz in this power-supply range.
See the Typical Characteristics for more information.
fCLK = 1.2 MHz, CS = VCC for 145 clock cycles out of every 160.
See the Power Dissipation section for more information regarding lower sample rates.
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ELECTRICAL CHARACTERISTICS: +VCC = 5 V
At –40°C to 85°C, +VCC = 5 V, VREF = 5 V, fSAMPLE = 200 kHz, and fCLK = 16 × fSAMPLE (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUT
Full-scale input span
Absolute input range
+In – (–In)
0
VREF
V
+In – GND
–0.2
VCC + 0.2
V
–In – GND
–0.2
+1.0
V
Capacitance
25
pF
Leakage current
±1
µA
12
Bits
SYSTEM PERFORMANCE
Resolution
No missing codes
11
Integral linearity error
–2
Differential linearity error
Bits
+2
LSB (1)
±0.8
Offset error
–3
Gain error
–4
LSB
+3
LSB
+4
LSB
Noise
33
µVrms
Power-supply rejection
70
dB
12
Clk Cycles
SAMPLING DYNAMICS
Conversion time
Acquisition time (2)
1.5
Clk Cycles
Throughput rate
200
kHz
DYNAMIC CHARACTERISTICS
Total harmonic distortion
VIN = 5 VPP at 10 kHz
–78
dB
SINAD
VIN = 5 VPP at 10 kHz
71
dB
Spurious-free dynamic range
VIN = 5 VPP at 10 kHz
79
dB
REFERENCE OUTPUT
Voltage range
0.05
Resistance
CS = GND, fSAMPLE = 0 Hz
5
CS = VCC
5
At code 710h
Current drain
VCC
40
fSAMPLE = 12.5 kHz
GΩ
GΩ
100
0.001
µA
µA
2.5
CS = VCC
V
3
µA
DIGITAL INPUT/OUTPUT
Logic family
Logic levels
CMOS
VIH
IIH = +5 µA
3.0
5.5
V
VIL
IIL = +5 µA
–0.3
0.8
V
VOH
IOH = –250 µA
VOL
IOL = 250 µA
3.5
V
0.4
Data format
V
Straight Binary
POWER-SUPPLY REQUIREMENTS
VCC
Specified performance
Quiescent current
fSAMPLE = 200 kHz
Power down current
CS = VCC
4.75
320
5.25
V
550
µA
10
µA
85
°C
TEMPERATURE RANGE
Specified performance
(1)
(2)
–40
LSB means least significant bit. With VREF equal to 5 V, one LSB is 1.22 mV.
Not production tested
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PIN CONFIGURATION
DGK PACKAGE
(TOP VIEW)
VREF
1
+In
2
8
+VCC
7
DCLOCK
ADS7822
-In
3
6
DOUT
GND
4
5
CS/SHDN
PIN ASSIGNMENTS
PIN
NAME
DESCRIPTION
NO.
VREF
1
Reference input
+In
2
Noninverting input
–In
3
Inverting input. Connect to ground or to remote ground sense point.
GND
4
Ground
CS/SHDN
5
Chip select when low; shutdown mode when high.
DOUT
6
The serial output data word is comprised of 12 bits of data. In operation, the data are valid on the falling edge of
DCLOCK. The second clock pulse after the falling edge of CS enables the serial output. After one null bit, the
data are valid for the next edges.
DCLOCK
7
Data clock synchronizes the serial data transfer and determines conversion speed.
+VCC
8
Power supply
6
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TYPICAL CHARACTERISTICS
TA = 25°C, VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, fCLK = 16 × fSAMPLE (unless otherwise noted)
DIFFERENTIAL LINEARITY ERROR
vs CODE
1.00
1.00
0.75
0.75
Differential Linearity Error (LSB)
Integral Linearity Error (LSB)
INTEGRAL LINEARITY ERROR
vs CODE
0.50
0.25
0.00
-0.25
-0.50
-0.75
-1.00
0.50
0.25
0.00
-0.25
-0.50
-0.75
-1.00
0
2048
4095
0
2048
4095
Code
Figure 1.
Figure 2.
SUPPLY CURRENT
vs TEMPERATURE
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
350
120
300
100
Supply Current (nA)
Supply Current (mA)
Code
250
200
150
80
60
40
20
100
0
50
-50
0
-25
25
50
75
100
-50
0
-25
25
50
75
Temperature (°C)
Temperature (°C)
Figure 3.
Figure 4.
QUIESCENT CURRENT
vs VCC
MAXIMUM SAMPLE RATE
vs VCC
400
100
1000
Sample Rate (kHz)
Quiescent Current (mA)
350
300
250
200
100
10
150
100
1
1
2
3
4
5
1
VCC (V)
2
3
4
5
VCC (V)
Figure 5.
Figure 6.
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TYPICAL CHARACTERISTICS (continued)
TA = 25°C, VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, fCLK = 16 × fSAMPLE (unless otherwise noted)
CHANGE IN OFFSET
vs REFERENCE VOLTAGE
CHANGE IN OFFSET
vs TEMPERATURE
1.2
0.6
VCC = 5V
0.4
0.8
Delta from 25°C (LSB)
Change in Offset (LSB)
1.0
0.6
0.4
0.2
0.0
-0.2
-0.4
0.2
0
-0.2
-0.4
-0.6
-0.6
-0.8
1
2
3
Reference Voltage (V)
4
5
-50
0
25
50
75
100
75
100
Temperature (°C)
Figure 7.
Figure 8.
CHANGE IN GAIN
vs REFERENCE VOLTAGE
CHANGE IN GAIN
vs TEMPERATURE
2.5
0.15
V
= 5V
VCC
CC = 5V
2.0
0.10
1.5
Delta from 25°C (LSB)
Change in Gain (LSB)
-25
1.0
0.5
0.0
-0.5
0.05
0
-0.05
-0.10
-1.0
-0.15
-1.5
1
2
3
Reference Voltage (V)
4
-50
5
25
50
Figure 9.
Figure 10.
EFFECTIVE NUMBER OF BITS
vs REFERENCE VOLTAGE
PEAK-TO-PEAK NOISE
vs REFERENCE VOLTAGE
10
VCC = 5V
11.75
VCC = 5V
9
Peak-to-Peak Noise (LSB)
Effective Number of Bits (rms)
0
Temperature (°C)
12.00
11.50
11.25
11.00
10.75
10.50
10.25
8
7
6
5
4
3
2
1
10.00
0
0.1
1
10
0.1
1
10
Reference Voltage (V)
Reference Voltage (V)
Figure 11.
8
-25
Figure 12.
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TYPICAL CHARACTERISTICS (continued)
TA = 25°C, VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, fCLK = 16 × fSAMPLE (unless otherwise noted)
SPURIOUS FREE DYNAMIC RANGE AND
SIGNAL-TO-NOISE RATIO vs FREQUENCY
TOTAL HARMONIC DISTORTION
vs FREQUENCY
100
0
Spurious Free Dynamic Range
SFDR and SNR (dB)
80
70
60
Signal-to-Noise Ratio
50
40
30
20
-30
-40
-50
-60
-70
-80
-90
0
-100
10
100
1
10
100
Frequency (kHz)
Frequency (kHz)
Figure 13.
Figure 14.
SIGNAL-TO-(NOISE+DISTORTION)
vs FREQUENCY
SIGNAL-TO-(NOISE+DISTORTION)
vs INPUT LEVEL
Signal-to-(Noise Ratio + Distortion) (dB)
100
Signal-to-(Noise + Distortion) (dB)
-20
10
1
90
80
70
60
50
40
30
20
10
0
1
10
80
70
60
50
40
30
20
10
0
100
-40
-35
-30
-25
-20
-15
-10
-5
Frequency (kHz)
Input Level (dB)
Figure 15.
Figure 16.
REFERENCE CURRENT
vs SAMPLE RATE
REFERENCE CURRENT vs TEMPERATURE
(Code = 710h)
14
14
12
12
Reference Current (A)
Reference Current (mA)
-10
Total Harmonic Distortion (dB)
90
10
8
6
4
0
10
8
6
4
2
2
0
0
15
30
45
60
75
-50
-25
0
25
50
75
100
Temperature (°C)
Sample Rate (kHz)
Figure 17.
Figure 18.
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TYPICAL CHARACTERISTICS (continued)
TA = 25°C, VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 75 kHz, fCLK = 16 × fSAMPLE (unless otherwise noted)
POWER-SUPPLY REJECTION
vs RIPPLE FREQUENCY
0
-10
-20
POWER-SUPPLY REJECTION
vs RIPPLE FREQUENCY
0
VCC = 2.7V
Ripple = 500mVPP
VIN = 1.25VDC
VREF = 2.5V
-10
-20
-30
PSR (dB)
PSR (dB)
-30
VCC = 5V
Ripple = 500mVPP
VIN = 2.5VDC
VREF = 5V
-40
-50
-60
-40
-50
-60
-70
-70
PSR (dB) = 20log(500mV/DVO)
where DVO = change in digital result
-80
-90
1k
10k
100k
1M
PSR (dB) = 20log(500mV/DVO)
where DVO = change in digital result
-80
-90
10M
10
1
1k
10k
100k
Ripple Frequency (Hz)
Ripple Frequency (Hz)
Figure 19.
Figure 20.
1M
10M
CHANGE IN INTEGRAL LINEARITY
AND DIFFERENTIAL LINEARITY
vs REFERENCE VOLTAGE
Delta from +2.5V Reference (LSB)
0.20
VCC = 5V
0.15
Change in Integral
Linearity (LSB)
0.10
0.05
0.00
Change in Differential
Linearity (LSB)
-0.05
-0.10
1
2
3
4
5
Reference Voltage (V)
Figure 21.
10
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THEORY OF OPERATION
The ADS7822 is a classic successive approximation register (SAR) A/D converter. The architecture is based on
capacitive redistribution that inherently includes a sample/hold function. The converter is fabricated on a 0.6µ
CMOS process. The architecture and process allow the ADS7822 to acquire and convert an analog signal at up
to 200,000 conversions per second while consuming very little power.
The ADS7822 requires an external reference, an external clock, and a single power source (VCC). The external
reference can be any voltage between 50 mV and VCC. The value of the reference voltage directly sets the range
of the analog input. The reference input current depends on the conversion rate of the ADS7822.
The external clock can vary between 10 kHz (625 Hz throughput) and 3.2 MHz (200 kHz throughput). The duty
cycle of the clock is essentially unimportant as long as the minimum high and low times are at least 400 ns for a
supply range between 2.7 V to 3.6 V, or 125 ns for a supply range between 4.75 V to 5.25 V. The minimum clock
frequency is set by the leakage on the capacitors internal to the ADS7822.
The analog input is provided to two input pins: +In and –In. When a conversion is initiated, the differential input
on these pins is sampled on the internal capacitor array. While a conversion is in progress, both inputs are
disconnected from any internal function.
The digital result of the conversion is clocked out by the DCLOCK input and is provided serially, most significant
bit first, on the DOUT pin. The digital data that is provided on the DOUT pin is for the conversion currently in
progress—there is no pipeline delay. It is possible to continue to clock the ADS7822 after the conversion is
complete and to obtain the serial data least significant bit first. See the Digital Interface section for more
information.
ANALOG INPUT
The +In and –In input pins allow for a pseudo-differential input signal. Unlike some converters of this type, the
–In input is not resampled later in the conversion cycle. When the converter goes into the hold mode, the voltage
difference between +In and –In is captured on the internal capacitor array.
The range of the –In input is limited to –0.2 V to 1 V. Because of this, the differential input can be used to reject
only small signals that are common to both inputs. Thus, the –In input is best used to sense a remote signal
ground that may move slightly with respect to the local ground potential.
The input current on the analog inputs depends on a number of factors: sample rate, input voltage, source
impedance, and power-down mode. Essentially, the current into the ADS7822 charges the internal capacitor
array during the sample period. After this capacitance has been fully charged, there is no further input current.
The source of the analog input voltage must be able to charge the input capacitance (25 pF) to a 12-bit settling
level within 1.5 clock cycles. When the converter goes into the hold mode or while it is in the power-down mode,
the input impedance is greater than 1 GΩ.
Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, the
–In input should not drop below GND – 200 mV or exceed GND + 1 V. The +In input should always remain within
the range of GND – 200 mV to VCC + 200 mV. Outside of these ranges, the converter linearity may not meet
specifications.
REFERENCE INPUT
The external reference sets the analog input range. The ADS7822 operates with a reference in the range of
50 mV to VCC. There are several important implications of this.
As the reference voltage is reduced, the analog voltage weight of each digital output code is reduced. This is
often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 4096. This
means that any offset or gain error inherent in the A/D converter will appear to increase, in terms of LSB size, as
the reference voltage is reduced.
The noise inherent in the converter will also appear to increase with lower LSB size. With a 2.5-V reference, the
internal noise of the converter typically contributes only 0.32 LSB peak-to-peak of potential error to the output
code. When the external reference is 50 mV, the potential error contribution from the internal noise will be 50
times larger—16 LSBs. The errors due to the internal noise are gaussian in nature and can be reduced by
averaging consecutive conversion results.
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For more information regarding noise, consult the typical characteristic curves Effective Number of Bits vs
Reference Voltage and Peak-to-Peak Noise vs Reference Voltage. Note that the effective number of bits (ENOB)
figure is calculated based on the converter signal-to-(noise + distortion) ratio with a 1-kHz 0-dB input signal.
SINAD is related to ENOB as follows:
SINAD = 6.02 × ENOB + 1.76
With lower reference voltages, extra care should be taken to provide a clean layout including adequate
bypassing, a clean power supply, a low-noise reference, and a low-noise input signal. Because the LSB size is
lower, the converter will also be more sensitive to external sources of error such as nearby digital signals and
electromagnetic interference.
DIGITAL INTERFACE
Signal Levels
The digital inputs of the ADS7822 can accommodate logic levels up to 6 V regardless of the value of VCC. Thus,
the ADS7822 can be powered at 3V and still accept inputs from logic powered at 5 V.
The CMOS digital output (DOUT) will swing 0 V to VCC. If VCC is 3 V and this output is connected to a 5-V CMOS
logic input, then that IC may require more supply current than normal and may have a slightly longer propagation
delay.
Serial Interface
The ADS7822 communicates with microprocessors and other digital systems via a synchronous 3-wire serial
interface, as shown in Figure 22 and Table 1. The DCLOCK signal synchronizes the data transfer with each bit
being transmitted on the falling edge of DCLOCK. Most receiving systems will capture the bitstream on the rising
edge of DCLOCK. However, if the minimum hold time for DOUT is acceptable, the system can use the falling edge
of DCLOCK to capture each bit.
tCYC
CS/SHDN
Power
Down
tSUCS
DCLOCK
tCSD
DOUT
Hi-Z
Null
Bit
B11 B10 B9
B8
(MSB)
tSMPL
B7
B6
B5
B4
B3
B2
B1 B0
tCONV
Null
Bit
Hi-Z
(1)
B11 B10
B9
B8
tDATA
Note: (1) After completing the data transfer, if further clocks are applied with CS LOW,
the A/D will output LSB-First data then followed with zeroes indefinitely.
tCYC
CS/SHDN
tSUCS
Power Down
DCLOCK
tCSD
DOUT
Hi-Z
tSMPL
Null
Bit
Hi-Z
B11 B10 B9
(MSB)
B8
B7
B6
B5
B4
B3
B2
B1
B0
B1
B2
B3
tCONV
B4
B5
B6
B7
B8
B9 B10 B11
(1)
tDATA
Note: (1) After completing the data transfer, if further clocks are applied with CS LOW,
the A/D will output zeroes indefinitely.
tDATA: During this time, the bias current and the comparator power down and the reference input
becomes a high impedance node, leaving the CLK running to clock out LSB-first data or zeroes.
Figure 22. Basic Timing Diagrams
12
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Table 1. Timing Specifications (–40°C to 85°C)
SYMBOL
tSMPL (1)
Analog input sample time
tCONV
Conversion time
tCYC (1)
Cycle time
tCSD (1)
CS falling to DCLOCK low
tSUCS
(1)
VCC = 2.7 V
DESCRIPTION
MIN
TYP
1.5
MIN
2.0
1.5
TYP
Clk
Cycles
2.0
Clk
Cycles
Clk
Cycles
16
0
CS falling to DCLOCK rising
UNITS
MAX
12
16
0.03
1000
DCLOCK falling to current DOUT not valid
tdDO (1)
DCLOCK falling to next DOUT valid
130
tdis
MAX
12
thDO (1)
(1)
VCC = 5 V
0.03
15
0
ns
1000
µs
15
ns
200
85
150
ns
CS rising to DOUT high impedance
40
80
25
50
ns
ten (1)
DCLOCK falling to DOUT enabled
75
175
50
100
ns
tf (1)
DOUT fall time
90
200
70
100
ns
DOUT rise time
110
200
60
100
ns
tr
(1)
(1)
Not production tested
1.4V
3kW
DOUT
VOH
DOUT
VOL
Test Point
tr
100pF
CLOAD
tf
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
Load Circuit for tdDO, tr, and tf
Test Point
DCLOCK
VIL
VCC
DOUT
tdDO
VOH
DOUT
tdis Waveform 2, ten
3kW
tdis Waveform 1
100pF
CLOAD
VOL
thDO
Load Circuit for tdis and ten
Voltage Waveforms for DOUT Delay Times, tdDO
VIH
CS/SHDN
DOUT
Waveform 1(1)
CS/SHDN
90%
DCLOCK
1
2
tdis
DOUT
Waveform 2(2)
10%
VOL
DOUT
B11
ten
Voltage Waveforms for tdis
Voltage Waveforms for ten
NOTES: (1) Waveform 1 is for an output with internal conditions such that the output
is HIGH unless disabled by the output control.
(2) Waveform 2 is for an output with internal conditions such that the output
is LOW unless disabled by the output control.
Figure 23. Timing Diagrams and Test Circuits for the Parameters in Table 1
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A falling CS signal initiates the conversion and data transfer. The first 1.5 to 2.0 clock periods of the conversion
cycle are used to sample the input signal. After the second falling DCLOCK edge, DOUT is enabled and outputs a
low value for one clock period. For the next 12 DCLOCK periods, DOUT outputs the conversion result, most
significant bit first.
After the least significant bit (B0) has been output, subsequent clocks repeat the output data, but in a least
significant bit first format. After the most significant bit (B11) has been repeated, DOUT becomes high
impedance. Subsequent clocks have no effect on the converter. A new conversion is initiated only when CS is
taken high and returned low.
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Data Format
The output data from the ADS7822 is in straight binary format, as shown in Table 2. This table represents the
ideal output code for the given input voltage and does not include the effects of offset, gain error, or noise.
Table 2. Ideal Input Voltages and Output Codes
DESCRIPTION
DIGITAL OUTPUT
STRAIGHT BINARY
ANALOG VALUE
Full-scale range
VREF
Least significant bit (LSB)
VREF/4096
BINARY CODE
HEX CODE
Full-scale
VREF – 1 LSB
1111 1111 1111
FFF
Midscale
VREF/2
1000 0000 0000
800
VREF/2 – 1 LSB
0111 1111 1111
7FF
0V
0000 0000 0000
000
Midscale – 1 LSB
Zero
POWER DISSIPATION
The architecture of the converter, the semiconductor fabrication process, and a careful design allow the
ADS7822 to convert at up to a 75-kHz rate while requiring very little power. Still, for the absolute lowest power
dissipation, there are several things to keep in mind.
The power dissipation of the ADS7822 scales directly with conversion rate. So, the first step to achieving the
lowest power dissipation is to find the lowest conversion rate that will satisfy the requirements of the system.
In addition, the ADS7822 goes into power-down mode under two conditions: when the conversion is complete
and whenever CS is high (see Figure 22). Ideally, each conversion should occur as quickly as possible;
preferably, at a 1.2MHz clock rate. This way, the converter spends the longest possible time in the power-down
mode. This is very important since the converter not only uses power on each DCLOCK transition (as is typical
for digital CMOS components), but also uses some current for the analog circuitry, such as the comparator. The
analog section dissipates power continuously, until the power-down mode is entered.
Figure 24 shows the current consumption of the ADS7822 versus sample rate. For this graph, the converter is
clocked at 1.2 MHz regardless of the sample rate—CS is high for the remaining sample period. Figure 25 also
shows current consumption versus sample rate. However, in this case, the DCLOCK period is 1/16th of the
sample period—CS is high for one DCLOCK cycle out of every 16.
1000
Supply Current (mA)
TA = 25°C
fCLK = 1.2MHz
100
VCC = 5.0V
VREF = 5.0V
VCC = 2.7V
VREF = 2.5V
10
1
0.1
1
10
100
Sample Rate (kHz)
Figure 24. Maintaining fCLK at the Highest Possible Rate Allows the Supply Current to Drop Linearly with
the Sample Rate
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Supply Current (mA)
1000
100
10
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 16 · fSAMPLE
1
0.1
1
10
100
Sample Rate (kHz)
Figure 25. Scaling fCLK Reduces the Supply Current Only Slightly with the Sample Rate
There is an important distinction between the power-down mode that is entered after a conversion is complete
and the full power-down mode that is enabled when CS is high. While both shut down the analog section, the
digital section is completely shutdown only when CS is high. Thus, if CS is left low at the end of a conversion and
the converter is continually clocked, the power consumption will not be as low as when CS is high; see Figure 26
for more information.
10.0
Supply Current (mA)
8.0
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 16 · fSAMPLE
6.0
CS LOW (GND)
4.0
2.0
0.0
CS HIGH (VCC)
0.050
0.00
0.1
1
10
100
Sample Rate (kHz)
Figure 26. Shutdown Current with CS High is Typically 50nA, Regardless of the Clock. Shutdown Current
with CS Low varies with Sample Rate.
Power dissipation can also be reduced by lowering the power-supply voltage and the reference voltage. The
ADS7822 operates over a VCC range of 2.0 V to 5.25 V. It will run up to a 200-kHz throughput rate over a supply
range of 4.75 V to 5.25 V; therefore, it can be clocked at up to 3.2 MHz. However, at voltages below 2.7 V, the
converter does not run at a 75-kHz sample rate. See the Typical Characteristic curves for more information
regarding power-supply voltage and maximum sample rate.
Short Cycling
Another way of saving power is to use the CS signal to short-cycle the conversion. Because the ADS7822 places
the latest data bit on the DOUT line as it is generated, the converter can easily be short-cycled. This term means
that the conversion can be terminated at any time. For example, if only eight bits of the conversion result are
needed, then the conversion can be terminated (by pulling CS high) after the eighth bit has been clocked out.
This technique can be used to lower the power dissipation (or to increase the conversion rate) in those
applications where an analog signal is being monitored until some condition becomes true. For example, if the
signal is outside a predetermined range, the full 12-bit conversion result may not be needed. If so, the conversion
can be terminated after the first n-bits, where n might be as low as 3 or 4. This results in lower power dissipation
in both the converter and the rest of the system, because they spend more time in the power-down mode.
16
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LAYOUT
For optimum performance, care should be taken with the physical layout of the ADS7822 circuitry. This is
particularly true if the reference voltage is low and/or the conversion rate is high. At a 75 kHz conversion rate, the
ADS7822 makes a bit decision every 830ns. If the supply range is limited to 4.75 V to 5.25 V, then up to a
200-kHz conversion rate can be used, which reduces the bit decision time to 312 ns. That is, for each
subsequent bit decision, the digital output must be updated with the results of the last bit decision, the capacitor
array appropriately switched and charged, and the input to the comparator settled to a 12-bit level all within one
clock cycle.
The basic SAR architecture is sensitive to spikes on the power supply, reference, and ground connections that
occur just prior to latching the comparator output. Thus, during any single conversion for an n-bit SAR converter,
there are n windows in which large external transient voltages can easily affect the conversion result. Such
spikes might originate from switching power supplies, digital logic, and high-power devices, to name a few. This
particular source of error can be very difficult to track down if the glitch is almost synchronous to the converter
DCLOCK signal because the phase difference between the two changes with time and temperature, causing
sporadic misoperation.
With this in mind, power to the ADS7822 should be clean and well-bypassed. A 0.1-µF ceramic bypass capacitor
should be placed as close to the ADS7822 package as possible. In addition, a 1-µF to 10-µF capacitor and a 5-Ω
or 10-Ω series resistor can be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1-µF capacitor. Again, a series resistor and large capacitor
can be used to lowpass filter the reference voltage. If the reference voltage originates from an op amp, be careful
that the op amp can drive the bypass capacitor without oscillation (the series resistor can help in this case). Keep
in mind that while the ADS7822 draws very little current from the reference on average, there are still
instantaneous current demands placed on the external reference circuitry.
Also, keep in mind that the ADS7822 offers no inherent rejection of noise or voltage variation in regards to the
reference input. This is of particular concern when the reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital results. While high-frequency noise can be filtered out as
described in the previous paragraph, voltage variation due to the line frequency (50 Hz or 60 Hz), can be difficult
to remove.
The GND pin on the ADS7822 should be placed on a clean ground point. In many cases, this will be the analog
ground. Avoid connecting the GND pin too close to the grounding point for a microprocessor, microcontroller, or
digital signal processor. If needed, run a ground trace directly from the converter to the power-supply connection
point. The ideal layout will include an analog ground plane for the converter and associated analog circuitry.
APPLICATION CIRCUITS
Figure 27 and Figure 28 show some typical application circuits for the ADS7822. Figure 27 uses an ADS7822
and a multiplexer to provide for a flexible data acquisition circuit. A resistor string provides for various voltages at
the multiplexer input. The selected voltage is buffered and driven into VREF. As shown in Figure 27, the input
range of the ADS7822 is programmable to 100 mV, 200 mV, 300 mV, or 400 mV. The 100-mV range would be
useful for sensors such as the thermocouple shown.
Figure 28 shows a basic data acquisition system. The ADS7822 input range is 0 V to VCC, as the reference input
is connected directly to the power supply. The 5-Ω resistor and 1-µF to 10-µF capacitor filter the microcontroller
noise on the supply, as well as any high-frequency noise from the supply itself. The exact values should be
picked such that the filter provides adequate rejection of the noise.
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+3V
+3V
+3V
R8
26kW
D1
R1
150kW
R9
1kW
OPA237
C2
0.1mF
R3
500kW
R6
1MW
R2
59kW
TC1
0.4V
R7
5W
0.3V
U2
C1
10mF
VREF
Mux
0.2V
DCLOCK
C3
0.1mF
TC2
ADS7822
DOUT
A0
CS/SHDN
A1
Thermocouple
TC3
R4
1kW
C4
10mF
ISO Thermal Block
R10
1kW
U1
R5
500W
U3
C5
0.1mF
R11
1kW
0.1V
R12
1kW
P
3-Wire
Interface
U4
Figure 27. Thermocouple Application Using a Mux to Scale the Input Range of the ADS7822
+2.7V to +3.6V
5W
+ 1mF to
10mF
ADS7822
VREF
VCC
+In
CS
-In
DOUT
0.1mF
GND
+ 1mF to
10mF
Microcontroller
DCLOCK
Figure 28. Basic Data Acquisition System
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PACKAGE OPTION ADDENDUM
www.ti.com
16-Aug-2012
PACKAGING INFORMATION
Orderable Device
ADS7822IDGKRQ1
Status
(1)
Package Type Package
Drawing
ACTIVE
VSSOP
DGK
Pins
Package Qty
8
2500
Eco Plan
(2)
Green (RoHS
& no Sb/Br)
Lead/
Ball Finish
Call TI
MSL Peak Temp
(3)
Samples
(Requires Login)
Level-2-260C-1 YEAR
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
OTHER QUALIFIED VERSIONS OF ADS7822-Q1 :
• Catalog: ADS7822
NOTE: Qualified Version Definitions:
• Catalog - TI's standard catalog product
Addendum-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
16-Aug-2012
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
ADS7822IDGKRQ1
Package Package Pins
Type Drawing
VSSOP
DGK
8
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
2500
330.0
12.4
Pack Materials-Page 1
5.3
B0
(mm)
K0
(mm)
P1
(mm)
3.4
1.4
8.0
W
Pin1
(mm) Quadrant
12.0
Q1
PACKAGE MATERIALS INFORMATION
www.ti.com
16-Aug-2012
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
ADS7822IDGKRQ1
VSSOP
DGK
8
2500
367.0
367.0
35.0
Pack Materials-Page 2
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