ADS7844
12-Bit, 8-Channel Serial Output Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
SINGLE SUPPLY: 2.7V to 5V
8-CHANNEL SINGLE-ENDED OR
4-CHANNEL DIFFERENTIAL INPUT
UP TO 200kHz CONVERSION RATE
±1 LSB MAX INL AND DNL
NO MISSING CODES
72dB SINAD
SERIAL INTERFACE
20-LEAD QSOP AND
20-LEAD SSOP PACKAGES
ALTERNATE SOURCE FOR MAX147
DESCRIPTION
The ADS7844 is an 8-channel, 12-bit sampling analog-to-
digital converter (ADC) with a synchronous serial interface.
Typical power dissipation is 3mW at a 200kHz throughput
rate and a +5V supply. The reference voltage (VREF) can be
varied between 100mV and VCC, providing a corresponding
input voltage range of 0V to VREF. The device includes a
shutdown mode that reduces power dissipation to under
1µW. The ADS7844 is ensured down to 2.7V operation.
Low power, high speed, and onboard multiplexer make the
ADS7844 ideal for battery-operated systems such as personal
digital assistants, portable multichannel data loggers, and
measurement equipment. The serial interface also provides
low-cost isolation for remote data acquisition. The ADS7844
is available in a 20-lead QSOP package and the MAX147
equivalent 20-lead SSOP package and is ensured over the
–40°C to +85°C temperature range.
APPLICATIONS
DATA ACQUISITION
TEST AND MEASUREMENT
INDUSTRIAL PROCESS CONTROL
PERSONAL DIGITAL ASSISTANTS
BATTERY-POWERED SYSTEMS
CDAC
SAR
Comparator
Eight
Channel
Multiplexer Serial
Interface
and
Control
CH4
CH5
CH6
CH7
COM
VREF
CH0
CH1
CH2
CH3 CS
SHDN
DIN
DOUT
BUSY
DCLK
ADS7844
ADS7844
SBAS100A – JANUARY 1998 – REVISED OCTOBER 2003
www.ti.com
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
Copyright © 1998-2003, Texas Instruments Incorporated
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.
ADS7844
2SBAS100A
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SPECIFICATION: +5V
At TA = –40°C to +85°C, +VCC = +5V, VREF = +5V, fSAMPLE = 200kHz, and fCLK = 16 • fSAMPLE = 3.2MHz, unless otherwise noted.
ADS7844E, N ADS7844EB, NB
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
ANALOG INPUT
Full-Scale Input Span Positive Input - Negative Input 0 VREF ✻✻V
Absolute Input Range Positive Input –0.2 +VCC +0.2 ✻✻V
Negative Input –0.2 +1.25 ✻✻V
Capacitance 25 pF
Leakage Current ±1µA
SYSTEM PERFORMANCE
Resolution 12 Bits
No Missing Codes 12 Bits
Integral Linearity Error ±2±1 LSB(1)
Differential Linearity Error ±0.8 ±0.5 ±1 LSB
Offset Error ±3LSB
Offset Error Match 0.15 1.0 ✻✻ LSB
Gain Error ±4±3 LSB
Gain Error Match 0.1 1.0 ✻✻ LSB
Noise 30 µVrms
Power Supply Rejection 70 dB
SAMPLING DYNAMICS
Conversion Time 12 Clk Cycles
Acquisition Time 3 Clk Cycles
Throughput Rate 200 kHz
Multiplexer Settling Time 500 ns
Aperture Delay 30 ns
Aperture Jitter 100 ps
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2) VIN = 5VPP at 10kHz –76 –78 dB
Signal-to-(Noise + Distortion) VIN = 5VPP at 10kHz 71 72 dB
Spurious Free Dynamic Range VIN = 5VPP at 10kHz 76 78 dB
Channel-to-Channel Isolation VIN = 5VPP at 50kHz 120 dB
REFERENCE INPUT
Range 0.1 +VCC ✻✻V
Resistance DCLK Static 5 G
Input Current 45 100 ✻✻ µA
fSAMPLE = 12.5kHz 2.5 µA
DCLK Static 0.001 3 ✻✻ µA
DIGITAL INPUT/OUTPUT
Logic Family CMOS
Logic Levels
VIH | IIH | +5µA 3.0 5.5 ✻✻V
VIL | IIL | +5µA –0.3 +0.8 ✻✻V
VOH IOH = –250µA 3.5 V
VOL IOL = 250µA 0.4 V
Data Format Straight Binary
POWER SUPPLY REQUIREMENTS
+VCC Specified Performance 4.75 5.25 ✻✻V
Quiescent Current 550 900 µA
fSAMPLE = 12.5kHz 300 µA
Power-Down Mode(3), CS = +VCC 3µA
Power Dissipation 4.5 mW
TEMPERATURE RANGE
Specified Performance –40 +85 ✻✻°C
Same specifications as ADS7844E, ADS7844N.
NOTE: (1) LSB means Least Significant Bit. With VREF equal to +5.0V, one LSB is 1.22mV. (2) First five harmonics of the test frequency. (3) Auto power-down mode
(PD1 = PD0 = 0) active or SHDN = GND.
ADS7844 3
SBAS100A www.ti.com
SPECIFICATION: +2.7V
At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
ADS7844E, N ADS7844EB, NB
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
ANALOG INPUT
Full-Scale Input Span Positive Input - Negative Input 0 VREF ✻✻V
Absolute Input Range Positive Input –0.2 +VCC +0.2 ✻✻V
Negative Input –0.2 +0.2 ✻✻V
Capacitance 25 pF
Leakage Current ±1µA
SYSTEM PERFORMANCE
Resolution 12 Bits
No Missing Codes 12 Bits
Integral Linearity Error ±2±1 LSB(1)
Differential Linearity Error ±0.8 ±0.5 ±1 LSB
Offset Error ±3LSB
Offset Error Match 0.15 1.0 ✻✻ LSB
Gain Error ±4±3 LSB
Gain Error Match 0.1 1.0 ✻✻ LSB
Noise 30 µVrms
Power Supply Rejection 70 dB
SAMPLING DYNAMICS
Conversion Time 12 Clk Cycles
Acquisition Time 3 Clk Cycles
Throughput Rate 125 kHz
Multiplexer Settling Time 500 ns
Aperture Delay 30 ns
Aperture Jitter 100 ps
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2) VIN = 2.5VPP at 10kHz –75 –77 dB
Signal-to-(Noise + Distortion) VIN = 2.5VPP at 10kHz 71 72 dB
Spurious Free Dynamic Range VIN = 2.5VPP at 10kHz 78 80 dB
Channel-to-Channel Isolation VIN = 2.5VPP at 50kHz 100 dB
REFERENCE INPUT
Range 0.1 +VCC ✻✻V
Resistance DCLK Static 5 G
Input Current 13 40 ✻✻ µA
fSAMPLE = 12.5kHz 2.5 µA
DCLK Static 0.001 3 ✻✻ µA
DIGITAL INPUT/OUTPUT
Logic Family CMOS
Logic Levels
VIH | IIH | +5µA+V
CC • 0.7 5.5 ✻✻V
VIL | IIL | +5µA –0.3 +0.8 ✻✻V
VOH IOH = –250µA+V
CC • 0.8 V
VOL IOL = 250µA 0.4 V
Data Format Straight Binary
POWER SUPPLY REQUIREMENTS
+VCC Specified Performance 2.7 3.6 ✻✻V
Quiescent Current 280 650 ✻✻ µA
fSAMPLE = 12.5kHz 220 µA
Power-Down Mode(3), CS = +VCC 3µA
Power Dissipation 1.8 mW
TEMPERATURE RANGE
Specified Performance –40 +85 ✻✻°C
Same specifications as ADS7844E, ADS7844N.
NOTE: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 610mV. (2) First five harmonics of the test frequency. (3) Auto power-down mode
(PD1 = PD0 = 0) active or SHDN = GND.
ADS7844
4SBAS100A
www.ti.com
PIN CONFIGURATION
Top View
PIN DESCRIPTIONS
PIN NAME DESCRIPTION
1 CH0 Analog Input Channel 0.
2 CH1 Analog Input Channel 1.
3 CH2 Analog Input Channel 2.
4 CH3 Analog Input Channel 3.
5 CH4 Analog Input Channel 4.
6 CH5 Analog Input Channel 5.
7 CH6 Analog Input Channel 6.
8 CH7 Analog Input Channel 7.
9 COM
Ground reference for analog inputs. Sets zero code
voltage in single ended mode. Connect this pin to ground
or ground reference point.
10 SHDN Shutdown. When LOW, the device enters a very low
power shutdown mode.
11 VREF Voltage Reference Input. See Specification Table for
ranges.
12 +VCC Power Supply, 2.7V to 5V.
13 GND Ground
14 GND Ground
15 DOUT Serial Data Output. Data is shifted on the falling edge of
DCLK. This output is high impedance when
CS is high.
16 BUSY Busy Output. Busy goes low when the DIN control bits
are being read and also when the device is converting.
The Output is high impedance when CS is High.
17 DIN Serial Data Input. If CS is LOW, data is latched on rising
edge of DCLK.
18 CS Chip Select Input. Active LOW. Data will not be clocked
into DIN unless CS is low. When CS is high DOUT is high
impedance.
19 CLK External Clock Input. The clock speed determines the
conversion rate by the equation fCLK = 16 • fSAMPLE.
20 +VCC Power Supply
1
2
3
4
5
6
7
8
9
10
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
SHDN
+V
CC
D
CLK
CS
D
IN
BUSY
D
OUT
GND
GND
+V
CC
V
REF
20
19
18
17
16
15
14
13
12
11
ADS7844
MINIMUM
RELATIVE MAXIMUM SPECIFIED
ACCURACY GAIN ERROR TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT (LSB) (LSB) RANGE PACKAGE-LEAD DESIGNATOR NUMBER MEDIA, QUANTITY
ADS7844E ±2±440°C to +85°C QSOP-20 DBQ ADS7844E Rails, 56
" " " " " " ADS7844E/2K5 Tape and Reel, 2500
ADS7844N " " " SSOP-20 DB ADS7844N Rails, 68
" " " " " " ADS7844N/1K Tape and Reel,1000
ADS7844EB ±1±340°C to +85°C QSOP-20 DBQ ADS7844EB Rails, 56
" " " " " " ADS7844EB/2K5 Tape and Reel, 2500
ADS7844NB " " " SSOP-20 DB ADS7844NB Rails, 68
" " " " " " ADS7844NB/1K Tape and Reel, 1000
NOTES: (1) For the most current package and ordering information, see the Package Option Addendum at the end of this data sheet.
PACKAGE/ORDERING INFORMATION(1)
ABSOLUTE MAXIMUM RATINGS(1)
+VCC to GND ........................................................................ 0.3V to +6V
Analog Inputs to GND ............................................ 0.3V to +VCC + 0.3V
Digital Inputs to GND ........................................................... 0.3V to +6V
Power Dissipation .......................................................................... 250mW
Maximum Junction Temperature................................................... +150°C
Operating Temperature Range ........................................40°C to +85°C
Storage Temperature Range .........................................65°C to +150°C
Lead Temperature (soldering, 10s)............................................... +300°C
NOTE: (1) Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to absolute maximum
conditions for extended periods may affect device reliability. ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Texas Instru-
ments 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.
ADS7844 5
SBAS100A www.ti.com
TYPICAL PERFORMANCE CURVES:+5V
At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 200kHz, and fCLK = 16 fSAMPLE = 3.2MHz, unless otherwise noted.
0
20
40
60
80
100
120
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1,123Hz, 0.2dB)
0 10025 7550
Frequency (kHz)
Amplitude (dB)
0
20
40
60
80
100
120
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 10.3kHz, 0.2dB)
0 10025 7550
Frequency (kHz)
Amplitude (dB)
SIGNAL-TO-NOISE RATIO AND SIGNAL-TO-
(NOISE+DISTORTION) vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
SNR and SINAD (dB)
74
73
72
71
70
69
68
SINAD
SNR
SPURIOUS FREE DYNAMIC RANGE AND TOTAL
HARMONIC DISTORTION vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
SFDR (dB)
THD (dB)
85
80
75
70
65
85
80
75
70
65
THD
SFDR
12.0
11.8
11.6
11.4
11.2
11.0
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
Effective Number of Bits
CHANGE IN SIGNAL-TO-(NOISE+DISTORTION)
vs TEMPERATURE
2040 100
Temperature (°C)
Delta from +25°C (dB)
0.4
0.2
0.0
0.2
0.4
0.6
0.6
020 40 60 80
fIN = 10kHz, 0.2dB
ADS7844
6SBAS100A
www.ti.com
0
20
40
60
80
100
120
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1,129Hz, 0.2dB)
0 62.515.6 46.931.3
Frequency (kHz)
Amplitude (dB)
0
20
40
60
80
100
120
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 10.6kHz, 0.2dB)
0 62.515.6 46.931.3
Frequency (kHz)
Amplitude (dB)
SIGNAL-TO-NOISE RATIO AND SIGNAL-TO-
(NOISE+DISTORTION) vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
SNR and SINAD (dB)
78
74
70
66
62
58
54
SINAD
SNR
THD
SFDR
SPURIOUS FREE DYNAMIC RANGE AND TOTAL
HARMONIC DISTORTION vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
SFDR (dB)
THD (dB)
90
85
80
75
70
65
60
55
50
90
85
80
75
70
65
60
55
50
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
101 100
Input Frequency (kHz)
Effective Number of Bits
12.0
11.5
11.0
10.5
10.0
9.5
9.0
TYPICAL PERFORMANCE CURVES:+2.7V
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 fSAMPLE = 2MHz, unless otherwise noted.
CHANGE IN SIGNAL-TO-(NOISE+DISTORTION)
vs TEMPERATURE
2040 100
Temperature (˚C)
Delta from +25°C (dB)
0.2
0.0
0.2
0.4
0.6
0.8
0.4
020 40 60 80
fIN = 10kHz, 0.2dB
ADS7844 7
SBAS100A www.ti.com
SUPPLY CURRENT vs TEMPERATURE
2040 10020 0 40
Temperature (˚C)
Supply Current (µA)
400
350
300
250
200
150
100 60 80
POWER DOWN SUPPLY CURRENT
vs TEMPERATURE
2040 10020 0 40
Temperature (˚C)
Supply Current (nA)
140
120
100
80
60
40
20 60 80
Output Code
1.00
0.75
0.50
0.25
0.00
0.25
0.50
0.75
1.00
INTEGRAL LINEARITY ERROR vs CODE
800
H
FFF
H
000
H
ILE (LSB)
Output Code
1.00
0.75
0.50
0.25
0.00
0.25
0.50
0.75
1.00
DIFFERENTIAL LINEARITY ERROR vs CODE
800
H
FFF
H
000
H
DLE (LSB)
CHANGE IN GAIN vs TEMPERATURE
2040 10020 0 40
Temperature (˚C)
Delta from +25˚C (LSB)
0.15
0.10
0.05
0.00
0.05
0.10
0.15 60 80
CHANGE IN OFFSET vs TEMPERATURE
2040 10020 0 40
Temperature (˚C)
Delta from +25˚C (LSB)
0.6
0.4
0.2
0.0
0.2
0.4
0.6 60 80
TYPICAL PERFORMANCE CURVES:+2.7V (CONT)
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 fSAMPLE = 2MHz, unless otherwise noted.
ADS7844
8SBAS100A
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REFERENCE CURRENT vs SAMPLE RATE
750 12525 50 100
Sample Rate (kHz)
Reference Current (µA)
14
12
10
8
6
4
2
0
REFERENCE CURRENT vs TEMPERATURE
2040 10020 0 40
Temperature (˚C)
Reference Current (µA)
18
16
14
12
10
8
660 80
SUPPLY CURRENT vs +V
CC
3.5252.5 4
+V
CC
(V)
Supply Current (µA)
320
300
280
260
240
220
200
180 4.53
f
SAMPLE
= 12.5kHz
V
REF
= +V
CC
MAXIMUM SAMPLE RATE vs +V
CC
3.5252.5 4
+V
CC
(V)
Sample Rate (Hz)
1M
100k
10k
1k 4.53
V
REF
= +V
CC
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 fSAMPLE = 2MHz, unless otherwise noted.
ADS7844 9
SBAS100A www.ti.com
THEORY OF OPERATION
The ADS7844 is a classic successive approximation register
(SAR) analog-to-digital (A/D) converter. The architecture is
based on capacitive redistribution which inherently includes
a sample/hold function. The converter is fabricated on a
0.6µs CMOS process.
The basic operation of the ADS7844 is shown in Figure 1.
The device requires an external reference and an external
clock. It operates from a single supply of 2.7V to 5.25V. The
external reference can be any voltage between 100mV and
+VCC. The value of the reference voltage directly sets the
input range of the converter. The average reference input
current depends on the conversion rate of the ADS7844.
The analog input to the converter is differential and is
provided via an eight-channel multiplexer. The input can be
provided in reference to a voltage on the COM pin (which
is generally ground) or differentially by using four of the
eight input channels (CH0 - CH7). The particular configura-
tion is selectable via the digital interface.
A2 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7
00 0+ININ
00 1 +ININ
01 0 +ININ
01 1 +ININ
10 0IN +IN
10 1 IN +IN
11 0 IN +IN
11 1 IN +IN
TABLE II. Differential Channel Control (SGL/DIF LOW).
TABLE I. Single-Ended Channel Selection (SGL/DIF HIGH).
FIGURE 1. Basic Operation of the ADS7844.
A2 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM
000+IN IN
100 +IN IN
001 +IN IN
101 +IN IN
010 +IN IN
110 +IN IN
011 +IN IN
111 +ININ
ANALOG INPUT
Figure 2 shows a block diagram of the input multiplexer on
the ADS7844. The differential input of the converter is
derived from one of the eight inputs in reference to the COM
pin or four of the eight inputs. Table I and Table II show the
relationship between the A2, A1, A0, and SGL/DIF control
bits and the configuration of the analog multiplexer. The
control bits are provided serially via the DIN pin, see the
Digital Interface section of this data sheet for more details.
When the converter enters the hold mode, the voltage
difference between the +IN and –IN inputs (see Figure 2) is
captured on the internal capacitor array. The voltage on the
–IN input is limited between –0.2V and 1.25V, allowing the
input to reject small signals which are common to both the
+IN and –IN input. The +IN input has a range of –0.2V to
+VCC + 0.2V.
The input current on the analog inputs depends on the
conversion rate of the device. During the sample period, the
source must charge the internal sampling capacitor (typi-
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
SHDN
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
+VCC
DCLK
CS
DIN
BUSY
DOUT
GND
GND
+VCC
VREF
Serial/Conversion Clock
Chip Select
Serial Data In
Serial Data Out
+2.7V to +5V
1µF to 10µF
ADS7844
Single-ended
or differential
analog inputs
1µF to 10µF
0.1µF
ADS7844
10 SBAS100A
www.ti.com
cally 25pF). After the capacitor has been fully charged, there
is no further input current. The rate of charge transfer from
the analog source to the converter is a function of conversion
rate.
REFERENCE INPUT
The external reference sets the analog input range. The
ADS7844 will operate with a reference in the range of
100mV to +VCC. Keep in mind that the analog input is the
difference between the +IN input and the –IN input as shown
in Figure 2. For example, in the single-ended mode, a 1.25V
reference, and with the COM pin grounded, the selected input
channel (CH0 - CH7) will properly digitize a signal in the
range of 0V to 1.25V. If the COM pin is connected to 0.5V,
the input range on the selected channel is 0.5V to 1.75V.
There are several critical items concerning the reference input
and its wide voltage range. As the reference voltage is re-
duced, the analog voltage weight of each digital output code
is also reduced. This is often referred to as the LSB (least
significant bit) size and is equal to the reference voltage
divided by 4096. 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. For example, if the offset of a
given converter is 2 LSBs with a 2.5V reference, then it will
typically be 10 LSBs with a 0.5V reference. In each case, the
actual offset of the device is the same, 1.22mV.
Likewise, the noise or uncertainty of the digitized output will
increase with lower LSB size. With a reference voltage of
100mV, the LSB size is 24µV. This level is below the
internal noise of the device. As a result, the digital output
code will not be stable and vary around a mean value by a
number of LSBs. The distribution of output codes will be
gaussian and the noise can be reduced by simply averaging
consecutive conversion results or applying a digital filter.
With a lower reference voltage, care should be taken to
provide a clean layout including adequate bypassing, a clean
(low noise, low ripple) 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 nearby digital
signals and electromagnetic interference.
The voltage into the VREF input is not buffered and directly
drives the capacitor digital-to-analog converter (CDAC)
portion of the ADS7844. Typically, the input current is
13µA with a 2.5V reference. This value will vary by
microamps depending on the result of the conversion. The
reference current diminishes directly with both conversion
rate and reference voltage. As the current from the reference
is drawn on each bit decision, clocking the converter more
quickly during a given conversion period will not reduce
overall current drain from the reference.
DIGITAL INTERFACE
Figure 3 shows the typical operation of the ADS7844’s
digital interface. This diagram assumes that the source of the
digital signals is a microcontroller or digital signal processor
with a basic serial interface (note that the digital inputs are
over-voltage tolerant up to 5.5V, regardless of +VCC). Each
communication between the processor and the converter
consists of eight clock cycles. One complete conversion can
be accomplished with three serial communications, for a
total of 24 clock cycles on the DCLK input.
The first eight clock cycles are used to provide the control
byte via the DIN pin. When the converter has enough
information about the following conversion to set the input
multiplexer appropriately, it enters the acquisition (sample)
mode. After three more clock cycles, the control byte is
complete and the converter enters the conversion mode. At
this point, the input sample/hold goes into the hold mode.
The next twelve clock cycles accomplish the actual analog-
to-digital conversion. A thirteenth clock cycle is needed for
the last bit of the conversion result. Three more clock cycles
are needed to complete the last byte (DOUT will be LOW).
These will be ignored by the converter.
Control Byte
Also shown in Figure 3 is the placement and order of the
control bits within the control byte. Tables III and IV give
detailed information about these bits. The first bit, the ‘S’ bit,
must always be HIGH and indicates the start of the control
byte. The ADS7844 will ignore inputs on the DIN pin until
the start bit is detected. The next three bits (A2 - A0) select
the active input channel or channels of the input multiplexer
(see Tables I and II and Figure 2).
FIGURE 2. Simplified Diagram of the Analog Input.
Converter
+IN
IN
CH0
CH1
CH2
CH3
A2-A0
(shown 000B)(1)
SGL/DIF
(shown HIGH)
CH4
CH5
CH6
CH7
COM
NOTE: (1) See T ruth Tables, Table 1,
and Table 2 for address coding.
ADS7844 11
SBAS100A www.ti.com
FIGURE 3. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK delay required with dedicated
serial port.
1
DCLK
CS
81
11
DOUT
BUSY
SDIN
CONTROL BITS
S
CONTROL BITS
1098765 43210 11 10 9
81 18
FIGURE 4. Conversion Timing, 16-Clocks per Conversion, 8-bit Bus Interface. No DCLK delay required with dedicated
serial port.
Bit 7 Bit 0
(MSB) Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 (LSB)
SA2A1A0
SGL/DIF
PD1 PD0
TABLE III. Order of the Control Bits in the Control Byte.
TABLE IV.Descriptions of the Control Bits within the
Control Byte.
BIT NAME DESCRIPTION
7 S Start Bit. Control byte starts with first HIGH bit on
DIN. A new control byte starts with every 15th clock
cycle.
6 - 4 A2 - A0 Channel Select Bits. Along with the SGL/DIF bit,
these bits control the setting of the multiplexer input
as detailed in Tables I and II.
3Not Used.
2 SGL/DIF Single-Ended/Differential Select Bit. Along with bits
A2 - A0, this bit controls the setting of the multiplexer
input as detailed in Tables I and II.
1 - 0 PD1 - PD0 Power-Down Mode Select Bits. See Table V for
details.
The SGL/DIF bit controls the multiplexer input mode: either
single-ended (HIGH) or differential (LOW). In single-ended
mode, the selected input channel is referenced to the COM
pin. In differential mode, the two selected inputs provide a
differential input. See Tables I and II and Figure 2 for more
information. The last two bits (PD1 - PD0) select the power-
down mode as shown in Table V. If both inputs are HIGH,
the device is always powered up. If both inputs are LOW, the
device enters a power-down mode between conversions.
When a new conversion is initiated, the device will resume
normal operation instantly—no delay is needed to allow the
device to power up and the very first conversion will be
valid.
16-Clocks per Conversion
The control bits for conversion n+1 can be overlapped with
conversion ‘n’ to allow for a conversion every 16 clock
cycles, as shown in Figure 4. This figure also shows possible
serial communication occurring with other serial peripherals
between each byte transfer between the processor and the
converter. This is possible provided that each conversion
completes within 1.6ms of starting. Otherwise, the signal
that has been captured on the input sample/hold may droop
enough to affect the conversion result. In addition, the
ADS7844 is fully powered while other serial communica-
tions are taking place.
tACQ
AcquireIdle Conversion Idle
1
DCLK
CS
81
11
DOUT
BUSY
(MSB)
(START)
(LSB)
A2S
DIN
A1 A0
SGL/
DIF
PD1 PD0
1098765 4 3210 Zero Filled...
81 8
ADS7844
12 SBAS100A
www.ti.com
Digital Timing
Figure 5 and Tables VI and VII provide detailed timing for
the digital interface of the ADS7844.
15-Clocks per Conversion
Figure 6 provides the fastest way to clock the ADS7844.
This method will not work with the serial interface of most
microcontrollers and digital signal processors as they are
generally not capable of providing 15 clock cycles per serial
transfer. However, this method could be used with field
programmable gate arrays (FPGAs) or application specific
integrated circuits (ASICs). Note that this effectively in-
creases the maximum conversion rate of the converter be-
yond the values given in the specification tables, which
assume 16 clock cycles per conversion.
PD1 PD0 Description
0 0 Power-down between conversions. When each
conversion is finished, the converter enters a low
power mode. At the start of the next conversion,
the device instantly powers up to full power. There
is no need for additional delays to assure full
operation and the very first conversion is valid.
0 1 Reserved for future use.
1 0 Reserved for future use.
1 1 No power-down between conversions, device al-
ways powered.
TABLE V. Power-Down Selection.
SYMBOL DESCRIPTION MIN TYP MAX UNITS
tACQ Acquisition Time 1.5 µs
tDS DIN Valid Prior to DCLK Rising 100 ns
tDH DIN Hold After DCLK HIGH 10 ns
tDO DCLK Falling to DOUT Valid 200 ns
tDV CS Falling to DOUT Enabled 200 ns
tTR CS Rising to DOUT Disabled 200 ns
tCSS CS Falling to First DCLK Rising 100 ns
tCSH CS Rising to DCLK Ignored 0 ns
tCH DCLK HIGH 200 ns
tCL DCLK LOW 200 ns
tBD DCLK Falling to BUSY Rising 200 ns
tBDV CS Falling to BUSY Enabled 200 ns
tBTR CS Rising to BUSY Disabled 200 ns
TABLE VI. Timing Specifications (+VCC = +2.7V to 3.6V,
TA = –40°C to +85°C, CLOAD = 50pF).
SYMBOL DESCRIPTION MIN TYP MAX UNITS
tACQ Acquisition Time 900 ns
tDS DIN Valid Prior to DCLK Rising 50 ns
tDH DIN Hold After DCLK HIGH 10 ns
tDO DCLK Falling to DOUT Valid 100 ns
tDV CS Falling to DOUT Enabled 70 ns
tTR CS Rising to DOUT Disabled 70 ns
tCSS CS Falling to First DCLK Rising 50 ns
tCSH CS Rising to DCLK Ignored 0 ns
tCH DCLK HIGH 150 ns
tCL DCLK LOW 150 ns
tBD DCLK Falling to BUSY Rising 100 ns
tBDV CS Falling to BUSY Enabled 70 ns
tBTR CS Rising to BUSY Disabled 70 ns
TABLE VII. Timing Specifications (+VCC = +4.75V to
+5.25V, TA = –40°C to +85°C, CLOAD = 50pF).
FIGURE 6. Maximum Conversion Rate, 15-Clocks per Conversion.
FIGURE 5. Detailed Timing Diagram.
PD0
tBDV
tDH
tCH tCL
tDS
tCSS
tDV
tBD tBD
tTR
tBTR
tD0 tCSH
DCLK
CS
11
DOUT
BUSY
DIN
10
1
DCLK
CS
11DOUT
BUSY
A2SDIN A1 A0
SGL/
DIF
PD1 PD0
109876543210 111098765432
A1 A0
15 1 15 1
A2SA1A0
SGL/
DIF
PD1 PD0
A2S
ADS7844 13
SBAS100A www.ti.com
Data Format
The ADS7844 output data is in straight binary format as
shown in Figure 7. This figure shows the ideal output code
for the given input voltage and does not include the effects
of offset, gain, or noise.
Output Code
0V
FS = Full-Scale Voltage = V
REF
1 LSB = V
REF
/4096
FS 1 LSB
11...111
11...110
11...101
00...010
00...001
00...000
1 LSB
Note 1: Voltage at converter input, after
multiplexer: +IN(IN). See Figure 2.
Input Voltage
(1)
(V)
FIGURE 7. Ideal Input Voltages and Output Codes.
POWER DISSIPATION
There are three power modes for the ADS7844: full power
(PD1 - PD0 = 11B), auto power-down (PD1 - PD0 = 00B),
and shutdown (SHDN LOW). The affects of these modes
varies depending on how the ADS7844 is being operated. For
example, at full conversion rate and 16 clocks per conver-
sion, there is very little difference between full power mode
and auto power-down. Likewise, if the device has entered
auto power-down, a shutdown (SHDN LOW) will not lower
power dissipation.
When operating at full-speed and 16-clocks per conversion
(as shown in Figure 4), the ADS7844 spends most of its time
acquiring or converting. There is little time for auto power-
down, assuming that this mode is active. Thus, the differ-
ence between full power mode and auto power-down is
negligible. If the conversion rate is decreased by simply
slowing the frequency of the DCLK input, the two modes
remain approximately equal. However, if the DCLK fre-
quency is kept at the maximum rate during a conversion, but
conversion are simply done less often, then the difference
between the two modes is dramatic. Figure 8 shows the
difference between reducing the DCLK frequency (“scal-
ing” DCLK to match the conversion rate) or maintaining
DCLK at the highest frequency and reducing the number of
conversion per second. In the later case, the converter
spends an increasing percentage of its time in power-down
mode (assuming the auto power-down mode is active).
If DCLK is active and CS is LOW while the ADS7844 is in
auto power-down mode, the device will continue to dissipate
some power in the digital logic. The power can be reduced
to a minimum by keeping CS HIGH. The differences in
supply current for these two cases are shown in Figure 9.
FIGURE 8. Supply Current vs Directly Scaling the Fre-
quency of DCLK with Sample Rate or Keeping
DCLK at the Maximum Possible Frequency.
10k 100k1k 1M
fSAMPLE (Hz)
Supply Current (µA)
100
10
1
1000
fCLK = 2MHz
fCLK = 16 fSAMPLE
TA = 25°C
+VCC = +2.7V
VREF = +2.5V
PD1 = PD0 = 0
FIGURE 9. Supply Current vs State of CS.
10k 100k1k 1M
f
SAMPLE
(Hz)
Supply Current (µA)
0.00
0.09
14
0
2
4
6
8
10
12
CS LOW
(GND)
CS HIGH (+V
CC
)
T
A
= 25°C
+V
CC
= +2.7V
V
REF
= +2.5V
f
CLK
= 16 f
SAMPLE
PD1 = PD0 = 0
Operating the ADS7844 in auto power-down mode will
result in the lowest power dissipation, and there is no
conversion time “penalty” on power-up. The very first
conversion will be valid. SHDN can be used to force an
immediate power-down.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7844 circuitry. This is particu-
larly true if the reference voltage is low and/or the conver-
sion rate is high.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connec-
tions, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “win-
dows” in which large external transient voltages can easily
affect the conversion result. Such glitches might originate
from switching power supplies, nearby digital logic, and
ADS7844
14 SBAS100A
www.ti.com
high power devices. The degree of error in the digital output
depends on the reference voltage, layout, and the exact
timing of the external event. The error can change if the
external event changes in time with respect to the DCLK
input.
With this in mind, power to the ADS7844 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the device as possible. In addition, a
1µF to 10µF capacitor and a 5 or 10 series resistor may
be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1µF
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, make sure that it can
drive the bypass capacitor without oscillation (the series
resistor can help in this case). The ADS7844 draws very
little current from the reference on average, but it does place
larger demands on the reference circuitry over short periods
of time (on each rising edge of DCLK during a conversion).
The ADS7844 architecture 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 fre-
quency noise can be filtered out as discussed in the previous
paragraph, voltage variation due to line frequency (50Hz or
60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power supply
entry point. The ideal layout will include an analog ground
plane dedicated to the converter and associated analog
circuitry.
PACKAGE OPTION ADDENDUM
www.ti.com 16-Aug-2012
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status (1) Package Type Package
Drawing Pins Package Qty Eco Plan (2) Lead/
Ball Finish MSL Peak Temp (3) Samples
(Requires Login)
ADS7844E ACTIVE SSOP DBQ 20 50 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844E/2K5 ACTIVE SSOP DBQ 20 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844E/2K5G4 ACTIVE SSOP DBQ 20 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844EB ACTIVE SSOP DBQ 20 50 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844EB/2K5 ACTIVE SSOP DBQ 20 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844EB/2K5G4 ACTIVE SSOP DBQ 20 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844EBG4 ACTIVE SSOP DBQ 20 50 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844EG4 ACTIVE SSOP DBQ 20 50 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844N ACTIVE SSOP DB 20 70 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844N/1K ACTIVE SSOP DB 20 1000 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844N/1KG4 ACTIVE SSOP DB 20 1000 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844NB ACTIVE SSOP DB 20 70 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844NB/1K ACTIVE SSOP DB 20 1000 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844NB/1KG4 ACTIVE SSOP DB 20 1000 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844NBG4 ACTIVE SSOP DB 20 70 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7844NG4 ACTIVE SSOP DB 20 70 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
(1) The marketing status values are defined as follows:
PACKAGE OPTION ADDENDUM
www.ti.com 16-Aug-2012
Addendum-Page 2
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.
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
ADS7844E/2K5 SSOP DBQ 20 2500 330.0 16.4 6.5 9.0 2.1 8.0 16.0 Q1
ADS7844EB/2K5 SSOP DBQ 20 2500 330.0 16.4 6.5 9.0 2.1 8.0 16.0 Q1
ADS7844N/1K SSOP DB 20 1000 330.0 16.4 8.2 7.5 2.5 12.0 16.0 Q1
ADS7844NB/1K SSOP DB 20 1000 330.0 16.4 8.2 7.5 2.5 12.0 16.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 16-Aug-2012
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
ADS7844E/2K5 SSOP DBQ 20 2500 367.0 367.0 38.0
ADS7844EB/2K5 SSOP DBQ 20 2500 367.0 367.0 38.0
ADS7844N/1K SSOP DB 20 1000 367.0 367.0 38.0
ADS7844NB/1K SSOP DB 20 1000 367.0 367.0 38.0
PACKAGE MATERIALS INFORMATION
www.ti.com 16-Aug-2012
Pack Materials-Page 2
MECHANICAL DATA
MSSO002E – JANUARY 1995 – REVISED DECEMBER 2001
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
DB (R-PDSO-G**) PLASTIC SMALL-OUTLINE
4040065 /E 12/01
28 PINS SHOWN
Gage Plane
8,20
7,40
0,55
0,95
0,25
38
12,90
12,30
28
10,50
24
8,50
Seating Plane
9,907,90
30
10,50
9,90
0,38
5,60
5,00
15
0,22
14
A
28
1
2016
6,50
6,50
14
0,05 MIN
5,905,90
DIM
A MAX
A MIN
PINS **
2,00 MAX
6,90
7,50
0,65 M
0,15
0°ā8°
0,10
0,09
0,25
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Body dimensions do not include mold flash or protrusion not to exceed 0,15.
D. Falls within JEDEC MO-150
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