TAOS ADC122S101EVAL 2 channel, 1 msps 12-bit a/d converter Datasheet

ADC122S101
2 Channel, 1 MSPS 12-Bit A/D Converter
General Description
Features
The ADC122S101 is a low-power, two-channel CMOS 12-bit
analog-to-digital converter with a high-speed serial interface.
Unlike the conventional practice of specifying performance
at a single sample rate only, the ADC122S101 is fully specified over a sample rate range of 500 kSPS to 1 MSPS. The
converter is based on a successive-approximation register
architecture with an internal track-and-hold circuit. It can be
configured to accept one or two input signals at inputs IN1
and IN2.
The output serial data is straight binary, and is compatible
with several standards, such as SPI™, QSPI™, MICROWIRE, and many common DSP serial interfaces.
n
n
n
n
The ADC122S101 operates with a single supply that can
range from +2.7V to +5.25V. Normal power consumption
using a +3V or +5V supply is 4.3 mW and 13.1 mW, respectively. The power-down feature reduces the power consumption to just 0.14 µW using a +3V supply, or 0.32 µW using a
+5V supply.
The ADC122S101 is packaged in an 8-lead MSOP package.
Operation over the industrial temperature range of −40˚C to
+85˚C is guaranteed.
Specified over a range of sample rates.
Two input channels
Variable power management
Single power supply with 2.7V - 5.25V range
Key Specifications
n
n
n
n
DNL
INL
SNR
Power Consumption
— 3V Supply
— 5V Supply
+0.9 / −0.6 LSB (typ)
± 0.64 LSB (typ)
72.4 dB (typ)
4.3 mW (typ)
13.1 mW (typ)
Applications
n Portable Systems
n Remote Data Acquisition
n Instrumentation and Control Systems
Pin-Compatible Alternatives by Resolution and Speed
All devices are fully pin and function compatible.
Resolution
Specified for Sample Rate Range of:
50 to 200 kSPS
200 to 500 kSPS
500 kSPS to 1 MSPS
12-bit
ADC122S021
ADC122S051
ADC122S101
10-bit
ADC102S021
ADC102S051
ADC102S101
8-bit
ADC082S021
ADC082S051
ADC082S101
Connection Diagram
20125205
Ordering Information
Order Code
Temperature Range
Description
Top Mark
ADC122S101CIMM
−40˚C to +85˚C
8-Lead MSOP Package
X24C
ADC122S101CIMMX
−40˚C to +85˚C
8-Lead MSOP Package, Tape & Reel
X24C
ADC122S101EVAL
Evaluation Board
TRI-STATE ® is a trademark of National Semiconductor Corporation
QSPI™ and SPI™ are trademarks of Motorola, Inc.
© 2005 National Semiconductor Corporation
DS201252
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ADC122S101 2 Channel, 1 MSPS 12-Bit A/D Converter
April 2005
ADC122S101
Block Diagram
20125207
Pin Descriptions and Equivalent Circuits
Pin No.
Symbol
Description
ANALOG I/O
5,4
IN1 and IN2
Analog inputs. These signals can range from 0V to VA.
DIGITAL I/O
8
SCLK
Digital clock input. This clock directly controls the conversion
and readout processes.
7
DOUT
Digital data output. The output samples are clocked out of this
pin on falling edges of the SCLK pin.
6
DIN
Digital data input. The ADC122S101’s Control Register is
loaded through this pin on rising edges of the SCLK pin.
1
CS
Chip select. On the falling edge of CS, a conversion process
begins. Conversions continue as long as CS is held low.
2
VA
Positive supply pin. This pin should be connected to a quiet
+2.7V to +5.25V source and bypassed to GND with a 1 µF
capacitor and a 0.1 µF monolithic capacitor located within 1
cm of the power pin.
3
GND
POWER SUPPLY
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The ground return for the analog supply and signals.
2
Operating Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Temperature Range
−40˚C ≤ TA ≤ +85˚C
VA Supply Voltage
+2.7V to +5.25V
Digital Input Pins Voltage Range
−0.3V to 6.5V
Analog Supply Voltage VA
Voltage on Any Pin to GND
Clock Frequency
−0.3V to VA +0.3V
Analog Input Voltage
± 10 mA
± 20 mA
Input Current at Any Pin (Note 3)
Package Input Current (Note 3)
Power Consumption at TA = 25˚C
2500V
250V
Junction Temperature
+150˚C
Storage Temperature
−65˚C to +150˚C
0V to VA
Package Thermal Resistance
See (Note 4)
ESD Susceptibility (Note 5)
Human Body Model
Machine Model
−0.3V to VA
0.8 MHz to 16 MHz
Package
θJA
8-lead MSOP
250˚C / W
Soldering process must comply with National Semiconductor’s Reflow Temperature Profile specifications. Refer to
www.national.com/packaging. (Note 6)
ADC122S101 Converter Electrical Characteristics
(Note 9)
The following specifications apply for VA = +2.7 V to 5.25 V, GND = 0V, fSCLK = 8 MHz to 16 MHz, fSAMPLE = 500 kSPS to 1
MSPS, CL = 35 pF unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C.
Symbol
Parameter
Conditions
Limits
(Note 7)
Units
12
Bits
+0.64
+1.6
LSB (max)
−0.64
−1.2
LSB (min)
+0.9
+1.6
LSB (max)
Typical
STATIC CONVERTER CHARACTERISTICS
Resolution with No Missing Codes
INL
Integral Non-Linearity
DNL
Differential Non-Linearity
−0.6
−1.0
LSB (min)
VOFF
Offset Error
+0.44
± 1.3
LSB (max)
OEM
Channel to Channel Offset Error
Match
± 0.1
± 1.0
LSB (max)
FSE
Full-Scale Error
−0.34
± 1.5
LSB (max)
FSEM
Channel to Channel Full-Scale Error
Match
± 0.1
± 1.0
LSB (max)
DYNAMIC CONVERTER CHARACTERISTICS
SINAD
Signal-to-Noise Plus Distortion Ratio
VA = +2.7 to 5.25V,
fIN = 40.3 kHz, −0.02 dBFS
72
69.2
dB (min)
SNR
Signal-to-Noise Ratio
VA = +2.7 to 5.25V
fIN = 40.3 kHz, −0.02 dBFS
72.4
70.6
dB (min)
THD
Total Harmonic Distortion
VA = +2.7 to 5.25V,
fIN = 40.3 kHz, −0.02 dBFS
−82
−75
dB (max)
SFDR
Spurious-Free Dynamic Range
VA = +2.7 to 5.25V
fIN = 40.3 kHz, −0.02 dBFS
83
76
dB (min)
ENOB
Effective Number of Bits
VA = +2.7 to 5.25V
11.7
11.2
Bits (min)
Channel-to-Channel Crosstalk
VA = +5.25V
fIN = 40.3 kHz
−86
dB
Intermodulation Distortion, Second
Order Terms
VA = +5.25V,
fa = 40.161 kHz, fb = 41.015 kHz
−87
dB
Intermodulation Distortion, Third
Order Terms
VA = +5.25V
fa = 40.161 kHz, fb = 41.015 kHz
−88
dB
VA = +5V
11
MHz
VA = +3V
8
MHz
IMD
FPBW
-3 dB Full Power Bandwidth
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ADC122S101
Absolute Maximum Ratings (Notes 1, 2)
ADC122S101
ADC122S101 Converter Electrical Characteristics
(Note 9) (Continued)
The following specifications apply for VA = +2.7 V to 5.25 V, GND = 0V, fSCLK = 8 MHz to 16 MHz, fSAMPLE = 500 kSPS to 1
MSPS, CL = 35 pF unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C.
Symbol
Parameter
Conditions
Typical
Limits
(Note 7)
Units
±1
µA (max)
ANALOG INPUT CHARACTERISTICS
VIN
Input Range
IDCL
DC Leakage Current
CINA
Input Capacitance
0 to VA
± 0.02
V
Track Mode
33
pF
Hold Mode
3
pF
DIGITAL INPUT CHARACTERISTICS
VIH
Input High Voltage
VIL
Input Low Voltage
IIN
Input Current
CIND
Digital Input Capacitance
VA = +5.25V
2.4
V (min)
VA = +3.6V
2.1
V (min)
0.8
V (max)
± 0.1
± 10
µA (max)
2
4
pF (max)
ISOURCE = 200 µA
VA − 0.03
VA − 0.5
V (min)
ISOURCE = 1 mA
VA − 0.1
VIN = 0V or VIN = VA
DIGITAL OUTPUT CHARACTERISTICS
VOH
Output High Voltage
VOL
Output Low Voltage
IOZH,
IOZL
TRI-STATE ® Leakage Current
COUT
TRI-STATE ® Output Capacitance
ISINK = 200 µA
0.02
ISINK = 1 mA
0.1
V
0.4
V (max)
V
± 0.01
±1
µA (max)
2
4
pF (max)
Output Coding
Straight (Natural) Binary
POWER SUPPLY CHARACTERISTICS (CL = 10 pF)
VA
Analog Supply Voltage
Supply Current, Normal Mode
(Operational, CS low)
IA
Supply Current, Shutdown (CS high)
PD
Power Consumption, Normal Mode
(Operational, CS low)
Power Consumption, Shutdown (CS
high)
2.7
V (min)
5.25
V (max)
VA = +5.25V
fSAMPLE = 1 MSPS, fIN = 40.3 kHz
2.5
3.0
mA (max)
VA = +3.6V,
fSAMPLE = 1 MSPS, fIN = 40.3 kHz
1.2
1.6
mA (max)
VA = +5.25V
fSAMPLE = 0 kSPS
60
nA
VA = +3.6V,
fSAMPLE = 0 kSPS
38
nA
VA = +5.25V
13.1
15.8
mW (max)
VA = +3.6V
4.3
5.8
mW (max)
VA = +5.25V
0.32
µW
VA = +3.6V
0.14
µW
AC ELECTRICAL CHARACTERISTICS
fSCLK
Maximum Clock Frequency
fS
Sample Rate
tCONV
Conversion Time
8
(Note 8)
(Note 8)
MHz (min)
16
MHz (max)
500
kSPS (min)
1
MSPS (max)
13
SCLK cycles
30
% (min)
70
% (max)
DC
SCLK Duty Cycle
fSCLK = 16 MHz
tACQ
Track/Hold Acquisition Time
Full-Scale Step Input
3
SCLK cycles
Throughput Time
Acquisition Time + Conversion Time
16
SCLK cycles
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50
The following specifications apply for VA = +2.7V to 5.25V, GND = 0V, fSCLK = 8 MHz to 16 MHz, fSAMPLE = 500 kSPS to 1
MSPS, CL = 35 pF, Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C.
Symbol
Parameter
Conditions
Typical
VA = +3.0V
−3.5
VA = +5.0V
−0.5
VA = +3.0V
+4.5
VA = +5.0V
+1.5
Limits
(Note 7)
Units
10
ns (min)
10
ns (min)
30
ns (max)
30
ns (max)
tCSU
Setup Time SCLK High to CS Falling Edge
(Note 10)
tCLH
Hold time SCLK Low to CS Falling Edge
(Note 10)
tEN
Delay from CS Until DOUT active
tACC
Data Access Time after SCLK Falling Edge
tSU
Data Setup Time Prior to SCLK Rising Edge
+3
10
ns (min)
tH
Data Valid SCLK Hold Time
+3
10
ns (min)
tCH
SCLK High Pulse Width
0.5 x
tSCLK
0.3 x
tSCLK
ns (min)
tCL
SCLK Low Pulse Width
0.5 x
tSCLK
0.3 x
tSCLK
ns (min)
20
ns (max)
Output Falling
tDIS
CS Rising Edge to DOUT High-Impedance
Output Rising
VA = +3.0V
+4
VA = +5.0V
+2
VA = +3.0V
+14.5
VA = +5.0V
+13
VA = +3.0V
1.8
VA = +5.0V
1.3
VA = +3.0V
1.0
VA = +5.0V
1.0
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed
specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test
conditions.
Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified.
Note 3: When the input voltage at any pin exceeds the power supply (that is, VIN < GND or VIN > VA), the current at that pin should be limited to 10 mA. The 20
mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 10 mA to two. The Absolute
Maximum Rating specification does not apply to the VA pin. The current into the VA pin is limited by the Analog Supply Voltage specification.
Note 4: The absolute maximum junction temperature (TJmax) for this device is 150˚C. The maximum allowable power dissipation is dictated by TJmax, the
junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA)/θJA. The values
for maximum power dissipation listed above will be reached only when the device is operated in a severe fault condition (e.g. when input or output pins are driven
beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided.
Note 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through zero ohms.
Note 6: Reflow temperature profiles are different for lead-free and non-lead-free packages.
Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: This is the frequency range over which the electrical performance is guaranteed. The device is functional over a wider range which is specified under
Operating Ratings.
Note 9: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 10: Clock may be in any state (high or low) when CS is asserted, with the restrictions on setup and hold time given by tCSU and tCLH.
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ADC122S101
ADC122S101 Timing Specifications
ADC122S101
Timing Diagrams
20125251
ADC122S101 Operational Timing Diagram
20125208
Timing Test Circuit
20125206
ADC122S101 Serial Timing Diagram
20125250
SCLK and CS Timing Parameters
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ACQUISITION TIME is the time required to acquire the input
voltage. That is, it is time required for the hold capacitor to
charge up to the input voltage.
OFFSET ERROR is the deviation of the first code transition
(000...000) to (000...001) from the ideal (i.e. GND + 0.5
LSB).
SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in
dB, of the rms value of the input signal to the rms value of the
sum of all other spectral components below one-half the
sampling frequency, not including harmonics or d.c.
SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD)
Is the ratio, expressed in dB, of the rms value of the input
signal to the rms value of all of the other spectral components below half the clock frequency, including harmonics
but excluding d.c.
APERTURE DELAY is the time between the fourth falling
SCLK edge of a conversion and the time when the input
signal is acquired or held for conversion.
CONVERSION TIME is the time required, after the input
voltage is acquired, for the ADC to convert the input voltage
to a digital word.
CROSSTALK is the coupling of energy from one channel
into the other channel, or the amount of signal energy from
one analog input that appears at the measured analog input.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of
the maximum deviation from the ideal step size of 1 LSB.
SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the input
signal and the peak spurious signal where a spurious signal
is any signal present in the output spectrum that is not
present at the input, excluding d.c.
DUTY CYCLE is the ratio of the time that a repetitive digital
waveform is high to the total time of one period. The specification here refers to the SCLK.
EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE
BITS) is another method of specifying Signal-to-Noise and
Distortion or SINAD. ENOB is defined as (SINAD − 1.76) /
6.02 and says that the converter is equivalent to a perfect
ADC of this (ENOB) number of bits.
TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB or dBc, of the rms total of the first five
harmonic components at the output to the rms level of the
input signal frequency as seen at the output. THD is calculated as
FULL POWER BANDWIDTH is a measure of the frequency
at which the reconstructed output fundamental drops 3 dB
below its low frequency value for a full scale input.
GAIN ERROR is the deviation of the last code transition
(111...110) to (111...111) from the ideal (VREF − 1.5 LSB),
after adjusting for offset error.
INTEGRAL NON-LINEARITY (INL) is a measure of the
deviation of each individual code from a line drawn from
negative full scale (1⁄2 LSB below the first code transition)
through positive full scale (1⁄2 LSB above the last code
transition). The deviation of any given code from this straight
line is measured from the center of that code value.
INTERMODULATION DISTORTION (IMD) is the creation of
additional spectral components as a result of two sinusoidal
frequencies being applied to the ADC input at the same time.
It is defined as the ratio of the power in the second and third
order intermodulation products to the sum of the power in
both of the original frequencies. IMD is usually expressed in
dB.
where Af1 is the RMS power of the input frequency at the
output and Af2 through Af6 are the RMS power in the first 5
harmonic frequencies.
THROUGHPUT TIME is the minimum time required between
the start of two successive conversion. It is the acquisition
time plus the conversion time. In the case of the
ADC122S101, this is 16 SCLK periods.
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ADC122S101
MISSING CODES are those output codes that will never
appear at the ADC outputs. The ADC122S101 is guaranteed
not to have any missing codes.
Specification Definitions
ADC122S101
Typical Performance Characteristics
TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz
to 16 MHz, fIN = 40.3 kHz unless otherwise stated.
DNL - VA = 3.0V
INL - VA = 3.0V
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20125221
DNL - VA = 5.0V
INL - VA = 5.0V
20125262
20125263
DNL vs. Supply
INL vs. Supply
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DNL vs. Clock Frequency
INL vs. Clock Frequency
20125224
20125225
DNL vs. Clock Duty Cycle
INL vs. Clock Duty Cycle
20125226
20125227
DNL vs. Temperature
INL vs. Temperature
20125228
20125229
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ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
SNR vs. Supply
THD vs. Supply
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SNR vs. Clock Frequency
THD vs. Clock Frequency
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20125236
SNR vs. Clock Duty Cycle
THD vs. Clock Duty Cycle
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10
SNR vs. Input Frequency
THD vs. Input Frequency
20125233
20125238
SNR vs. Temperature
THD vs. Temperature
20125234
20125239
SFDR vs. Supply
SINAD vs. Supply
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ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
SFDR vs. Clock Frequency
SINAD vs. Clock Frequency
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20125246
SFDR vs. Clock Duty Cycle
SINAD vs. Clock Duty Cycle
20125242
20125247
SFDR vs. Input Frequency
SINAD vs. Input Frequency
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20125248
12
SFDR vs. Temperature
SINAD vs. Temperature
20125244
20125249
ENOB vs. Supply
ENOB vs. Clock Frequency
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20125253
ENOB vs. Clock Duty Cycle
ENOB vs. Input Frequency
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ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
ADC122S101
Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS to 1 MSPS, fSCLK = 8 MHz to
16 MHz, fIN = 40.3 kHz unless otherwise stated. (Continued)
ENOB vs. Temperature
Spectral Response - 3V, 500 kSPS
20125256
20125259
Spectral Response - 5V, 500 kSPS
Spectral Response - 3V, 1 MSPS
20125260
20125264
Spectral Response - 5V, 1 MSPS
Power Consumption vs. Throughput
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1.0 ADC122S101 OPERATION
The ADC122S101 is a successive-approximation analog-todigital converter designed around a charge-redistribution
digital-to-analog converter. Simplified schematics of the
ADC122S101 in both track and hold modes are shown in
Figures 1, 2, respectively. In Figure 1, the ADC122S101 is in
track mode: switch SW1 connects the sampling capacitor to
one of two analog input channels through the multiplexer,
and SW2 balances the comparator inputs. The
ADC122S101 is in this state for the first three SCLK cycles
after CS is brought low.
Figure 2 shows the ADC122S101 in hold mode: switch SW1
connects the sampling capacitor to ground, maintaining the
20125209
FIGURE 1. ADC122S101 in Track Mode
20125210
FIGURE 2. ADC122S101 in Hold Mode
During the first 3 cycles of SCLK, the ADC is in the track
mode, acquiring the input voltage. For the next 13 SCLK
cycles the conversion is accomplished and the data is
clocked out, MSB first, starting on the 5th clock. If there are
more than one conversion in a frame, the ADC will re-enter
the track mode on the falling edge of SCLK after the N*16th
rising edge of SCLK, and re-enter the hold/convert mode on
the N*16+4th falling edge of SCLK, where "N" is an integer.
When CS is brought high, SCLK is internally gated off. If
SCLK is stopped in the low state while CS is high, the
subsequent fall of CS will generate a falling edge of the
internal version of SCLK, putting the ADC into the track
mode. This is seen by the ADC as the first falling edge of
SCLK. If SCLK is stopped with SCLK high, the ADC enters
the track mode on the first falling edge of SCLK after the
falling edge of CS.
During each conversion, data is clocked into the DIN pin on
the first 8 rising edges of SCLK after the fall of CS. For each
2.0 USING THE ADC122S101
An ADC122S101 timing diagram and a serial interface timing
diagram for the ADC122S101 are shown in the Timing Diagrams section. CS is chip select, which initiates conversions
and frames the serial data transfers. SCLK (serial clock)
controls both the conversion process and the timing of serial
data. DOUT is the serial data output pin, where a conversion
result is sent as a serial data stream, MSB first. Data to be
written to the ADC122S101’s Control Register is placed on
DIN, the serial data input pin. New data is written to DIN with
each conversion.
A serial frame is initiated on the falling edge of CS and ends
on the rising edge of CS. Each frame must contain an integer
multiple of 16 rising SCLK edges. The ADC output data
(DOUT) is in a high impedance state when CS is high and is
active when CS is low. Thus, CS acts as an output enable.
Additionally, the device goes into a power down state when
CS is high and also between continuous conversion cycles.
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ADC122S101
sampled voltage, and switch SW2 unbalances the comparator. The control logic then instructs the charge-redistribution
DAC to add fixed amounts of charge to the sampling capacitor until the comparator is balanced. When the comparator is
balanced, the digital word supplied to the DAC is the digital
representation of the analog input voltage. The
ADC122S101 is in this state for the fourth through sixteenth
SCLK cycles after CS is brought low.
The time when CS is low is considered a serial frame. Each
of these frames should contain an integer multiple of 16
SCLK cycles, during which time a conversion is performed
and clocked out at the DOUT pin and data is clocked into the
DIN pin to indicate the multiplexer address for the next
conversion.
Applications Information
ADC122S101
Applications Information
There are no power-up delays or dummy conversions required with the ADC122S101. The ADC is able to sample
and convert an input to full conversion immediately following
power up. The first conversion result after power-up will be
that of IN1.
(Continued)
conversion, it is necessary to clock in the data indicating the
input that is selected for the conversion after the current one.
See Tables 1, 2 and Table 3.
If CS and SCLK go low simultaneously, it is the following
rising edge of SCLK that is considered the first rising edge
for clocking data into DIN.
TABLE 1. Control Register Bits
Bit 7 (MSB)
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
DONTC
DONTC
ADD2
ADD1
ADD0
DONTC
DONTC
DONTC
TABLE 2. Control Register Bit Descriptions
Bit #:
Symbol:
7 - 6, 2 - 0
Description
DONTC
3
ADD0
4
ADD1
5
ADD2
Don’t care. The value of these bits do not affect the device.
These three bits determine which input channel will be sampled and
converted in the next track/hold cycle. The mapping between codes and
channels is shown in Table 3.
TABLE 3. Input Channel Selection
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ADD2
ADD1
ADD0
Input Channel
x
0
0
IN1 (Default)
x
0
1
IN2
x
1
x
Not allowed. The output signal at the DOUT
pin is indeterminate if ADD1 is high.
16
LSB values. The LSB width for the ADC122S101 is VA/4096.
The ideal transfer characteristic is shown in Figure 3. The
transition from an output code of 0000 0000 0000 to a code
of 0000 0000 0001 is at 1/2 LSB, or a voltage of VA/8192.
Other code transitions occur at steps of one LSB.
(Continued)
3.0 ADC122S101 TRANSFER FUNCTION
The output format of the ADC122S101 is straight binary.
Code transitions occur midway between successive integer
20125211
FIGURE 3. Ideal Transfer Characteristic
4.0 TYPICAL APPLICATION CIRCUIT
A typical application of the ADC122S101 is shown in Figure
4. Power is provided in this example by the National Semiconductor LP2950 low-dropout voltage regulator, available in
a variety of fixed and adjustable output voltages. The power
supply pin is bypassed with a capacitor network located
close to the ADC122S101. Because the reference for the
ADC122S101 is the supply voltage, any noise on the supply
will degrade device noise performance. To keep noise off the
supply, use a dedicated linear regulator for this device, or
provide sufficient decoupling from other circuitry to keep
noise off the ADC122S101 supply pin. Because of the
ADC122S101’s low power requirements, it is also possible to
use a precision reference as a power supply to maximize
performance. The four-wire interface is also shown connected to a microprocessor or DSP.
20125213
FIGURE 4. Typical Application Circuit
17
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ADC122S101
Applications Information
ADC122S101
Applications Information
The user may trade off throughput for power consumption by
simply performing fewer conversions per unit time. The
Power Consumption vs. Sample Rate curve in the Typical
Performance Curves section shows the typical power consumption of the ADC122S101 versus throughput. To calculate the power consumption, simply multiply the fraction of
time spent in the normal mode by the normal mode power
consumption , and add the fraction of time spent in shutdown
mode multiplied by the shutdown mode power dissipation.
(Continued)
5.0 ANALOG INPUTS
An equivalent circuit for one of the ADC122S101’s input
channels is shown in Figure 5. Diodes D1 and D2 provide
ESD protection for the analog inputs. At no time should any
input go beyond (VA + 300 mV) or (GND − 300 mV), as these
ESD diodes will begin conducting, which could result in
erratic operation.
The capacitor C1 in Figure 5 has a typical value of 3 pF, and
is mainly the package pin capacitance. Resistor R1 is the on
resistance of the multiplexer and track / hold switch, and is
typically 500 ohms. Capacitor C2 is the ADC122S101 sampling capacitor and is typically 30 pF. The ADC122S101 will
deliver best performance when driven by a low-impedance
source to eliminate distortion caused by the charging of the
sampling capacitance. This is especially important when
using the ADC122S101 to sample AC signals. Also important
when sampling dynamic signals is a band-pass or low-pass
filter to reduce harmonics and noise, improving dynamic
performance.
7.1 Power Management
When the ADC122S101 is operated continuously in normal
mode, the maximum throughput is fSCLK/16. Throughput
may be traded for power consumption by running fSCLK at its
maximum 16 MHz and performing fewer conversions per
unit time, putting the ADC122S101 into shutdown mode
between conversions. A plot of typical power consumption
versus throughput is shown in the Typical Performance
Curves section. To calculate the power consumption for a
given throughput, multiply the fraction of time spent in the
normal mode by the normal mode power consumption and
add the fraction of time spent in shutdown mode multiplied
by the shutdown mode power consumption. Generally, the
user will put the part into normal mode and then put the part
back into shutdown mode. Note that the curve of power
consumption vs. throughput is nearly linear. This is because
the power consumption in the shutdown mode is so small
that it can be ignored for all practical purposes.
7.2 Power Supply Noise Considerations
The charging of any output load capacitance requires current from the power supply, VA. The current pulses required
from the supply to charge the output capacitance will cause
voltage variations on the supply. If these variations are large
enough, they could degrade SNR and SINAD performance
of the ADC. Furthermore, discharging the output capacitance when the digital output goes from a logic high to a logic
low will dump current into the die substrate, which is resistive. Load discharge currents will cause "ground bounce"
noise in the substrate that will degrade noise performance if
that current is large enough. The larger is the output capacitance, the more current flows through the die substrate and
the greater is the noise coupled into the analog channel,
degrading noise performance.
To keep noise out of the power supply, keep the output load
capacitance as small as practical. If the load capacitance is
greater than 35 pF, use a 100 Ω series resistor at the ADC
output, located as close to the ADC output pin as practical.
This will limit the charge and discharge current of the output
capacitance and improve noise performance.
20125214
FIGURE 5. Equivalent Input Circuit
6.0 DIGITAL INPUTS AND OUTPUTS
The ADC122S101’s digital output DOUT is limited by, and
cannot exceed, the supply voltage, VA. The digital input pins
are not prone to latch-up and, and although not recommended, SCLK, CS and DIN may be asserted before VA
without any latch-up risk.
7.0 POWER SUPPLY CONSIDERATIONS
The ADC122S101 is fully powered-up whenever CS is low,
and fully powered-down whenever CS is high, with one
exception: the ADC122S101 automatically enters powerdown mode between the 16th falling edge of a conversion
and the 1st falling edge of the subsequent conversion (see
Timing Diagrams).
The ADC122S101 can perform multiple conversions back to
back; each conversion requires 16 SCLK cycles. The
ADC122S101 will perform conversions continuously as long
as CS is held low.
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18
ADC122S101 2 Channel, 1 MSPS 12-Bit A/D Converter
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead MSOP
Order Number ADC122S101CIMM, ADC122S101CIMMX
NS Package Number P0MUA08A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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