ADC121S705
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SNAS355B – DECEMBER 2006 – REVISED MARCH 2013
ADC121S705 12-Bit, 500 kSPS to 1 MSPS, Differential Input, Micro Power A/D Converter
Check for Samples: ADC121S705
FEATURES
DESCRIPTION
•
•
The ADC121S705 is a 12-bit, 500 kSPS to 1 MSPS
sampling Analog-to-Digital (A/D) converter that
features a fully differential, high impedance analog
input and an external reference. The reference
voltage can be varied from 1.0V to VA, with a
corresponding resolution between 244µV and VA
divided by 4096.
1
2
•
•
•
True Differential Inputs
Ensured performance from 500 kSPS to 1
MSPS
External Reference
Wide Input Common-Mode Voltage Range
SPI™/QSPI™/MICROWIRE/DSP Compatible
Serial Interface
APPLICATIONS
•
•
•
•
•
•
Automotive Navigation
Portable Systems
Medical Instruments
Instrumentation and Control Systems
Motor Control
Direct Sensor Interface
Operating from a single 5V supply, the supply current
when operating at 1 MSPS is typically 2.3 mA. The
supply current drops down to 0.3 µA typically when
the ADC121S705 enters power-down mode. The
ADC121S705 is available in the VSSOP-8 package.
Operation is ensured over the industrial temperature
range of −40°C to +105°C and clock rates of 8 MHz
to 16 MHz.
KEY SPECIFICATIONS
•
•
•
•
•
•
•
The output serial data is binary 2's complement and
is compatible with several standards, such as SPI™,
QSPI™, MICROWIRE, and many common DSP
serial interfaces. The differential input, low power
consumption, and small size make the ADC121S705
ideal for direct connection to transducers in battery
operated systems or remote data acquisition
applications.
Conversion Rate: 500 kSPS to 1 MSPS
INL: ± 0.95 LSB (Max)
DNL: ±0.95 LSB (Max)
Offset Error: ±3.0 LSB (Max)
Gain Error: ±6.5 LSB (Max)
SINAD: 69.5 db (Min)
Power Consumption at VA = 5V
– Active, 1 MSPS: 11.5 mW (Typ)
– Active, 500 kSPS: 9.0 mW (Typ)
– Power-Down: 1.5 µW (Typ)
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Connection Diagram
VA
VREF
1
8
+IN
2
7
SCLK
- IN
3
6
DOUT
GND
4
5
CS
ADC121S705
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 © 2006–2013, Texas Instruments Incorporated
ADC121S705
SNAS355B – DECEMBER 2006 – REVISED MARCH 2013
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Block Diagram
SAR
CONTROL
VREF
SERIAL
INTERFACE
+IN
S/H
CDAC
-IN
COMPARATOR
PIN DESCRIPTIONS
Pin No.
2
Symbol
Description
1
VREF
Voltage Reference Input. A voltage reference between 1V and VA must be applied to this
input. VREF must be decoupled to GND with a minimum ceramic capacitor value of 0.1 µF.
A bulk capacitor value of 1.0 µF to 10 µF in parallel with the 0.1 µF is recommended for
enhanced performance.
2
+IN
Non-Inverting Input. +IN is the positive analog input for the differential signal applied to the
ADC121S705.
3
−IN
Inverting Input. −IN is the negative analog input for the differential signal applied to the
ADC121S705.
4
GND
Ground. GND is the ground reference point for all signals applied to the ADC121S705.
5
CS
6
DOUT
Serial Data Output. The conversion result is provided on DOUT. The serial data output word
is comprised of 4 null bits and 12 data bits (MSB first). During a conversion, the data is
outputted on the falling edges of SCLK and is valid on the rising edges.
7
SCLK
Serial Clock. SCLK is used to control data transfer and serves as the conversion clock.
8
VA
Chip Select Bar. CS is active low. The ADC121S705 is in Normal Mode when CS is LOW
and Power-Down Mode when CS is HIGH. A conversion begins on the fall of CS.
Power Supply input. A voltage source between 4.5V and 5.5V must be applied to this input.
VA must be decoupled to GND with a ceramic capacitor value of 0.1 µF in parallel with a
bulk capacitor value of 1.0 µF to 10 µF.
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Absolute Maximum Ratings (1) (2) (3)
−0.3V to 6.5V
Analog Supply Voltage VA
−0.3V to (VA +0.3V)
Voltage on Any Pin to GND
Input Current at Any Pin (4)
±10 mA
Package Input Current (4)
±50 mA
See (5)
Power Consumption at TA = 25°C
Human Body Model
ESD Susceptibility (6)
2500V
Machine Model
250v
Charge Device Model
750V
Junction Temperature
+150°C
Storage Temperature
−65°C to +150°C
(1)
(2)
(3)
(4)
(5)
(6)
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 ensure specific performance limits. For ensured specifications and test conditions, see the
Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance characteristics may
degrade when the device is not operated under the listed test conditions. Operation of the device beyond the maximum Operating
Ratings is not recommended.
All voltages are measured with respect to GND = 0V, unless otherwise specified.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
When the input voltage at any pin exceeds the power supplies (that is, VIN < GND or VIN > VA), the current at that pin should be limited
to 10 mA. The 50 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 five.
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 ADC121S705 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). Such conditions should always be avoided.
Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is a 220 pF capacitor discharged
through 0 Ω. Charge device model simulates a pin slowly acquiring charge (such as from a device sliding down the feeder in an
automated assembler) then rapidly being discharged.
Operating Ratings (1) (2)
−40°C ≤ TA ≤ +105°C
Operating Temperature Range
Supply Voltage, VA
+4.5V to +5.5V
Reference Voltage, VREF
1.0V to VA
Input Common-Mode Voltage, VCM
See Figure 59
Digital Input Pins Voltage Range
0 to VA
Clock Frequency
8 MHz to 16 MHz
−VREF to +VREF
Differential Analog Input Voltage
(1)
(2)
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 ensure specific performance limits. For ensured specifications and test conditions, see the
Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance characteristics may
degrade when the device is not operated under the listed test conditions. Operation of the device beyond the maximum Operating
Ratings is not recommended.
All voltages are measured with respect to GND = 0V, unless otherwise specified.
Package Thermal Resistance (1) (2)
(1)
(2)
Package
θJA
8-lead VSSOP
200°C / W
Soldering process must comply with Texas Instruments' Reflow Temperature Profile specifications. Refer to www.ti.com/packaging.
Reflow temperature profiles are different for lead-free packages.
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ADC121S705 Converter Electrical Characteristics (1)
The following specifications apply for VA = +4.5V to 5.5V, VREF = 2.5V, fSCLK = 8 to 16 MHz, fIN = 100 kHz, CL = 25 pF, unless
otherwise noted. Boldface limits apply for TA = TMIN to TMAX; all other limits are at TA = 25°C.
Symbol
Parameter
Conditions
Units (2)
Typical
Limits
12
Bits
±0.6
±0.95
LSB (max)
STATIC CONVERTER CHARACTERISTICS
Resolution with No Missing Codes
INL
Integral Non-Linearity
DNL
Differential Non-Linearity
±0.4
±0.95
LSB (max)
OE
Offset Error
−0.4
±3
LSB (max)
Positive Full-Scale Error
+0.1
±2
LSB (max)
Negative Full-Scale Error
-1.0
±6
LSB (max)
Gain Error
+1.0
±6.5
LSB (max)
FSE
GE
DYNAMIC CONVERTER CHARACTERISTICS
SINAD
Signal-to-Noise Plus Distortion Ratio
fIN = 100 kHz, −0.1 dBFS
72.2
69.5
dBc (min)
SNR
Signal-to-Noise Ratio
fIN = 100 kHz, −0.1 dBFS
72.8
71
dBc (min)
THD
Total Harmonic Distortion
fIN = 100 kHz, −0.1 dBFS
−81.6
−72
dBc (max)
SFDR
Spurious-Free Dynamic Range
fIN = 100 kHz, −0.1 dBFS
83.9
72
dBc (min)
ENOB
Effective Number of Bits
fIN = 100 kHz, −0.1 dBFS
11.7
11.25
bits (min)
−3 dB Full Power Bandwidth
Output at 70.7%FS Differential Input
with FS Input
Single-Ended Input
FPBW
26
MHz
22
MHz
ANALOG INPUT CHARACTERISTICS
VIN
Differential Input Range
IDCL
DC Leakage Current
CINA
Input Capacitance
CMRR
Common Mode Rejection Ratio
VREF
Reference Voltage Range
IREF
Reference Current
−VREF
VIN = VREF or VIN = -VREF
V (min)
+VREF
V (max)
±1
µA (max)
In Track Mode
17
pF
In Hold Mode
3
pF
See Specification Definitions for the test
condition
76
dB
1.0
V (min)
VA
V (max)
CS low, fSCLK = 16 MHz,
fS = 1 MSPS, output = FF8h
55
µA
CS low, fSCLK = 8 MHz,
fS = 500 kSPS, output = FF8h
28
µA
CS high, fSCLK = 0
0.2
µA
DIGITAL INPUT CHARACTERISTICS
VIH
Input High Voltage
2.6
3.6
V (min)
VIL
Input Low Voltage
2.5
1.5
V (max)
IIN
Input Current
±1
µA (max)
CIND
Input Capacitance
2
4
pF (max)
ISOURCE = 200 µA
VA − 0.12
VA − 0.2
V (min)
ISOURCE = 1 mA
VA − 0.16
ISINK = 200 µA
0.01
0.4
V (max)
ISINK = 1 mA
0.05
±1
µA (max)
4
pF (max)
VIN = 0V or VA
DIGITAL OUTPUT CHARACTERISTICS
VOH
Output High Voltage
VOL
Output Low Voltage
IOZH, IOZL TRI-STATE Leakage Current
Force 0V or VA
COUT
Force 0V or VA
TRI-STATE Output Capacitance
2
Output Coding
(1)
(2)
4
V
V
Binary 2'S Complement
Data sheet min/max specification limits are specified by design, test, or statistical analysis.
Tested limits are specified to AOQL (Average Outgoing Quality Level).
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ADC121S705 Converter Electrical Characteristics(1) (continued)
The following specifications apply for VA = +4.5V to 5.5V, VREF = 2.5V, fSCLK = 8 to 16 MHz, fIN = 100 kHz, CL = 25 pF, unless
otherwise noted. Boldface limits apply for TA = TMIN to TMAX; all other limits are at TA = 25°C.
Symbol
Parameter
Conditions
Typical
Limits
Units (2)
POWER SUPPLY CHARACTERISTICS
VA
Analog Supply Voltage
IVA
(Normal)
Supply Current, Normal Mode
(Operational)
IVA (PD)
Supply Current, Power Down Mode (CS
high)
PWR
(Normal)
Power Consumption, Normal Mode
(Operational)
PWR
(PD)
Power Consumption, Power Down Mode
(CS high)
PSRR
Power Supply Rejection Ratio
4.5
V (min)
5.5
V (max)
3
mA (max)
fSCLK = 16 MHz, fS = 1 MSPS,
fIN = 100 kHz
2.3
fSCLK = 8 MHz, fS = 500 kSPS,
fIN = 100 kHz
1.8
mA
fSCLK = 16 MHz
56
µA (max)
fSCLK = 0
(3)
0.3
2
µA (max)
fSCLK = 16 MHz, fS = 1 MSPS,
fIN = 100 kHz, VA = 5.0V
11.5
mW
fSCLK = 8 MHz, fS = 500 kSPS,
fIN = 100 kHz, VA = 5.0V
9.0
mW
fSCLK = 16 MHz, VA = 5.0V
280
µW
fSCLK = 0, VA = 5.0V
1.5
µW
See the Specification Definitions for the
test condition
−85
dB
AC ELECTRICAL CHARACTERISTICS
fSCLK
Maximum Clock Frequency
fSCLK
fS
tACQ
20
16
Minimum Clock Frequency
0.8
8
MHz (max)
Maximum Sample Rate (4)
1.25
1
MSPS (min)
2.5
SCLK cycles
(min)
3.0
SCLK cycles
(max)
13
SCLK cycles
Track/Hold Acquisition Time
tCONV
Conversion Time
tAD
Aperture Delay
(3)
(4)
See Specification Definitions
6
MHz (min)
ns
Data sheet min/max specification limits are specified by design, test, or statistical analysis.
While the maximum sample rate is fSCLK/16, the actual sample rate may be lower than this by having the CS rate slower than fSCLK/16.
ADC121S705 Timing Specifications (1)
The following specifications apply for VA = +4.5V to 5.5V, VREF = 2.5V, fSCLK = 8 MHz to 16 MHz, CL = 25 pF, Boldface limits
apply for TA = TMIN to TMAX: all other limits TA = 25°C.
Symbol
(1)
(2)
Parameter
Conditions
Typical
Limits
Units
5
ns (min)
5
ns (min)
2.5
ns (min)
tCSH
CS Hold Time after an SCLK rising edge
tCSSU
CS Setup Time prior to an SCLK rising edge
tDH
DOUT Hold time after an SCLK Falling edge
7
18
22
ns (max)
20
ns (max)
20
ns (max)
tDA
DOUT Access time after an SCLK Falling edge
tDIS
DOUT Disable Time after the rising edge of CS (2)
tEN
DOUT Enable Time after the falling edge of CS
tCH
SCLK High Time
25
ns (min)
tCL
SCLK Low Time
25
ns (min)
tr
DOUT Rise Time
7
ns
tf
DOUT Fall Time
7
ns
8
Data sheet min/max specification limits are specified by design, test, or statistical analysis.
tDIS is the time for DOUT to change 10% while being loaded by the Timing Test Circuit.
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Timing Diagrams
tCONV
tACQ
tPWRDWN
CS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
SCLK
tCH
tCL
tDIS
tEN
DB11 DB10 DB9
DOUT
DB8
DB7
DB6 DB5
DB4
DB3 DB2
DB1
DB0
HI-Z
HI-Z
LSB
MSB
Figure 1. ADC121S705 Single Conversion Timing Diagram
tACQ
tCONV
tACQ
tPD
tCONV
CS
1
2
3
4
5
6
7
8
9
DB8
DB7
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
SCLK
DOUT
DB11 DB10 DB9
DB6
DB5
DB4 DB3
DB2
DB1
DB0
DB11 DB10 DB9
LSB
MSB
DB8
HI-Z
MSB
Figure 2. ADC121S705 Continuous Conversion Timing Diagram
3.5V
DOUT
IOL
2 mA
1.0V
tf
tr
TO OUTPUT
PIN
1.6V
CL
25 pF
IOH
2 mA
Figure 3. Timing Test Circuit
Figure 4. Rise and Fall Times
SCLK
SCLK
1
2
VIL
tCSH
tDA
tCSSU
3.5V
DOUT
CS
1.0V
tDH
Figure 5. Hold and Access Times
Figure 6. Valid CS Assertion Times
CS
VIH
90%
90%
DOUT
10%
tDIS
90%
DOUT
10%
10%
Figure 7. Voltage Waveform for tDIS
6
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Specification Definitions
APERTURE DELAY is the time between the fourth falling edge of SCLK and the time when the input signal is
acquired or held for conversion.
COMMON MODE REJECTION RATIO (CMRR) is a measure of how well in-phase signals common to both input
pins are rejected.
To calculate CMRR, the change in output offset is measured while the common mode input voltage is changed
from 2V to 3V.
CMRR = 20 LOG ( Δ Common Input / Δ Output Offset)
(1)
CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input
voltage to a digital word.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1
LSB.
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.
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 from the ideal slope of the transfer function. It is the difference between Positive
Full-Scale Error and Negative Full-Scale Error and can be calculated as:
Gain Error = Positive Full-Scale Error − Negative Full-Scale Error
(2)
INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from
negative full scale (½ LSB below the first code transition) through positive full scale (½ 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.
MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC121S705 is
ensured not to have any missing codes.
NEGATIVE FULL-SCALE ERROR is the difference between the differential input voltage at which the output
code transitions from negative full scale to the next code and −VREF + 0.5 LSB
OFFSET ERROR is the difference between the differential input voltage at which the output code transitions from
code 000h to 001h and 1/2 LSB.
POSITIVE FULL-SCALE ERROR is the difference between the differential input voltage at which the output
code transitions to positive full scale and VREF minus 1.5 LSB.
POWER SUPPLY REJECTION RATIO (PSRR) is a measure of how well a change in supply voltage is rejected.
PSRR is calculated from the ratio of the change in offset error for a given change in supply voltage, expressed in
dB. For the ADC121S705, VA is changed from 4.5V to 5.5V.
PSRR = 20 LOG (ΔOffset / ΔVA)
(3)
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.
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SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the desired signal
amplitude to the amplitude of the peak spurious spectral component, where a spurious spectral component is
any signal present in the output spectrum that is not present at the input and may or may not be a harmonic.
TOTAL HARMONIC DISTORTION (THD) is the ratio 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, expressed in dB. THD is calculated as
THD = 20 ‡ log10
A f 22 +
+ A f 62
A f 12
(4)
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.
8
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Typical Performance Characteristics
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
DNL - 1 MSPS
INL - 1 MSPS
Figure 8.
Figure 9.
DNL vs. VA
INL vs. VA
Figure 10.
Figure 11.
OFFSET ERROR vs. VA
GAIN ERROR vs. VA
Figure 12.
Figure 13.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
10
DNL vs. VREF
INL vs. VREF
Figure 14.
Figure 15.
OFFSET ERROR vs. VREF
GAIN ERROR vs. VREF
Figure 16.
Figure 17.
DNL vs. SCLK FREQUENCY
INL vs. SCLK FREQUENCY
Figure 18.
Figure 19.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
OFFSET ERROR vs. SCLK FREQUENCY
GAIN ERROR vs. SCLK FREQUENCY
Figure 20.
Figure 21.
DNL vs. SCLK DUTY CYCLE
INL vs. SCLK DUTY CYCLE
Figure 22.
Figure 23.
OFFSET ERROR vs. SCLK DUTY CYCLE
GAIN ERROR vs. SCLK DUTY CYCLE
Figure 24.
Figure 25.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
12
DNL vs. TEMPERATURE
INL vs. TEMPERATURE
Figure 26.
Figure 27.
OFFSET ERROR vs. TEMPERATURE
GAIN ERROR vs. TEMPERATURE
Figure 28.
Figure 29.
SNR vs. VA
THD vs. VA
Figure 30.
Figure 31.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
SINAD vs. VA
SFDR vs. VA
Figure 32.
Figure 33.
SNR vs. VREF
THD vs. VREF
Figure 34.
Figure 35.
SINAD vs. VREF
SFDR vs. VREF
Figure 36.
Figure 37.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
14
SNR vs. SCLK FREQUENCY
THD vs. SCLK FREQUENCY
Figure 38.
Figure 39.
SINAD vs. SCLK FREQUENCY
SFDR vs. SCLK FREQUENCY
Figure 40.
Figure 41.
SNR vs. SCLK DUTY CYCLE
THD vs. SCLK DUTY CYCLE
Figure 42.
Figure 43.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
SINAD vs. SCLK DUTY CYCLE
SFDR vs. SCLK DUTY CYCLE
Figure 44.
Figure 45.
SNR vs. INPUT FREQUENCY
THD vs. INPUT FREQUENCY
Figure 46.
Figure 47.
SINAD vs. INPUT FREQUENCY
SFDR vs. INPUT FREQUENCY
Figure 48.
Figure 49.
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ADC121S705
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
16
SNR vs. TEMPERATURE
THD vs. TEMPERATURE
Figure 50.
Figure 51.
SINAD vs. TEMPERATURE
SFDR vs. TEMPERATURE
Figure 52.
Figure 53.
SUPPLY CURRENT vs. SCLK FREQUENCY
SUPPLY CURRENT vs. TEMPERATURE
Figure 54.
Figure 55.
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Typical Performance Characteristics (continued)
VA = 5.0V, VREF = 2.5V, TA = +25°C, fSAMPLE = 1 MSPS, fSCLK = 16 MHz, fIN = 100 kHz unless otherwise stated.
REF. CURRENT vs. SCLK FREQUENCY
REF. CURRENT vs. TEMPERATURE
Figure 56.
Figure 57.
SPECTRAL RESPONSE - 1 MSPS
Figure 58.
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ADC121S705
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Functional Description
The ADC121S705 analog-to-digital converter uses a successive approximation register (SAR) architecture based
upon capacitive redistribution containing an inherent sample/hold function. The architecture and process allow
the ADC121S705 to acquire and convert an analog signal at sample rates up to 1 MSPS while consuming very
little power.
The ADC121S705 requires an external reference, external clock, and a single +5V power source that can be as
low as +4.5V. The external reference can be any voltage between 1V and VA. The value of the reference voltage
determines the range of the analog input, while the reference input current depends upon the conversion rate.
The external clock can take on values as indicated in the Electrical Characteristics of this data sheet. The duty
cycle of the clock is essentially unimportant, provided the minimum clock high and low times are met. The
minimum clock frequency is set by internal capacitor leakage. Each conversion requires 16 SCLK cycles to
complete. If less than 12 bits of conversion data are required, CS can be brought high at any point during the
conversion. This procedure of terminating a conversion prior to completion is often referred to as short cycling.
The analog input is presented to the two input pins: +IN and –IN. Upon initiation of a conversion, the differential
input at these pins is sampled on the internal capacitor array. The inputs are disconnected from the internal
circuitry while a conversion is in progress.
The digital conversion result is clocked out by the SCLK input and is provided serially, most significant bit first, at
the DOUT pin. The digital data that is provided at the DOUT pin is that of the conversion currently in progress. With
CS held low after the conversion is complete, the ADC121S705 continuously converts the analog input. The
digital data on DOUT can be clocked into the receiving device on the SCLK rising edges. See Serial Digital
Interface and Timing Diagrams for more information.
REFERENCE INPUT
The externally supplied reference voltage sets the analog input range. The ADC121S705 will operate with a
reference voltage in the range of 1V to VA.
As the reference voltage is reduced, the range of input voltages corresponding to each digital output code is
reduced. That is, a smaller analog input range corresponds to one LSB (Least Significant Bit). The size of one
LSB is equal to twice the reference voltage divided by 4096. When the LSB size goes below the noise floor of
the ADC121S705, the noise will span an increasing number of codes and overall performance will suffer. For
example, dynamic signals will have their SNR degrade, while D.C. measurements will have their code
uncertainty increase. Since the noise is Gaussian in nature, the effects of this noise can be reduced by averaging
the results of a number of consecutive conversions.
Additionally, since offset and gain errors are specified in LSB, any offset and/or gain errors inherent in the A/D
converter will increase in terms of LSB size as the reference voltage is reduced.
The reference input and the analog inputs are connected to the capacitor array through a switch matrix when the
input is sampled. Hence, the only current required at the reference and at the analog inputs is a series of
transient spikes.
Lower reference voltages will decrease the current pulses at the reference input and will slightly decrease the
average input current. The reference current changes only slightly with temperature. See Figure 56 and
Figure 57 in Typical Performance Characteristics for additional details.
18
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ANALOG SIGNAL INPUTS
The ADC121S705 has a differential input, and the effective input voltage that is digitized is (+IN) − (−IN). As is
the case with all differential input A/D converters, operation with a fully differential input signal or voltage will
provide better performance than with a single-ended input. Yet, the ADC121S705 can be presented with a
single-ended input.
The current required to recharge the input sampling capacitor will cause voltage spikes at +IN and −IN. Do not
try to filter out these noise spikes. Rather, ensure that the transient settles out during the acquisition period (three
SCLK cycles after the fall of CS).
Differential Input Operation
With a fully differential input voltage or signal, a positive full scale output code (0111 1111 1111b or 7FFh) will be
obtained when (+IN) − (−IN) ≥ VREF − 1.5 LSB. A negative full scale code (1000 0000 0000b or 800h) will be
obtained when (+IN) − (−IN) ≤ −VREF + 0.5 LSB. This ignores gain, offset and linearity errors, which will affect the
exact differential input voltage that will determine any given output code.
Single-Ended Input Operation
For single-ended operation, the non-inverting input (+IN) of the ADC121S705 should be driven with a signal or
voltages that have a maximum to minimum value range that is equal to or less than twice the reference voltage.
The inverting input (−IN) should be biased at a stable voltage that is halfway between these maximum and
minimum values.
Since the design of the ADC121S705 is optimized for a differential input, the performance degrades slightly when
driven with a single-ended input. Linearity characteristics such as INL and DNL typically degrade by 0.1 LSB and
dynamic characteristics such as SINAD typically degrades by 2 dB. Note that single-ended operation should only
be used if the performance degradation (compared with differential operation) is acceptable.
Input Common Mode Voltage
The allowable input common mode voltage (VCM) range depends upon the supply and reference voltages used
for the ADC121S705. The ranges of VCM are depicted in Figure 59 and Figure 60. The minimum and maximum
common mode voltages for differential and single-ended operation are shown in Table 1.
6
6
Single-Ended Input
5
COMMON-MODE VOLTAGE (V)
COMMON-MODE VOLTAGE (V)
Differential Input
VA = 5.0V
3.75
2.5
1.25
0
-1
0.0
1.0
2.0 2.5 3.0
4.0
5
VA = 5.0V
3.75
2.5
1.25
0
-1
0.0
5.0
0.75
VREF (V)
1.25
1.75
2.5
VREF (V)
Figure 59. VCM range for Differential Input
operation
Figure 60. VCM range for single-ended operation
Table 1. Allowable VCM Range
Input Signal
Differential
Single-Ended
Minimum VCM
Maximum VCM
VREF / 2
VA − VREF / 2
VREF
VA − VREF
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SERIAL DIGITAL INTERFACE
The ADC121S705 communicates via a synchronous 3-wire serial interface as shown in Timing Diagrams. CS,
chip select, 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.
A serial frame is initiated on the falling edge of CS and ends on the rising edge of CS. The ADC121S705's
DOUT pin is in a high impedance state when CS is high and is active when CS is low; thus CS acts as an output
enable.
During the first three cycles of SCLK, the ADC121S705 is in acquisition mode (tACQ), acquiring the input voltage.
For the next thirteen SCLK cycles (tCONV), the conversion is accomplished and the data is clocked out. SCLK
falling edges one through four clock out leading zeros while falling edges five through sixteen clock out the
conversion result, MSB first. If there is more than one conversion in a frame (continuous conversion mode), the
ADC121S705 will re-enter acquisition mode on the falling edge of SCLK after the N*16th rising edge of SCLK
and re-enter the conversion mode on the N*16+4th falling edge of SCLK as shown in Figure 2. "N" is an integer
value.
The ADC121S705 can enter acquisition mode under three different conditions. The first condition involves CS
going low (asserted) with SCLK high. In this case, the ADC121S705 enters acquisition mode on the first falling
edge of SCLK after CS is asserted. In the second condition, CS goes low with SCLK low. Under this condition,
the ADC121S705 automatically enters acquisition mode and the falling edge of CS is seen as the first falling
edge of SCLK. In the third condition, CS and SCLK go low simultaneously and the ADC121S705 enters
acquisition mode. While there is no timing restriction with respect to the falling edges of CS and SCLK, see
Figure 6 for setup and hold time requirements for the falling edge of CS with respect to the rising edge of SCLK.
CS Input
The CS (chip select bar) input is CMOS compatible and is active low. The ADC121S705 is in normal mode when
CS is low and power-down mode when CS is high. CS frames the conversion window. The falling edge of CS
marks the beginning of a conversion and the rising of CS marks the end of a conversion window. Multiple
conversions can occur within a given conversion frame with each conversion requiring sixteen SCLK cycles.
SCLK Input
The SCLK (serial clock) is used as the conversion clock and to clock out the conversion results. This input is
CMOS compatible. Internal settling time requirements limit the maximum clock frequency while internal capacitor
leakage limits the minimum clock frequency. The ADC121S705 offers ensured performance with the clock rates
indicated in the electrical table.
Data Output
The output data format of the ADC121S705 is two’s complement, as shown in Table 2. This table indicates the
ideal output code for the given input voltage and does not include the effects of offset, gain error, linearity errors,
or noise. Each data output bit is sent on the falling edge of SCLK.
While most receiving systems will capture the digital output bits on the rising edge of SCLK, the falling edge of
SCLK may be used to capture each bit if the minimum hold time (tDH) for DOUT is acceptable. See Figure 5 for
DOUT hold and access times.
DOUT is enabled on the falling edge of CS and disabled on the rising edge of CS. If CS is raised prior to the 16th
falling edge of SCLK, the current conversion is aborted and DOUT will go into its high impedance state. A new
conversion will begin when CS is taken LOW.
Table 2. Ideal Output Code vs. Input Voltage
Description
Analog Input, (+IN) − (−IN)
2's Complement Binary Output
2's Comp. Hex Code
VREF − 1.5 LSB
0111 1111 1111
7FF
+ Full Scale
Midscale
Midscale − 1 LSB
− Full Scale
20
0V
0000 0000 0000
000
0V − 1 LSB
1111 1111 1111
FFF
−VREF − 0.5 LSB
1000 0000 0000
800
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APPLICATIONS INFORMATION
OPERATING CONDITIONS
We recommend that the following conditions be observed for operation of the ADC121S705:
−40°C ≤ TA ≤ +105°C
+4.5V ≤ VA ≤ +5.5V
1V ≤ VREF ≤ VA
8 MHz ≤ fCLK ≤ 16 MHz
VCM: See Input Common Mode Voltage
POWER CONSUMPTION
The architecture, design, and fabrication process allow the ADC121S705 to operate at conversion rates up to 1
MSPS while consuming very little power. The ADC121S705 consumes the least amount of power while operating
in power down mode. For applications where power consumption is critical, the ADC121S705 should be
operated in power down mode as often as the application will tolerate. To further reduce power consumption,
stop the SCLK while CS is high.
Short Cycling
Another way of saving power is to short cycle the conversion process. This is done by pulling CS high after the
last required bit is received from the ADC121S705 output. This is possible because the ADC121S705 places the
latest converted data bit on DOUT as it is generated. If only 8-bits of the conversion result are needed, for
example, the conversion can be terminated by pulling CS high after the 8th bit has been clocked out. Halting the
conversion after the last needed bit is outputted is called short cycling.
Short cycling can be used to lower the power consumption in those applications that do not need a full 12-bit
resolution, or where an analog signal is being monitored until some condition occurs. For example, it may not be
necessary to use the full 12-bit resolution of the ADC121S705 as long as the signal being monitored is within
certain limits. In some circumstances, the conversion could be terminated after the first few bits. This will lower
power consumption in the converter since the ADC121S705 spends more time in power down mode and less
time in the conversion mode.
Burst Mode Operation
Normal operation of the ADC121S705 requires the SCLK frequency to be sixteen times the sample rate and the
CS rate to be the same as the sample rate. However, in order to minimize power consumption in applications
requiring sample rates below 500 kSPS, the ADC121S705 should be run with an SCLK frequency of 16 MHz and
a CS rate as slow as the system requires. When this is accomplished, the ADC121S705 is operating in burst
mode. The ADC121S705 enters into power down mode at the end of each conversion, minimizing power
consumption. This causes the converter to spend the longest possible time in power down mode. Since power
consumption scales directly with conversion rate, minimizing power consumption requires determining the lowest
conversion rate that will satisfy the requirements of the system.
TIMING CONSIDERATIONS
Proper operation requires that the fall of CS not occur simultaneously with a rising edge of SCLK. If the fall of CS
occurs during the rising edge of SCLK, the data might be clocked out one bit early. Whether or not the data is
clocked out early depends upon how close the CS transition is to the SCLK transition, the device temperature,
and characteristics of the individual device. To ensure that the data is always clocked out at a given time (the 5th
falling edge of SCLK), it is essential that the fall of CS always meet the timing requirement specified in the
Timing Specifications.
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PCB LAYOUT AND CIRCUIT CONSIDERATIONS
For best performance, care should be taken with the physical layout of the printed circuit board. This is especially
true with a low reference voltage or when the conversion rate is high. At high clock rates there is less time for
settling, so it is important that any noise settles out before the conversion begins.
Power Supply
Any ADC architecture is sensitive to spikes on the power supply, reference, and ground pins. These spikes may
originate from switching power supplies, digital logic, high power devices, and other sources. Power to the
ADC121S705 should be clean and well bypassed. A 0.1 µF ceramic bypass capacitor and a 1 µF to 10 µF
capacitor should be used to bypass the ADC121S705 supply, with the 0.1 µF capacitor placed as close to the
ADC121S705 package as possible.
Voltage Reference
The reference source must have a low output impedance and needs to be bypassed with a minimum capacitor
value of 0.1 µF. A larger capacitor value of 1 µF to 10 µF placed in parallel with the 0.1 µF is preferred. While the
ADC121S705 draws very little current from the reference on average, there are higher instantaneous current
spikes at the reference input that must settle out while SCLK is high. Since these transient spikes can be as high
as 20 mA, it is important that the reference circuit be capable of providing this much current and settle out during
the first three clock periods (acquisition time).
The reference input of the ADC121S705, like all A/D converters, does not reject noise or voltage variations. Keep
this in mind if the reference voltage is derived from the power supply. Any noise and/or ripple from the supply
that is not rejected by the external reference circuitry will appear in the digital results. The use of an active
reference source is recommended. The LM4040 and LM4050 shunt reference families and the LM4132 and
LM4140 series reference families are excellent choices for a reference source.
Power and Ground Planes
A single ground plane and the use of two or more power planes is recommended. The power planes should all
be in the same board layer and will define the analog, digital, and high power board areas. Lines associated with
these areas should always be routed within their respective areas.
The GND pin on the ADC121S705 should be connected to the ground plane at a quiet point. Avoid connecting
the GND pin too close to the ground point of a microprocessor, microcontroller, digital signal processor, or other
high power digital device.
22
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APPLICATION CIRCUITS
The following figures are examples of the ADC121S705 in typical application circuits. These circuits are basic
and will generally require modification for specific circumstances.
Data Acquisition
Figure 61 shows a basic low cost, low power data acquisition circuit. Maximum clock rate with a minimum
sample rate can reduce the power consumption further.
+5V
5: to 10:
+
2 k:
ADC121S705
+
LM4040-2.5
10 PF
VREF
VA
+ 1.0 PF to
4.7 PF
0.1 PF
4.7 PF
+IN
SCLK
- IN
DOUT
GND
CSB
Microcontroller
Figure 61. Low cost, low power Data Acquisition System
Pressure Sensor
Figure 62 shows an example of interfacing a pressure sensor to the ADC121S705. A digital-to-analog converter
(DAC) is used to bias the pressure sensor. The DAC081S101 provides a means for dynamically adjusting the
sensitivity of the sensor. A shunt reference voltage of 2.5V is used as the reference for the ADC121S705. The
ADC121S705, DAC081S101, and the LM4040 are all powered from the same voltage source.
VA
LMP7701
DAC081S101
+
-
470 pF
200 k:
VA
SCLK
ADC121S705
REF
Pressure
Sensor
0.2 mV/PSI
+
LMP7701
DIN
180 :
2 k:
200 k:
SYNCB
180 :
DOUT
CSB
VA
470 pF
AV = 100 V/V
MicroController
SCLK
2 k:
1 uF
LM4040-2.5
Figure 62. Interfacing the ADC121S705 for a Pressure Sensor
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REVISION HISTORY
Changes from Revision A (March 2013) to Revision B
•
24
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 23
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
ADC121S705CIMM/NOPB
NRND
VSSOP
DGK
8
1000
RoHS & Green
SN
Level-1-260C-UNLIM
0 to 0
X1AC
(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)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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