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19-3364; Rev 0; 8/04
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
General Description
The MAX1276/MAX1278 are low-power, high-speed, serial-output, 12-bit, analog-to-digital converters (ADCs) that operate at up to 1.8Msps and have an internal reference. These devices feature true-differential inputs, offering better noise immunity, distortion improvements, and a wider dynamic range over single-ended inputs. A standard SPITM/QSPITM/MICROWIRETM interface provides the clock necessary for conversion. These devices easily interface with standard digital signal processor (DSP) synchronous serial interfaces. The MAX1276/MAX1278 operate from a single +4.75V to +5.25V supply voltage. The MAX1276/MAX1278 include a 4.096V internal reference. The MAX1276 has a unipolar analog input, while the MAX1278 has a bipolar analog input. These devices feature a partial power-down mode and a full power-down mode for use between conversions, which lower the supply current to 2mA (typ) and 1A (max), respectively. Also featured is a separate power-supply input (VL), which allows direct interfacing to +1.8V to VDD digital logic. The fast conversion speed, low-power dissipation, excellent AC performance, and DC accuracy (1.25 LSB INL) make the MAX1276/MAX1278 ideal for industrial process control, motor control, and base-station applications. The MAX1276/MAX1278 come in a 12-pin TQFN package, and are available in the commercial (0C to +70C) and extended (-40C to +85C) temperature ranges. 1.8Msps Sampling Rate Only 55mW (typ) Power Dissipation Only 1A (max) Shutdown Current High-Speed, SPI-Compatible, 3-Wire Serial Interface 70dB S/(N + D) at 525kHz Input Frequency Internal True-Differential Track/Hold (T/H) Internal 4.096V Reference No Pipeline Delays Small 12-Pin TQFN Package
Features
MAX1276/MAX1278
Ordering Information
PART MAX1276CTC-T MAX1276ETC-T MAX1278CTC-T MAX1278ETC-T TEMP RANGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C PINPACKAGE 12 TQFN-12 12 TQFN-12 12 TQFN-12 12 TQFN-12 INPUT Unipolar Unipolar Bipolar Bipolar
Applications
Data Acquisition Bill Validation Motor Control Communications Portable Instruments
Typical Operating Circuit
4.75V TO +5.25V 10F 0.01F VDD DIFFERENTIAL + INPUT VOLTAGE AIN+ AIN+1.8V TO VDD
Pin Configuration
TOP VIEW
AIN+ 12 N.C. 11 SCLK 10
0.01F VL DOUT
10F
AINREF RGND
1 2 3
9 8 7
CNVST DOUT VL
MAX1276 MAX1278
MAX1276 MAX1278
REF 4.7F 0.01F RGND
C/DSP CNVST SCLK
GND
4 VDD
5 N.C.
6 GND
TQFN
SPI/QSPI are trademarks of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corp. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
ABSOLUTE MAXIMUM RATINGS
VDD to GND ..............................................................-0.3V to +6V VL to GND ................-0.3V to the lower of (VDD + 0.3V) and +6V Digital Inputs to GND .................-0.3V to the lower of (VDD + 0.3V) and +6V Digital Output to GND ....................-0.3V to the lower of (VL + 0.3V) and +6V Analog Inputs and REF to GND..........-0.3V to the lower of (VDD + 0.3V) and +6V RGND to GND .......................................................-0.3V to +0.3V Maximum Current into Any Pin............................................50mA Continuous Power Dissipation (TA = +70C) 12-Pin TQFN (derate 16.9mW/C above +70C) ......1349mW Operating Temperature Ranges MAX127_ CTC ...................................................0C to +70C MAX127_ ETC.................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-60C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDD = +5V 5%, VL = VDD, fSCLK = 28.8MHz, 50% duty cycle, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER DC ACCURACY Resolution Relative Accuracy Differential Nonlinearity Offset Error Offset-Error Temperature Coefficient Gain Error Gain Temperature Coefficient DYNAMIC SPECIFICATIONS (fIN = 525kHz sine wave, VIN = VREF, unless otherwise noted.) Signal-to-Noise Plus Distortion Total Harmonic Distortion Spurious-Free Dynamic Range Intermodulation Distortion Full-Power Bandwidth Full-Linear Bandwidth CONVERSION RATE Minimum Conversion Time Maximum Throughput Rate Minimum Throughput Rate Track-and-Hold Acquisition Time Aperture Delay Aperture Jitter External Clock Frequency ANALOG INPUTS (AIN+, AIN-) Differential Input Voltage Range Absolute Input Voltage Range VIN AIN+ - AIN-, MAX1276 AIN+ - AIN-, MAX1278 0 -VREF / 2 0 VREF +VREF / 2 VDD V V fSCLK (Note 7) tACQ (Note 5) (Note 6) tCONV (Note 4) 1.8 10 104 5 30 28.8 0.556 s Msps ksps ns ns ps MHz SINAD THD SFDR IMD (Note 1) Up to the 5th harmonic (Note 1) (Note 1) fIN1 = 250kHz, fIN2 = 300kHz -3dB point, small-signal method S/(N + D) > 68dB, single ended 69 70 -80 -83 -78 20 2.0 -76 -76 dB dB dB dB MHz MHz Offset nulled 2 1 6.0 INL DNL (Notes 1, 2) (Notes 1, 3) 12 -1.25 -1.0 +1.25 +1.0 6.0 Bits LSB LSB LSB ppm/C LSB ppm/C SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
ELECTRICAL CHARACTERISTICS (continued)
(VDD = +5V 5%, VL = VDD, fSCLK = 28.8MHz, 50% duty cycle, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER DC Leakage Current Input Capacitance Input Current (Average) REFERENCE OUTPUT (REF) REF Output Voltage Range Voltage Temperature Coefficient Load Regulation Line Regulation DIGITAL INPUTS (SCLK, CNVST) Input Voltage Low Input Voltage High Input Leakage Current POWER REQUIREMENTS Analog Supply Voltage Digital Supply Voltage Analog Supply Current, Normal Mode Analog Supply Current, Partial Power-Down Mode Analog Supply Current, Full Power-Down Mode VDD VL Static, fSCLK = 28.8MHz IDD Static, no SCLK Operational, 1.8Msps IDD IDD fSCLK = 28.8MHz No SCLK fSCLK = 28.8MHz No SCLK Operational, full-scale input at 1.8Msps Static, fSCLK = 28.8MHz Digital Supply Current (Note 8) Partial/full power-down mode, fSCLK = 28.8MHz Static, no SCLK, all modes Positive-Supply Rejection DIGITAL OUTPUT (DOUT) Output Load Capacitance Output Voltage Low Output Voltage High Output Leakage Current COUT VOL VOH IOL For stated timing performance ISINK = 5mA, VL 1.8V ISOURCE = 1mA, VL 1.8V Output high impedance VL - 0.5V 0.2 10 30 0.4 pF V V A PSR VDD = 5V 5%, full-scale input 4.75 1.8 8 5 10 2 2 1 0.3 1 0.4 0.2 0.1 0.2 1 2.5 1 0.5 1 3.0 A mV mA 5.25 VDD 11 7 13 mA A mA V V VIL VIH IIL 0.7 x VL Output high impedance 0.2 10 0.3 x VL V V A ISOURCE = 0 to 2mA ISINK = 0 to 200A VDD = 4.75V to 5.25V, static Static, TA = +25C 4.086 4.096 50 0.3 0.5 0.5 4.106 V ppm/C mV/mA mV/V Per input pin Time averaged at maximum throughput rate 16 75 SYMBOL CONDITIONS MIN TYP MAX 1 UNITS A pF A
MAX1276/MAX1278
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3
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
TIMING CHARACTERISTICS
(VDD = +5V 5%, VL = VDD, fSCLK = 28.8MHz, 50% duty cycle, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER SCLK Pulse-Width High SCLK Pulse-Width Low SCLK Rise to DOUT Transition DOUT Remains Valid After SCLK CNVST Fall to SCLK Fall CNVST Pulse Width Power-Up Time; Full Power-Down Restart Time; Partial Power-Down SYMBOL tCH tCL tDOUT tDHOLD tSETUP tCSW tPWR-UP tRCV CONDITIONS VL = 1.8V to VDD VL = 1.8V to VDD CL = 30pF, VL = 4.75V to VDD CL = 30pF, VL = 2.7V to VDD CL = 30pF, VL = 1.8V to VDD VL = 1.8V to VDD VL = 1.8V to VDD VL = 1.8V to VDD 4 10 20 2 16 MIN 15.6 15.6 14 17 24 ns ns ns ms Cycles ns TYP MAX UNITS ns ns
Note 1: -40C performance is guaranteed by design. Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the gain error and the offset error have been nulled. Note 3: No missing codes over temperature. Note 4: Conversion time is defined as the number of clock cycles (16) multiplied by the clock period. Note 5: At sample rates below 10ksps, the input full-linear bandwidth is reduced to 5kHz. Note 6: The listed value of three SCLK cycles is given for full-speed continuous conversions. Acquisition time begins on the 14th rising edge of SCLK and terminates on the next falling edge of CNVST. The IC idles in acquisition mode between conversions. Note 7: Undersampling at the maximum signal bandwidth requires the minimum jitter spec for SINAD performance. Note 8: Digital supply current is measured with the VIH level equal to VL, and the VIL level equal to GND.
VL
CNVST tCSW tSETUP SCLK tCL tCH
DOUT DOUT
6k
DOUT
tDHOLD tDOUT
6k GND a) HIGH-Z TO VOH, VOL TO VOH, AND VOH TO HIGH-Z
CL
CL GND b) HIGH-Z TO VOL, VOH TO VOL, AND VOL TO HIGH-Z
Figure 1. Detailed Serial-Interface Timing
Figure 2. Load Circuits for Enable/Disable Times
4
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1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
Typical Operating Characteristics
(VDD = +5V, VL = VDD, fSCLK = 28.8MHz, fSAMPLE = 1.8Msps, TA = TMIN to TMAX, unless otherwise noted. Typical values are measured at TA = +25C)
INTEGRAL NONLINEARITY vs. DIGITAL OUTPUT CODE (MAX1276)
MAX1276/78 toc01
INTEGRAL NONLINEARITY vs. DIGITAL OUTPUT CODE (MAX1278)
MAX1276/78 toc02
DIFFERENTIAL NONLINEARITY vs. DIGITAL OUTPUT CODE (MAX1276)
0.75 0.50 DNL (LSB) 0.25 0 -0.25 -0.50 -0.75 -1.00
MAX1276/78 toc03
1.00 0.75 0.50 INL (LSB) 0.25 0 -0.25 -0.50 -0.75 -1.00 0 1024 2048 3072
1.0 0.8 0.6 0.4 INL (LSB) 0.2 0 -0.2 -0.4 -0.6 -0.8
1.00
4096
-1.0 -2048
-1024
0
1024
2048
0
1024
2048
3072
4096
DIGITAL OUTPUT CODE
DIGITAL OUTPUT CODE
DIGITAL OUTPUT CODE
DIFFERENTIAL NONLINEARITY vs. DIGITAL OUTPUT CODE (MAX1278)
MAX1276/78 toc04
OFFSET ERROR vs. TEMPERATURE (MAX1276)
MAX1276/78 toc05
OFFSET ERROR vs. TEMPERATURE (MAX1278)
0.75 OFFSET ERROR (LSB) 0.50 0.25 0 -0.25 -0.50 -0.75 -1.00
MAX1276/78 toc06
1.0 0.8 0.6 0.4 DNL (LSB) 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -2048 -1024 0 1024
0.4 0.3 OFFSET ERROR (LSB) 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -40 -15 10 35 60
1.00
2048
85
-40
-15
10
35
60
85
DIGITAL OUTPUT CODE
TEMPERATURE (C)
TEMPERATURE (C)
GAIN ERROR vs. TEMPERATURE (MAX1276)
MAX1276/78 toc07
GAIN ERROR vs. TEMPERATURE (MAX1278)
MAX1276/78 toc08
DYNAMIC PERFORMANCE vs. INPUT FREQUENCY (MAX1276)
MAX1276/78 toc09
-0.5
-0.5
72.0 SNR 71.5
-1.0 GAIN ERROR (LSB)
-1.0 GAIN ERROR (LSB)
-1.5
-1.5
DYNAMIC PERFORMANCE (dB)
71.0 SINAD 70.5
-2.0
-2.0
-2.5
-2.5
-3.0 -40
-15
10
35
60
85
-3.0 -40
70.0 -15 10 35 60 85 100 200 300 400 500 TEMPERATURE (C) ANALOG INPUT FREQUENCY (kHz)
TEMPERATURE (C)
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5
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
Typical Operating Characteristics (continued)
(VDD = +5V, VL = VDD, fSCLK = 28.8MHz, fSAMPLE = 1.8Msps, TA = TMIN to TMAX, unless otherwise noted. Typical values are measured at TA = +25C)
DYNAMIC PERFORMANCE vs. INPUT FREQUENCY (MAX1278)
MAX1276/78 toc10
THD vs. INPUT FREQUENCY
MAX1276/78 toc11
72.00
-86
DYNAMIC PERFORMANCE (dB)
-88 MAX1276 THD (dB) MAX1278
71.75 SNR 71.50
-90
-92 71.25 SINAD 71.00 100 200 300 400 500 ANALOG INPUT FREQUENCY (kHz) -96 100 200 300 400 500 ANALOG INPUT FREQUENCY (kHz)
-94
SFDR vs. INPUT FREQUENCY
MAX1276/78 toc12
FFT PLOT (MAX1276)
fIN = 500kHz SINAD = 71.0dB SNR = 71.1dB THD = -87.1dB SFDR = 90.2dB
MAX1276/78 toc13
95
0 -20 -40 -60 -80 -100
93 MAX1276 SFDR (dB) 91
89
87
MAX1278 -120
85 100 200 300 400 500 ANALOG INPUT FREQUENCY (kHz)
AMPLITUDE (dB)
-140 0 150 300 450 600 750 900 ANALOG INPUT FREQUENCY (kHz)
FFT PLOT (MAX1278)
MAX1276/78 toc14
TOTAL HARMONIC DISTORTION vs. SOURCE IMPEDANCE
MAX1276/78 toc15
0 -20 AMPLITUDE (dB) -40 -60 -80 -100 -120 -140 0
fIN = 500kHz SINAD = 71.2dB SNR = 71.3dB THD = -95.5dB SFDR = 90.5dB
-50
-60 fIN = 500kHz THD (dB) -70
-80
-90
fIN = 100kHz
-100 150 300 450 600 750 900 10 100 SOURCE IMPEDANCE () 1000 ANALOG INPUT FREQUENCY (kHz)
6
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1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
Typical Operating Characteristics (continued)
(VDD = +5V, VL = VDD, fSCLK = 28.8MHz, fSAMPLE = 1.8Msps, TA = TMIN to TMAX, unless otherwise noted. Typical values are measured at TA = +25C)
TWO-TONE IMD PLOT (MAX1276)
MAX1276/78 toc16
TWO-TONE IMD PLOT (MAX1278)
fSAMPLE = 2Msps fIN1 = 250.039kHz fIN2 = 300.059kHz IMD = -81.8dB fIN1 -60 -80 -100 -120 -140 fIN2
MAX1276/78 toc17
0 -20 -40 fIN1 -60 -80 -100 -120 -140 0 200 400 fIN2
fSAMPLE = 2Msps fIN1 = 250.039kHz fIN2 = 300.059kHz IMD = -84.2dB
0 -20 -40
AMPLITUDE (dB)
600
800
1000
AMPLITUDE (dB)
0
200
400
600
800
1000
ANALOG INPUT FREQUENCY (kHz)
ANALOG INPUT FREQUENCY (kHz)
VDD/VL FULL POWER-DOWN SUPPLY CURRENT vs. TEMPERATURE
MAX1276/78 toc18
VL PARTIAL/FULL POWER-DOWN SUPPLY CURRENT vs. TEMPERATURE
MAX1276/78 toc19
1.0 VDD/VL SUPPLY CURRENT (A)
200
VL SUPPLY CURRENT (A)
0.8 VDD, NO SCLK 0.6 VDD, fSCLK = 28.8MHz
150 VL = 5V, fSCLK = 28.8MHz 100 VL = 3V, fSCLK = 28.8MHz 50
0.4 VL, NO SCLK 0.2
0 -40 -15 10 35 60 85 TEMPERATURE (C)
0 -40 -15 10 35 60 85 TEMPERATURE (C)
VDD SUPPLY CURRENT vs. TEMPERATURE
MAX1276/78 toc20
VDD SUPPLY CURRENT vs. CONVERSION RATE
MAX1276/78 toc21
12
12
VDD SUPPLY CURRENT (mA)
VDD SUPPLY CURRENT (mA)
9 CONVERSION, fSCLK = 28.8MHz 6 PARTIAL POWER-DOWN 3
9
6
3
0 -40 -15 10 35 60 85 TEMPERATURE (C)
0 0 500 1000 fSAMPLE (kHz) 1500 2000
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7
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
Typical Operating Characteristics (continued)
(VDD = +5V, VL = VDD, fSCLK = 28.8MHz, fSAMPLE = 1.8Msps, TA = TMIN to TMAX, unless otherwise noted. Typical values are measured at TA = +25C)
VL SUPPLY CURRENT vs. TEMPERATURE
MAX1276/78 toc22
VL SUPPLY CURRENT vs. CONVERSION RATE
MAX1276/78 toc23
1.0
1.0
VL SUPPLY CURRENT (mA)
CONVERSION, fSCLK = 28.8MHz 0.6
VL SUPPLY CURRENT (mA)
0.8
0.8 VL = 5V VL = 3V VL = 1.8V 0.2
0.6
0.4
FULL/PARTIAL POWER-DOWN, fCLK = 28.8MHz
0.4
0.2
0 -40 -15 10 35 60 85 TEMPERATURE (C)
0 0 500 1000 fSAMPLE (kHz) 1500 2000
REFERENCE VOLTAGE vs. TEMPERATURE
MAX1276/78 toc24
REFERENCE VOLTAGE vs. LOAD CURRENT (SOURCE)
MAX1276/78 toc25
4.12
4.10
REFERENCE VOLTAGE (V)
REFERENCE VOLTAGE (V)
4.09
4.10
4.08
4.08
4.07
4.06 -40 -15 10 35 60 85 TEMPERATURE (C)
4.06 0 2 4 6 8 10 LOAD CURRENT (mA)
REFERENCE VOLTAGE vs. LOAD CURRENT (SINK)
MAX1276/78 toc26
4.12
REFERENCE VOLTAGE (V)
4.11
4.10
4.09
4.08 0 100 200 300 400 500 LOAD CURRENT (A)
8
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1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
Pin Description
PIN 1 2 3 4 5, 11 6 7 8 9 10 12 -- NAME AINREF RGND VDD N.C. GND VL DOUT CNVST SCLK AIN+ EP Negative Analog Input Reference Voltage Output. Internal 4.096V reference output. Bypass REF with a 0.01F capacitor and a 4.7F capacitor to RGND. Reference Ground. Connect RGND to GND. Positive Analog Supply Voltage (+4.75V to +5.25V). Bypass VDD with a 0.01F capacitor and a 10F capacitor to GND. No Connection Ground. GND is internally connected to EP. Positive Logic Supply Voltage (1.8V to VDD). Bypass VL with a 0.01F capacitor and a 10F capacitor to GND. Serial Data Output. Data is clocked out on the rising edge of SCLK. Convert Start. Forcing CNVST high prepares the part for a conversion. Conversion begins on the falling edge of CNVST. The sampling instant is defined by the falling edge of CNVST. Serial Clock Input. Clocks data out of the serial interface. SCLK also sets the conversion speed. Positive Analog Input Exposed Paddle. EP is internally connected to GND. FUNCTION
MAX1276/MAX1278
VDD REF 4.096V TRACK AND HOLD AIN 12-BIT SAR ADC
VL
CIN+ RIN+ AIN+
CAPACITIVE DAC
REF AIN +
OUTPUT BUFFER
DOUT
VAZ
COMP
CONTROL LOGIC
AINCINRINACQUISITION MODE
CONTROL LOGIC AND TIMING RGND
CNVST SCLK
CIN+ AIN+ RIN+ CAPACITIVE DAC
MAX1276 MAX1278
GND
VAZ
COMP
CONTROL LOGIC
Figure 3. Functional Diagram
Detailed Description
The MAX1276/MAX1278 use an input T/H and successive-approximation register (SAR) circuitry to convert an analog input signal to a digital 12-bit output. The serial interface requires only three digital lines (SCLK, CNVST, and DOUT) and provides easy interfacing to microprocessors (Ps) and DSPs. Figure 3 shows the simplified internal structure for the MAX1276/MAX1278.
AINCINRINHOLD/CONVERSION MODE
Figure 4. Equivalent Input Circuit
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9
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
True-Differential Analog Input T/H
The equivalent circuit of Figure 4 shows the input architecture of the MAX1276/MAX1278, which is composed of a T/H, a comparator, and a switched-capacitor digital-to-analog converter (DAC). The T/H enters its tracking mode on the 14th SCLK rising edge of the previous conversion. Upon power-up, the T/H enters its tracking mode immediately. The positive input capacitor is connected to AIN+. The negative input capacitor is connected to AIN-. The T/H enters its hold mode on the falling edge of CNVST and the difference between the sampled positive and negative input voltages is converted. The time required for the T/H to acquire an input signal is determined by how quickly its input capacitance is charged. If the input signal's source impedance is high, the acquisition time lengthens. The acquisition time, tACQ, is the minimum time needed for the signal to be acquired. It is calculated by the following equation: tACQ 9 x (RS + RIN) x 16pF where RIN = 200, and RS is the source impedance of the input signal. Note: tACQ is never less than 104ns and any source impedance below 12 does not significantly affect the ADC's AC performance. signal bandwidth, making it possible to digitize highspeed transient events and measure periodic signals with bandwidths exceeding the ADC's sampling rate by using undersampling techniques. To avoid high-frequency signals being aliased into the frequency band of interest, anti-alias filtering is recommended.
Analog Input Protection
Internal protection diodes that clamp the analog input to VDD and GND allow the analog input pins to swing from GND - 0.3V to VDD + 0.3V without damage. Both inputs must not exceed VDD or be lower than GND for accurate conversions.
Serial Interface
Initialization After Power-Up and Starting a Conversion
Upon initial power-up, the MAX1276/MAX1278 require a complete conversion cycle to initialize the internal calibration. Following this initial conversion, the part is ready for normal operation. This initialization is only required after a hardware power-up sequence and is not required after exiting partial or full power-down mode. To start a conversion, pull CNVST low. At CNVST's falling edge, the T/H enters its hold mode and a conversion is initiated. SCLK runs the conversion and the data can then be shifted out serially on DOUT.
Input Bandwidth
The ADC's input-tracking circuitry has a 20MHz small-
CNVST tSETUP POWER-MODE SELECTION WINDOW tACQUIRE CONTINUOUS-CONVERSION SELECTION WINDOW 16
SCLK
1
2
3
4
8
14
HIGH IMPEDANCE DOUT
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
Figure 5. Interface-Timing Sequence
CNVST ONE 8-BIT TRANSFER SCLK 1ST SCLK RISING EDGE DOUT MODE REF 0 0 0 D11 D10 D9 D8 D7 DOUT GOES HIGH IMPEDANCE ONCE CNVST GOES HIGH CNVST MUST GO HIGH AFTER THE 3RD BUT BEFORE THE 14TH SCLK RISING EDGE
NORMAL ENABLED (4.096V)
PPD
Figure 6. SPI Interface--Partial Power-Down Mode 10 ______________________________________________________________________________________
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
Timing and Control
Conversion-start and data-read operations are controlled by the CNVST and SCLK digital inputs. Figures 1 and 5 show timing diagrams, which outline the serialinterface operation. A CNVST falling edge initiates a conversion sequence; the T/H stage holds the input voltage, the ADC begins to convert, and DOUT changes from high impedance to logic low. SCLK is used to drive the conversion process, and it shifts data out as each bit of the conversion is determined. SCLK begins shifting out the data after the 4th rising edge of SCLK. DOUT transitions t DOUT after each SCLK's rising edge and remains valid 4ns (tDHOLD) after the next rising edge. The 4th rising clock edge produces the MSB of the conversion at DOUT, and the MSB remains valid 4ns after the 5th rising edge. Since there are 12 data bits and 3 leading zeros, at least 16 rising clock edges are needed to shift out these bits. For continuous operation, pull CNVST high between the 14th and the 16th SCLK rising edges. If CNVST stays low after the falling edge of the 16th SCLK cycle, the DOUT line goes to a high-impedance state on either CNVST's rising edge or the next SCLK's rising edge. Full power-down mode is ideal for infrequent data sampling and very low supply current applications. The MAX1276/MAX1278 have to be in partial power-down mode in order to enter full power-down mode. Perform the SCLK/CNVST sequence described above to enter partial power-down mode. Then repeat the same sequence to enter full power-down mode (see Figure 7). Drive CNVST low, and allow at least 14 SCLK cycles to elapse before driving CNVST high to exit full powerdown mode. While in full power-down mode, the reference is disabled to minimize power consumption. Be sure to allow at least 2ms recovery time after exiting full power-down mode for the reference to settle. In partial/full power-down mode, maintain a logic low or a logic high on SCLK to minimize power consumption.
MAX1276/MAX1278
Transfer Function
Figure 8 shows the unipolar transfer function for the MAX1276. Figure 9 shows the bipolar transfer function for the MAX1278. The MAX1276 output is straight binary, while the MAX1278 output is two's complement.
Applications Information
Internal Reference
The MAX1276/MAX1278 have an on-chip voltage reference trimmed to 4.096V. The internal reference output is connected to REF and also drives the internal capacitive DAC. The output can be used as a reference voltage source for other components and can source up to 2mA Bypass REF with a 0.01F capacitor and a 4.7F capacitor to RGND. The internal reference is continuously powered up during both normal and partial power-down modes. In full power-down mode, the internal reference is disabled. Be sure to allow at least 2ms recovery time after hardware power-up or exiting full power-down mode for the reference to reach its intended value.
Partial Power-Down and Full Power-Down Modes
Power consumption can be reduced significantly by placing the MAX1276/MAX1278 in either partial power-down mode or full power-down mode. Partial power-down mode is ideal for infrequent data sampling and fast wakeup time applications. Pull CNVST high after the 3rd SCLK rising edge and before the 14th SCLK rising edge to enter and stay in partial power-down mode (see Figure 6). This reduces the supply current to 2mA. While in partial power-down mode, the reference remains enabled to allow valid conversions once the IC is returned to normal mode. Drive CNVST low and allow at least 14 SCLK cycles to elapse before driving CNVST high to exit partial power-down mode.
CNVST FIRST 8-BIT TRANSFER SCLK 1ST SCLK RISING EDGE DOUT 0 0 0 D11 D10 D9 1ST SCLK RISING EDGE D8 D7 PPD
EXECUTE PARTIAL POWER-DOWN TWICE SECOND 8-BIT TRANSFER
DOUT ENTERS TRI-STATE ONCE CNVST GOES HIGH
0
0
0
0
0
0
0
0 FPD DISABLED
MODE REF
NORMAL
RECOVERY
ENABLED (4.096V)
Figure 7. SPI Interface--Full Power-Down Mode ______________________________________________________________________________________ 11
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
How to Start a Conversion
OUTPUT CODE FULL-SCALE TRANSITION
111...111 111...110 111...101
FS = VREF ZS = 0 V 1 LSB = REF 4096 000...011 000...010 000...001 000...000 0 1 2 3 DIFFERENTIAL INPUT VOLTAGE (LSB) FS - 3/2 LSB FS
An analog-to-digital conversion is initiated by CNVST, clocked by SCLK, and the resulting data is clocked out on DOUT by SCLK. With SCLK idling high or low, a falling edge on CNVST begins a conversion. This causes the analog input stage to transition from track to hold mode, and for DOUT to transition from high impedance to being actively driven low. A total of 16 SCLK cycles are required to complete a normal conversion. If CNVST is low during the 16th falling SCLK edge, DOUT returns to high impedance on the next rising edge of CNVST or SCLK, enabling the serial interface to be shared by multiple devices. If CNVST returns high after the 14th, but before the 16th SCLK rising edge, DOUT remains active so continuous conversions can be sustained. The highest throughput is achieved when performing continuous conversions. Figure 10 illustrates a conversion using a typical serial interface.
Connection to Standard Interfaces
The MAX1276/MAX1278 serial interface is fully compatible with SPI/QSPI and MICROWIRE (see Figure 11). If a serial interface is available, set the CPU's serial interface in master mode so the CPU generates the serial clock. Choose a clock frequency up to 28.8MHz.
Figure 8. Unipolar Transfer Function (MAX1276 Only)
SPI and MICROWIRE
OUTPUT CODE V FS = REF 2 ZS = 0 -V - FS = REF 2 V 1 LSB = REF 4096 FULL-SCALE TRANSITION
011...111 011...110
000...010 000...001 000...000 111...111 111...110 111...101
When using SPI or MICROWIRE, the MAX1276/ MAX1278 are compatible with all four modes programmed with the CPHA and CPOL bits in the SPI or MICROWIRE control register. Conversion begins with a CNVST falling edge. DOUT goes low, indicating a conversion is in progress. Two consecutive 1-byte reads are required to get the full 12 bits from the ADC. DOUT transitions on SCLK rising edges. DOUT is guaranteed to be valid tDOUT later and remains valid until tDHOLD after the following SCLK rising edge. When using CPOL = 0 and CPHA = 0, or CPOL = 1 and CPHA = 1, the data is clocked into the P on the following rising edge. When using CPOL = 0 and CPHA = 1, or CPOL = 1 and CPHA = 0, the data is clocked into the P on the next falling edge. See Figure 11 for connections and Figures 12 and 13 for timing. See the Timing Characteristics section to determine the best mode to use.
100...001 100...000
QSPI
-FS 0 DIFFERENTIAL INPUT VOLTAGE (LSB) FS FS - 3/2 LSB
Figure 9. Bipolar Transfer Function (MAX1278 Only)
Unlike SPI, which requires two 1-byte reads to acquire the 12 bits of data from the ADC, QSPI allows the minimum number of clock cycles necessary to clock in the data. The MAX1276/MAX1278 require 16 clock cycles from the P to clock out the 12 bits of data. Figure 14 shows a transfer using CPOL = 1 and CPHA = 1. The conversion result contains three zeros, followed by the 12 data bits, and a trailing zero with the data in MSB-first format.
12
______________________________________________________________________________________
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
CNVST
SCLK
1
14
16
1
DOUT 0 0 0 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 0 0
Figure 10. Continuous Conversion with Burst/Continuous Clock
I/O SCK MISO
+3V TO +5V
CNVST SCLK DOUT
MAX1276 MAX1278
SS A) SPI
CS SCK MISO
+3V TO +5V
CNVST SCLK DOUT
MAX1276 MAX1278
SS B) QSPI
I/O SK SI
CNVST SCLK DOUT
MAX1276 MAX1278
C) MICROWIRE
Figure 11. Common Serial-Interface Connections to the MAX1276/MAX1278
______________________________________________________________________________________
13
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
CNVST
1 SCLK HIGH-Z
8
9
16
DOUT
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
HIGH-Z
Figure 12. SPI/MICROWIRE Serial-Interface Timing--Single Conversion (CPOL = CPHA = 0), (CPOL = CPHA = 1)
CNVST
SCLK
1
14
16
1
DOUT
0
0
0
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
0
0
Figure 13. SPI/MICROWIRE Serial-Interface Timing--Continuous Conversion (CPOL = CPHA = 0), (CPOL = CPHA = 1)
CNVST
SCLK HIGH-Z
2
16
HIGH-Z D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
DOUT
Figure 14. QSPI Serial-Interface Timing--Single Conversion (CPOL = 1, CPHA = 1)
DSP Interface to the TMS320C54_
The MAX1276/MAX1278 can be directly connected to the TMS320C54_ family of DSPs from Texas Instruments, Inc. Set the DSP to generate its own clocks or use external clock signals. Use either the standard or buffered serial port. Figure 15 shows the simplest interface between the MAX1276/MAX1278 and the TMS320C54_, where the transmit serial clock (CLKX) drives the receive serial clock (CLKR) and SCLK, and the transmit frame sync (FSX) drives the receive frame sync (FSR) and CNVST. For continuous conversion, set the serial port to trans-
mit a clock, and pulse the frame sync signal for a clock period before data transmission. The serial-port configuration (SPC) register should be set up with internal frame sync (TXM = 1), CLKX driven by an on-chip clock source (MCM = 1), burst mode (FSM = 1), and 16-bit word length (FO = 0). This setup allows continuous conversions provided that the data-transmit register (DXR) and the data-receive register (DRR) are serviced before the next conversion. Alternatively, autobuffering can be enabled when using the buffered serial port to execute conversions and read the data without CPU intervention. Connect the VL
14
______________________________________________________________________________________
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
pin to the TMS320C54_ supply voltage when the MAX1276/MAX1278 are operating with an analog supply voltage higher than the DSP supply voltage. The word length can be set to 8 bits with FO = 1 to implement the power-down modes. The CNVST pin must idle high to remain in either power-down state. Another method of connecting the MAX1276/MAX1278 to the TMS320C54_ is to generate the clock signals external to either device. This connection is shown in Figure 16, where serial clock (CLOCK) drives the CLKR, and SCLK and the convert signal (CONVERT) drive the FSR and CNVST. The serial port must be set up to accept an external receive-clock and external receive-frame sync. The SPC register should be written as follows: TXM = 0, external frame sync MCM = 0, CLKX is taken from the CLKX pin FSM = 1, burst mode FO = 0, data transmitted/received as 16-bit words This setup allows continuous conversion, provided that the DRR is serviced before the next conversion. Alternatively, autobuffering can be enabled when using the buffered serial port to read the data without CPU intervention. Connect the VL pin to the TMS320C54_ supply voltage when the MAX1276/MAX1278 are operating with an analog supply voltage higher than the DSP supply voltage. The MAX1276/MAX1278 can also be connected to the TMS320C54_ by using the data transmit (DX) pin to drive CNVST and the CLKX generated internally to drive SCLK. A pullup resistor is required on the CNVST signal to keep it high when DX goes high impedance and 0001hex should be written to the DXR continuously for continuous conversions. The power-down modes may be entered by writing 00FFhex to the DXR (see Figures 17 and 18).
MAX1276/MAX1278
VL
DVDD CLKX TMS320C54_ CLKR FSX FSR
MAX1276 SCLK MAX1278
CNVST
DOUT
DR
Figure 15. Interfacing to the TMS320C54_ Internal Clocks
VL
DVDD
MAX1276 MAX1278 SCLK
CNVST DOUT
TMS320C54_
CLKR FSR DR
DSP Interface to the ADSP21_ _ _
The MAX1276/MAX1278 can be directly connected to the ADSP21_ _ _ family of DSPs from Analog Devices, Inc. Figure 19 shows the direct connection of the MAX1276/MAX1278 to the ADSP21_ _ _. There are two modes of operation that can be programmed to interface with the MAX1276/MAX1278. For continuous conversions, idle CNVST low and pulse it high for one clock cycle during the LSB of the previous transmitted word. The ADSP21_ _ _ STCTL and SRCTL registers should be configured for early framing (LAFR = 0) and for an
CLOCK CONVERT
Figure 16. Interfacing to the TMS320C54_ External Clocks
CNVST
SCLK
1
1
DOUT
D0
0
0
0
0
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
0
0
Figure 17. DSP Interface--Continuous Conversion ______________________________________________________________________________________ 15
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
CNVST
SCLK
1
1
DOUT
0
0
0
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
0
0
0
Figure 18. DSP Interface--Single-Conversion, Continuous/Burst Clock
VL
VDDINT TCLK RCLK TFS RFS
MAX1276 SCLK MAX1278
CNVST
ADSP21_ _ _
and for an active-low frame (LTFS = 1, LRFS = 1) signal. This is also the best way to enter the power-down modes by setting the word length to 8 bits (SLEN = 1001). Connect the VL pin to the ADSP21_ _ _ supply voltage when the MAX1276/MAX1278 are operating with a supply voltage higher than the DSP supply voltage (see Figures 17 and 18).
DOUT
DR
Layout, Grounding, and Bypassing
For best performance, use PC boards. Wire-wrap boards are not recommended. Board layout should ensure that digital and analog signal lines are separated from each other. Do not run analog and digital (especially clock) lines parallel to one another, or digital lines underneath the ADC package. Figure 20 shows the recommended system ground connections. Establish a single-point analog ground (star ground point) at GND, separate from the logic ground. Connect all other analog grounds and DGND to this star ground point for further noise reduction. The ground return to the power supply for this ground should be low impedance and as short as possible for noise-free operation. High-frequency noise in the VDD power supply can affect the ADC's high-speed comparator. Bypass this supply to the single-point analog ground with 0.01F and 10F bypass capacitors. Minimize capacitor lead lengths for best supply-noise rejection.
Figure 19. Interfacing to the ADSP21_ _ _
SUPPLIES GND VL
10F 10F 0.1F
0.1F
VDD
GND RGND
VL
DGND DIGITAL CIRCUITRY
VL
MAX1276 MAX1278 Figure 20. Power-Supply Grounding Condition
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values on an actual transfer function from a straight line. This straight line can be either a best-straight-line fit or a line drawn between the end points of the transfer function, once offset and gain errors have been nullified. The static linearity parameters for the MAX1276/MAX1278 are measured using the end-points method.
active-high frame (LTFS = 0, LRFS = 0) signal. In this mode, the data-independent frame-sync bit (DITFS = 1) can be selected to eliminate the need for writing to the transmit-data register more than once. For single conversions, idle CNVST high and pulse it low for the entire conversion. The ADSP21_ _ _ STCTL and SRCTL registers should be configured for late framing (LAFR = 1)
16
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
______________________________________________________________________________________
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference
an actual step width and the ideal value of 1 LSB. A DNL error specification of 1 LSB or less guarantees no missing codes and a monotonic transfer function.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS sum of the first five harmonics of the input signal to the fundamental itself. This is expressed as: 2 2 2 V 2 + V3 + V4 + V5 2 THD = 20 x log V1 where V 1 is the fundamental amplitude, and V 2 through V5 are the amplitudes of the 2nd- through 5thorder harmonics.
MAX1276/MAX1278
Aperture Jitter
Aperture jitter (tAJ) is the sample-to-sample variation in the time between the samples.
Aperture Delay
Aperture delay (tAD) is the time defined between the falling edge of CNVST and the instant when an actual sample is taken.
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital samples, signal-to-noise ratio (SNR) is the ratio of the fullscale analog input (RMS value) to the RMS quantization error (residual error). The theoretical minimum analog-todigital noise is caused by quantization error, and results directly from the ADC's resolution (N bits): SNR = (6.02 x N + 1.76)dB In reality, there are other noise sources besides quantization noise, including thermal noise, reference noise, clock jitter, etc. Therefore, SNR is computed by taking the ratio of the RMS signal to the RMS noise, which includes all spectral components minus the fundamental, the first five harmonics, and the DC offset.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio of the RMS amplitude of the fundamental (maximum signal component) to the RMS value of the next-largest distortion component.
Full-Power Bandwidth
Full-power bandwidth is the frequency at which the input signal amplitude attenuates by 3dB for a full-scale input.
Full-Linear Bandwidth
Full-linear bandwidth is the frequency at which the signal to noise plus distortion (SINAD) is equal to 68dB.
Signal-to-Noise Plus Distortion
Signal-to-noise plus distortion (SINAD) is the ratio of the fundamental input frequency's RMS amplitude to the RMS equivalent of all other ADC output signals: SINAD(dB) = 20 x log (SignalRMS / NoiseRMS)
Intermodulation Distortion (IMD)
Any device with nonlinearities creates distortion products when two sine waves at two different frequencies (f1 and f2) are input into the device. Intermodulation distortion (IMD) is the total power of the IM2 to IM5 intermodulation products to the Nyquist frequency relative to the total input power of the two input tones, f1 and f2. The individual input tone levels are at -7dBFS. The intermodulation products are as follows: * 2nd-order intermodulation products (IM2): f1 + f2, f2 - f1 * 3rd-order intermodulation products (IM3): 2f 1 - f2, 2f2 - f1, 2f1 + f2, 2f2 + f1 * 4th-order intermodulation products (IM4): 3f 1 - f2, 3f2 - f1, 3f1 + f2, 3f2 + f1 * 5th-order intermodulation products (IM5): 3f1 - 2f2, 3f2 - 2f1, 3f1 + 2f2, 3f2 + 2f1
Effective Number of Bits
Effective number of bits (ENOB) indicates the global accuracy of an ADC at a specific input frequency and sampling rate. An ideal ADC's error consists of quantization noise only. With an input range equal to the full-scale range of the ADC, calculate the ENOB as follows: (SINAD - 1.76) ENOB = 6.02
Chip Information
TRANSISTOR COUNT: 13,016 PROCESS: BiCMOS PACKAGE CODE: T1244-3
______________________________________________________________________________________ 17
1.8Msps, Single-Supply, Low-Power, TrueDifferential, 12-Bit ADCs with Internal Reference MAX1276/MAX1278
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
1
2
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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