a FEATURES Complete Dual Matching ADC Low Power Dissipation: 225 mW (+3 V Supply) Single Supply: 2.7 V to 5.5 V Differential Nonlinearity Error: 0.1 LSB On-Chip Analog Input Buffers On-Chip Reference Signal-to-Noise Ratio: 49.2 dB Over Seven Effective Bits Spurious-Free Dynamic Range: -65 dB No Missing Codes Guaranteed 28-Lead SSOP Dual Channel 8-Bit Resolution CMOS ADC AD9281 FUNCTIONAL BLOCK DIAGRAM AVDD IINA IINB IREFB IREFT QREFB QREFT "I" ADC AVSS CLOCK I REGISTER AD9281 ASYNCHRONOUS MULTIPLEXER 1V "Q" ADC SLEEP SELECT REFSENSE QINA DVSS REFERENCE BUFFER VREF QINB DVDD Q REGISTER THREESTATE OUTPUT BUFFER DATA 8 BITS CHIP SELECT PRODUCT DESCRIPTION PRODUCT HIGHLIGHTS The AD9281 is a complete dual channel, 28 MSPS, 8-bit CMOS ADC. The AD9281 is optimized specifically for applications where close matching between two ADCs is required (e.g., I/Q channels in communications applications). The 28 MHz sampling rate and wide input bandwidth will cover both narrowband and spread-spectrum channels. The AD9281 integrates two 8-bit, 28 MSPS ADCs, two input buffer amplifiers, an internal voltage reference and multiplexed digital output buffers. 1. Dual 8-Bit, 28 MSPS ADC A pair of high performance 28 MSPS ADCs that are optimized for spurious free dynamic performance are provided for encoding of I and Q or diversity channel information. Each ADC incorporates a simultaneous sampling sample-andhold amplifier at its input. The analog inputs are buffered; no external input buffer op amp will be required in most applications. The ADCs are implemented using a multistage pipeline architecture that offers accurate performance and guarantees no missing codes. The outputs of the ADCs are ported to a multiplexed digital output buffer. 3. On-Chip Voltage Reference The AD9281 includes an on-chip compensated bandgap voltage reference pin programmable for 1 V or 2 V. The AD9281 is manufactured on an advanced low cost CMOS process, operates from a single supply from 2.7 V to 5.5 V, and consumes 225 mW of power (on 3 V supply). The AD9281 input structure accepts either single-ended or differential signals, providing excellent dynamic performance up to and beyond 14 MHz Nyquist input frequencies. 2. Low Power Complete CMOS Dual ADC function consumes a low 225 mW on a single supply (on 3 V supply). The AD9281 operates on supply voltages from 2.7 V to 5.5 V. 4. On-chip analog input buffers eliminate the need for external op amps in most applications. 5. Single 8-Bit Digital Output Bus The AD9281 ADC outputs are interleaved onto a single output bus saving board space and digital pin count. 6. Small Package The AD9281 offers the complete integrated function in a compact 28-lead SSOP package. 7. Product Family The AD9281 dual ADC is pin compatible with a dual 10-bit ADC (AD9201). REV. F Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/461-3113 (c)1999-2011 Analog Devices, Inc. All rights reserved. AD9281-SPECIFICATIONS Parameter (AVDD = +3 V, DVDD = +3 V, FSAMPLE = 28 MSPS, VREF = 2 V, INB = 0.5 V, TMIN to TMAX unless otherwise noted) Symbol Min RESOLUTION CONVERSION RATE DC ACCURACY Differential Nonlinearity Integral Nonlinearity Differential Nonlinearity (SE)1 Integral Nonlinearity (SE)1 Zero-Scale Error, Offset Error Full-Scale Error, Gain Error Gain Match Offset Match ANALOG INPUT Input Voltage Range Input Capacitance Aperture Delay Aperture Uncertainty (Jitter) Aperture Delay Match Input Bandwidth (-3 dB) Small Signal (-20 dB) Full Power (0 dB) INTERNAL REFERENCE Output Voltage (1 V Mode) Output Voltage Tolerance (1 V Mode) Output Voltage (2 V Mode) Output Voltage Tolerance (2 V Mode) Load Regulation (1 V Mode) Load Regulation (2 V Mode) POWER SUPPLY Operating Voltage Supply Current Power Consumption Power-Down Power Supply Rejection DYNAMIC PERFORMANCE2 Signal-to-Noise and Distortion f = 3.58 MHz f = 14 MHz Signal-to-Noise f = 3.58 MHz f = 14 MHz Total Harmonic Distortion f = 3.58 MHz f = 14 MHz Spurious Free Dynamic Range f = 3.58 MHz f = 14 MHz Two-Tone Intermodulation Distortion3 Differential Phase Differential Gain Crosstalk Rejection Typ Max Units Condition 8 Bits FS 28 MHz (32 MHz at +25C) DNL INL DNL INL EZS EFS 0.1 0.25 0.2 0.3 1 1.2 0.2 1.2 LSB LSB LSB LSB % FS % FS LSB LSB REFT = 1.0 V, REFB = 0.0 V AIN CIN tAP tAJ -0.5 1.0 1.5 3.2 5.4 AVDD/2 2 4 2 2 V pF ns ps ps 240 245 MHz MHz 1 10 2 15 10 15 V mV V mV mV mV REFT = 1.0 V, REFB = 0.0 V BW VREF VREF VREF AVDD DVDD IAVDD IDVDD PD 2.7 2.7 3 3 75 0.1 225 16 0.15 PSR 35 5.5 5.5 260 0.75 V V mA mA mW mW % FS REFSENSE = VREF REFSENSE = GND 1 mA Load Current 1 mA Load Current STBY = AVDD, Clock Low SINAD 46.4 49.1 48 dB dB 47.8 49.2 48.5 dB dB SNR THD -67.5 -60 -49.5 dB dB SFDR 49.6 IMD DP DG -2- 65 56 -58 0.2 0.08 -62 dB dB dB Degree % dB f = 44.9 MHz and 45.52 MHz NTSC 40 IRE Mod Ramp FS = 14.3 MHz REV. F AD9281 Parameter Symbol Min Typ Max Units Condition 1 DYNAMIC PERFORMANCE (SE) Signal-to-Noise and Distortion f = 3.58 MHz Signal-to-Noise f = 3.58 MHz Total Harmonic Distortion f = 3.58 MHz Spurious Free Dynamic Range f = 3.58 MHz SINAD 47.2 dB 48 dB -55 dB -58 dB SNR THD SFDR DIGITAL INPUTS High Input Voltage Low Input Voltage DC Leakage Current Input Capacitance VIH VIL IIN CIN LOGIC OUTPUT (with DVDD = 3 V) High Level Output Voltage (IOH = 50 A) Low Level Output Voltage (IOL = 1.5 mA) LOGIC OUTPUT (with DVDD = 5 V) High Level Output Voltage (IOH = 50 A) Low Level Output Voltage (IOL = 1.5 mA) Data Valid Delay MUX Select Delay Data Enable Delay Data High-Z Delay CLOCKING Clock Pulsewidth High Clock Pulsewidth Low Pipeline Latency 2.4 V V A pF 0.3 6 2 VOH 2.88 V VOL 0.095 V VOH 4.5 V VOL tOD tMD tED 0.4 11 7 13 V ns ns ns tDHZ 13 ns 3.0 ns ns Cycles tCH tCL 16.9 16.9 CL = 20 pF. Output Level to 90% of Final Value NOTES 1 SE is single ended input, REFT = 1.5 V, REFB = -0.5 V. 2 AIN differential 2 V p-p, REFT = 1.5 V, REFB = -0.5 V. 3 IMD referred to larger of two input signals. Specifications subject to change without notice. tOD CLOCK INPUT ADC SAMPLE #1 SELECT INPUT ADC SAMPLE #2 ADC SAMPLE #3 SAMPLE #1-3 Q CHANNEL OUTPUT I CHANNEL OUTPUT ENABLED SAMPLE #1 Q CHANNEL OUTPUT SAMPLE #1-2 Q CHANNEL OUTPUT SAMPLE #1-1 I CHANNEL OUTPUT Figure 1. ADC Timing REV. F ADC SAMPLE #5 t MD Q CHANNEL OUTPUT ENABLED SAMPLE #1-1 Q CHANNEL OUTPUT DATA OUTPUT ADC SAMPLE #4 -3- SAMPLE #1 I CHANNEL OUTPUT SAMPLE #2 Q CHANNEL OUTPUT AD9281 ABSOLUTE MAXIMUM RATINGS* Parameter With Respect to AVDD AVSS DVDD DVSS AVSS DVSS AVDD DVDD CLK AVSS Digital Outputs DVSS AINA, AINB AVSS VREF AVSS REFSENSE AVSS REFT, REFB AVSS Junction Temperature Storage Temperature Lead Temperature 10 sec PIN FUNCTION DESCRIPTIONS Pin Min Max Units -0.3 -0.3 -0.3 -6.5 -0.3 -0.3 -1.0 -0.3 -0.3 -0.3 +6.5 +6.5 +0.3 +6.5 AVDD + 0.3 DVDD + 0.3 AVDD + 0.3 AVDD + 0.3 AVDD + 0.3 AVDD + 0.3 +150 +150 V V V V V V V V V V C C +300 C -65 *Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum ratings for extended periods may effect device reliability. PIN CONFIGURATION DVSS CHIP-SELECT DVDD INA-Q DNC DNC INB-Q (LSB) D0 D1 D2 REFT-Q AD9281 TOP VIEW (Not to Scale) AVSS D5 REFB-I D6 REFT-I (MSB) D7 INB-I SELECT INA-I CLOCK 1 2 3 4 5 6 7 8 9 10 11 12 DVSS DVDD Digital Ground Digital Supply DNC Do not connect DNC Do not connect D0 D1 D2 D3 D4 D5 D6 D7 Bit 0 (LSB) Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 (MSB) 13 14 15 SELECT CLOCK SLEEP Hi I Channel Out, Lo Q Channel Out Clock Hi Power Down, Lo Normal Operation 16 17 18 19 20 21 22 23 24 25 26 27 28 INA-I INB-I REFT-I REFB-I AVSS REFSENSE VREF AVDD REFB-Q REFT-Q INB-Q INA-Q CHIP-SELECT I Channel, A Input I Channel, B Input Top Reference Decoupling, I Channel Bottom Reference Decoupling, I Channel Analog Ground Reference Select Internal Reference Output Analog Supply Bottom Reference Decoupling, Q Channel Top Reference Decoupling, Q Channel Q Channel B Input Q Channel A Input Hi-High Impedance, Lo-Normal Operation Integral nonlinearity refers to the deviation of each individual code from a line drawn from "zero" through "full scale." The point used as "zero" occurs 1/2 LSB before the first code transition. "Full scale" is defined as a level 1 1/2 LSBs beyond the last code transition. The deviation is measured from the center of each particular code to the true straight line. VREF REFSENSE Description INTEGRAL NONLINEARITY (INL) AVDD D4 Name DEFINITIONS OF SPECIFICATIONS REFB-Q D3 No. DIFFERENTIAL NONLINEARITY (DNL, NO MISSING CODES) SLEEP NC = NO CONNECT An ideal ADC exhibits code transitions that are exactly 1 LSB apart. DNL is the deviation from this ideal value. It is often specified in terms of the resolution for which no missing codes (NMC) are guaranteed. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD9281 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. -4- WARNING! ESD SENSITIVE DEVICE REV. F AD9281 AVDD DRVDD AVDD AVDD DRVSS AVSS DRVSS AVSS a. D0-D9 AVDD AVSS AVSS b. Three-State Standby AVDD AVDD AVDD AVDD AVSS c. CLK AVDD AVDD REFBS IN AVSS AVSS AVDD AVSS REFBF AVSS AVSS AVSS d. INA, INB e. Reference f. REFSENSE g. VREF Figure 2. Equivalent Circuits OFFSET ERROR The first transition should occur at a level 1 LSB above "zero." Offset is defined as the deviation of the actual first code transition from that point. scale. Gain error is the deviation of the actual difference between first and last code transitions and the ideal difference between the first and last code transitions. GAIN MATCH OFFSET MATCH The change in gain error between I and Q channels. The change in offset error between I and Q channels. PIPELINE DELAY (LATENCY) EFFECTIVE NUMBER OF BITS (ENOB) For a sine wave, SINAD can be expressed in terms of the number of bits. Using the following formula, N = (SINAD - 1.76)/6.02 The number of clock cycles between conversion initiation and the associated output data being made available. New output data is provided every rising clock edge. MUX SELECT DELAY It is possible to get a measure of performance expressed as N, the effective number of bits. The delay between the change in SELECT pin data level and valid data on output pins. Thus, effective number of bits for a device for sine wave inputs at a given input frequency can be calculated directly from its measured SINAD. POWER SUPPLY REJECTION TOTAL HARMONIC DISTORTION (THD) THD is the ratio of the rms sum of the first six harmonic components to the rms value of the measured input signal and is expressed as a percentage or in decibels. The specification shows the maximum change in full scale from the value with the supply at the minimum limit to the value with the supply at its maximum limit. APERTURE JITTER Aperture jitter is the variation in aperture delay for successive samples and is manifested as noise on the input to the A/D. SIGNAL-TO-NOISE RATIO (SNR) SNR is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the Nyquist frequency, excluding the first six harmonics and dc. The value for SNR is expressed in decibels. APERTURE DELAY SPURIOUS FREE DYNAMIC RANGE (SFDR) SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD) RATIO The difference in dB between the rms amplitude of the input signal and the peak spurious signal. GAIN ERROR The first code transition should occur for an analog value 1 LSB above nominal negative full scale. The last transition should occur for an analog value 1 LSB below the nominal positive full REV. F Aperture delay is a measure of the Sample-and-Hold Amplifier (SHA) performance and is measured from the rising edge of the clock input to when the input signal is held for conversion. S/N+D is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for S/N+D is expressed in decibels. -5- AD9281-Typical Characteristic Curves (AVDD = +3 V, DVDD = +3 V, FS = 28 MHz (50% duty cycle), 2 V input span from -0.5 V to +1.5 V, 2 V internal reference unless otherwise noted) 55 SNR - dB LSB 1 0 50 -0.5dB 45 -6dB 40 35 -20dB 30 -1 0 25 1.0E+05 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 CODE OFFSET Figure 3. Typical INL 1.0E+08 Figure 6. SNR vs. Input Frequency 55 SNR - dB 1 LSB 1.0E+06 1.0E+07 INPUT FREQUENCY - Hz 0 50 -0.5dB 45 -6dB 40 35 -20dB 30 -1 0 25 1.0E+05 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 CODE OFFSET Figure 4. Typical DNL 1.0E+06 1.0E+07 INPUT FREQUENCY - Hz 1.0E+08 Figure 7. SINAD vs. Input Frequency 1.00 -30 0.80 -35 0.60 -40 0.40 THD - dB IB - nA 0.20 0.00 -0.20 -45 -50 -55 -0.40 -6dB -60 -0.60 -65 -0.80 -1.00 -1.0 -20dB -0.5 0 0.5 1.0 INPUT VOLTAGE - Volts 1.5 -70 1.0E+05 2.0 Figure 5. Input Bias Current vs. Input Voltage -0.5dB 1.0E+06 1.0E+07 INPUT FREQUENCY - Hz 1.0E+08 Figure 8. THD vs. Input Frequency -6- REV. F AD9281 1.20E+07 70 65 55 8.00E+06 50 HITS THD - dB 10000000 1.00E+07 60 45 6.00E+06 40 4.00E+06 35 30 2.00E+06 25 20 1.00E+06 1.00E+07 CLOCK FREQUENCY - Hz 12050 0.00E+00 1.00E+08 Figure 9. THD vs. Clock Frequency N-1 800 N+1 N CODE Figure 12. Grounded Input Histogram 1.013 0 -3 1.012 AMPLITUDE - dB -6 VREF - Volts 1.011 1.010 -9 -12 -15 -18 -21 1.009 -24 1.008 -40 0 -20 20 40 60 TEMPERATURE - 8C 80 -27 1.00E+06 100 Figure 10. Voltage Reference Error vs. Temperature 50 235 -0.5dB 230 45 225 -6dB 220 SNR - dB POWER CONSUMPTION - mW 1.00E+09 Figure 13. Full Power Bandwidth 240 215 210 205 40 35 200 30 195 -20dB 190 185 0 4 8 12 16 20 24 CLOCK FREQUENCY - MHz 28 25 1.00E+05 32 Figure 11. Power Consumption vs. Clock Frequency REV. F 1.00E+07 1.00E+08 INPUT FREQUENCY - Hz 1.00E+06 1.00E+07 INPUT FREQUENCY - Hz 1.00E+08 Figure 14. SNR vs. Input Frequency (Single-Ended) -7- AD9281 10.0 converter to readily accommodate either single-ended or differential input signals. This differential structure makes the part capable of accommodating a wide range of input signals. FUND 0.0 -10.0 -20.0 The AD9281 also includes an on-chip bandgap reference and reference buffer. The reference buffer shifts the ground-referred reference to levels more suitable for use by the internal circuits of the converter. Both converters share the same reference and reference buffer. This scheme provides for the best possible gain match between the converters while simultaneously minimizing the channel-to-channel crosstalk. SNR - dB -30.0 -40.0 -50.0 -60.0 2ND 9TH -70.0 -80.0 3RD 8TH 4TH 7TH 5TH 6TH Each A/D converter has its own output latch, which updates on the rising edge of the input clock. A logic multiplexer, controlled through the SELECT pin, determines which channel is passed to the digital output pins. The output drivers have their own supply, allowing the part to be interfaced to a variety of logic families. The outputs can be placed in a high impedance state using the CHIP SELECT pin. -90.0 -100.0 -110.0 0.0E+0 2.0E+6 4.0E+6 6.0E+6 8.0E+6 10.0E+6 12.0E+6 14.0E+6 Figure 15a. Simultaneous Operation of I and Q Channels 10.0 0.0 FUND The AD9281 has great flexibility in its supply voltage. The analog and digital supplies may be operated from 2.7 V to 5.5 V, independently of one another. -10.0 -20.0 SNR - dB -30.0 ANALOG INPUT -40.0 Figure 16 shows an equivalent circuit structure for the analog input of one of the A/D converters. PMOS source-followers buffer the analog input pins from the charge kickback problems normally associated with switched capacitor ADC input structures. This produces a very high input impedance on the part, allowing it to be effectively driven from high impedance sources. This means that the AD9281 could even be driven directly by a passive antialias filter. -50.0 -60.0 -70.0 2ND 3RD 5TH 4TH -80.0 6TH 7TH 8TH -90.0 -100.0 -110.0 0.0E+0 2.0E+6 4.0E+6 6.0E+6 8.0E+6 10.0E+6 12.0E+6 14.0E+6 Figure 15b. Simultaneous Operation of I and Q Channels IINA BUFFER THEORY OF OPERATION The AD9281 integrates two A/D converters, two analog input buffers, an internal reference and reference buffer, and an output multiplexer. For clarity, this data sheet refers to the two converters as "I" and "Q." The two A/D converters simultaneously sample their respective inputs on the rising edge of the input clock. The two converters distribute the conversion operation over several smaller A/D sub-blocks, refining the conversion with progressively higher accuracy as it passes the result from stage to stage. As a consequence of the distributed conversion, each converter requires a small fraction of the 256 comparators used in a traditional flash-type 8-bit ADC. A sample-and-hold function within each of the stages permits the first stage to operate on a new input sample while the following stages continue to process previous samples. This results in a "pipeline processing" latency of three clock periods between when an input sample is taken and when the corresponding ADC output is updated into the output registers. SHA IINB BUFFER OUTPUT WORD ADC CORE +FS LIMIT +FS LIMIT = VREF +VREF/2 -FS LIMIT -FS LIMIT = VREF -VREF/2 VREF Figure 16. Equivalent Circuit for AD9281 Analog Inputs The source followers inside the buffers also provide a level-shift function of approximately 1 V, allowing the AD9281 to accept inputs at or below ground. One consequence of this structure is that distortion will result if the analog input comes within 1.4 V of the positive supply. For optimum high frequency distortion performance, the analog input signal should be centered according to Figure 27. The capacitance load of the analog input pin is 4 pF to the analog supplies (AVSS, AVDD). The AD9281 integrates input buffer amplifiers to drive the analog inputs of the converters. In most applications, these input amplifiers eliminate the need for external op amps for the input signals. The input structure is fully differential, but the SHA common-mode response has been designed to allow the Full-scale setpoints may be calculated according to the following algorithm (VREF may be internally or externally generated): -FS = VREF - (VREF/2) +FS = VREF + (VREF/2) VSPAN = VREF -8- REV. F AD9281 The AD9281 can accommodate a variety of input spans between 1 V and 2 V. For spans of less than 1 V, expect a proportionate degradation in SNR. Use of a 2 V span will provide the best noise performance. 1 V spans will provide lower distortion when using a 3 V analog supply. Users wishing to run with larger full-scales are encouraged to use a 5 V analog supply (AVDD). 1.5V ANALOG INPUT 1V 0V INPUT MIDSCALE VOLTAGE = 0.5V (1V) 5kV IINA I OR QREFT IINB I OR QREFB 0.1mF 0.1mF 10mF 0.1mF 5kV 10mF 0.1mF AD9281 I OR QREFT IINA 1kV IINB 1.0mF 0.1mF AD9281 0.1mF REFSENSE Figure 18. Example Configuration for 0.5 V-1.5 V ac Coupled Single-Ended Inputs Transformer Coupled Inputs Another option for input ac coupling is to use a transformer. This not only provides dc rejection, but also allows truly differential drive of the AD9281's analog inputs, which will provide the optimal distortion performance. Figure 19 shows a recommended transformer input drive configuration. Resistors R1 and R2 define the termination impedance of the transformer coupling. The center tap of the transformer secondary is tied to the common-mode voltage, establishing the dc bias point for the analog inputs. VREF REF SENSE 0.1mF 10mF 0.1mF I OR QREFB VREF Single-Ended Inputs: For single-ended input signals, the signal is applied to one input pin and the other input pin is tied to a midscale voltage. This midscale voltage defines the center of the full-scale span for the input signal. EXAMPLE: For a single-ended input range from 0 V to 1 V applied to IINA, we would configure the converter for a 1 V reference (see Figure 17) and apply 0.5 V to IINB. 0.1mF 0.5V 10mF Differential Inputs Use of differential input signals can provide greater flexibility in input ranges and bias points, as well as offering improvements in distortion performance, particularly for high frequency input signals. Users with differential input signals will probably want to take advantage of the differential input structure of the AD9281. Performance is still very good for single-ended inputs. Converting a single-ended input to a differential signal for application to the converter is probably only worth considering for very high frequency input signals. QINA IINB QINB R2 AD9281 COMMON MODE VOLTAGE Figure 17. Example Configuration for 0 V-1 V SingleEnded Input Signal Note that since the inputs are high impedance, this reference level can easily be generated with an external resistive divider with large resistance values (to minimize power dissipation). A decoupling capacitor is recommended on this input to minimize the high frequency noise-coupling onto this pin. Decoupling should occur close to the ADC. IINA R1 0.1mF I OR QREFT 0.1mF 10mF VREF 0.1mF 10mF I OR QREFB 0.1mF REFSENSE Figure 19. Example Configuration for Transformer Coupled Inputs Crosstalk: The internal layout of the AD9281, as well as its pinout, was configured to minimize the crosstalk between the two input signals. Users wishing to minimize high frequency crosstalk should take care to provide the best possible decoupling for input pins (see Figure 20). R and C values will make a pole dependant on antialiasing requirements. Decoupling is also required on reference pins and power supplies (see Figure 21). IINA QINA AD9281 IINB AC-Coupled Inputs If the signal of interest has no dc component, ac coupling can be easily used to define an optimum bias point. Figure 18 illustrates one recommended configuration. The voltage chosen for the dc bias point (in this case the 1 V reference) is applied to both IINA and IINB pins through 1 k resistors (R1 and R2). IINA is coupled to the input signal through Capacitor C1, while IINB is decoupled to ground through Capacitor C2. QINB Figure 20. Input Loading V ANALOG V DIGITAL AVDD 10mF 0.1mF DVDD AD9281 0.1mF 10mF 0.1mF 10mF I OR QREFT I OR QREFB 0.1mF 0.1mF Figure 21. Reference and Power Supply Decoupling REV. F -9- AD9281 REFERENCE AND REFERENCE BUFFER The reference and buffer circuitry on the AD9281 is configured for maximum convenience and flexibility. An illustration of the equivalent reference circuit is show in Figure 26. The user can select from five different reference modes through appropriate pin-strapping (see Table I below). These pin strapping options cause the internal circuitry to reconfigure itself for the appropriate operating mode. Externally Set Voltage Mode (Figure 24)--this mode uses the on-chip reference, but scales the exact reference level though the use of an external resistor divider network. VREF is wired to the top of the network, with the REFSENSE wired to the tap point in the resistor divider. The reference level (and input full scale) will be equal to 1 V x (R1 + R2)/R1. This method can be used for voltage levels from 0.7 V to 2.5 V. 1mF Table I. Table of Modes 0.1mF Mode Input Span REFSENSE Pin Figure 1V 2V Programmable External 1V 2V 1 + (R1/R2) = External Ref VREF AGND See Figure AVDD R2 22 23 24 25 1V 0V 5kV 10mF IINA QINA IINB QINB 1V 0.1mF 5kV 10mF I OR QREFT 0.1mF 10mF 5kV IINB QINB 0V AD9281 VREF QINA IINB QINB 0V AD9281 0.1mF I OR QREFT 10mF 0.1mF 0.1mF I OR QREFB AVDD 2V IINA VREF 0.1mF 2V 0.1mF 0.1mF 5kV 1V EXT REFERENCE 0.1mF 2 V Mode (Figure 23)--provides a 2 V reference and 2 V input full scale. Recommended for noise sensitive applications on 5 V supplies. The part is placed in 2 V reference mode by grounding (shorting to AVSS) the REFSENSE pin. 10mF 0.1mF 1V 0V VREF QINA 10mF External Reference Mode (Figure 25)--in this mode, the onchip reference is disabled, and an external reference applied to the VREF pin. This mode is achieved by tying the REFSENSE pin to AVDD. Figure 22. 0 V to 1 V Input 5kV 0.1mF I OR QREFB Figure 24. Programmable Reference AD9281 IINA 0.1mF I OR QREFT 1V I OR QREFB 0V AD9281 0V REFSENSE 10mF AVSS VREF = 1 + R2 R1 0.1mF 5kV 1V +- REFSENSE R1 1 V Mode (Figure 22)--provides a 1 V reference and 1 V input full scale. Recommended for applications wishing to optimize high frequency performance, or any circuit on a supply voltage of less than 4 V. The part is placed in this mode by shorting the REFSENSE pin to the VREF pin. 1V + - VREF REFSENSE 10mF 0.1mF Figure 25. External Reference Reference Buffer--The reference buffer structure takes the voltage on the VREF pin and level-shifts and buffers it for use by various sub-blocks within the two A/D converters. The two converters share the same reference buffer amplifier to maintain the best possible gain match between the two converters. In the interests of minimizing high frequency crosstalk, the buffered references for the two converters are separately decoupled on the IREFB, IREFT, QREFB and QREFT pins, as illustrated in Figure 26. 0.1mF I OR QREFT 10mF 0.1mF 0.1mF I OR QREFB REFSENSE 10mF 0.1mF Figure 23. 0 V to 2 V Input -10- REV. F AD9281 feature chip capacitors located close to the converter IC. The capacitors are connected to either IREFT/IREFB or QREFT/ QREFB. A connection to both sides is not required. ADC CORE 0.1mF 10mF QREFT IREFT 0.1mF 0.1mF 1.0mF 1V IREFB VREF 0.1mF 0.1mF QREFB 0.1mF 10mF 0.1mF 10kV 10kV Attention to the common-mode point of the analog input voltage can improve the performance of the AD9281. Figure 27 illustrates THD as a function of common-mode voltage (center point of the analog input span) and power supply. Inspection of the curves will yield the following conclusions: REFSENSE AVSS COMMON-MODE PERFORMANCE 1. An AD9281 running with AVDD = 5 V is the easiest to drive. INTERNAL CONTROL LOGIC 2. Differential inputs are the most insensitive to common-mode voltage. AD9281 Figure 26. Reference Buffer Equivalent Circuit and External Decoupling Recommendation 3. An AD9281 powered by AVDD = 3 V and a single ended input, should have a 1 V span with a common-mode voltage of 0.75 V. For best results in both noise suppression and robustness against crosstalk, the 4-capacitor buffer decoupling arrangement shown in Figure 26 is recommended. This decoupling should -3 -15 -13 2V -25 -33 -43 THD - dB THD - dB -23 2V -35 1V -45 -53 1V -55 -63 -73 -0.5 0 0.5 CML - V 1 -65 -0.5 1.5 a. Differential Input, 3 V Supplies 0 0.5 CML - V 1 1.5 c. Single-Ended Input, 3 V Supplies -35 -15 -40 -25 -50 -55 2V THD - dB THD - dB -45 1V -35 2V -45 1V -60 -55 -65 -70 -0.5 0 0.5 1 CML - V 1.5 2 -65 -0.5 2.5 b. Differential Input, 5 V Supplies 0 0.5 1 CML - V 1.5 d. Single-Ended Input, 5 V Supplies Figure 27. THD vs. CML Input Span and Power Supply (Analog Input = 1 MHz) REV. F 2 -11- 2.5 AD9281 DIGITAL INPUTS AND OUTPUTS SELECT Each of the AD9281 digital control inputs, CHIP SELECT, CLOCK, SELECT and SLEEP are referenced to AVDD and AVSS. Switching thresholds will be AVDD/2. When the select pin is held LOW, the output word will present the "Q" level. When the select pin is held HIGH, the "I" level will be presented to the output word (see Figure 1). The format of the digital output is straight binary. A low power mode feature is provided such that for STBY = HIGH and the clock disabled, the static power of the AD9281 will drop below 22 mW. The AD9281's select and clock pins may be driven by a common signal source. The data will change in 5 ns to 11 ns after the edges of the input pulse. The user must make sure the interface latches have sufficient hold time for the AD9281's delays (see Figure 28). CLOCK INPUT The AD9281 clock input is internally buffered with an inverter powered from the AVDD pin. This feature allows the AD9281 to accommodate either +5 V or +3.3 V CMOS logic input signal swings with the input threshold for the CLK pin nominally at AVDD/2. CLOCK CLOCK SOURCE CLK The pipelined architecture of the AD9281 operates on both rising and falling edges of the input clock. To minimize duty cycle variations the logic family recommended to drive the clock input is high speed or advanced CMOS (HC/HCT, AC/ACT) logic. CMOS logic provides both symmetrical voltage threshold levels and sufficient rise and fall times to support 28 MSPS operation. Running the part at slightly faster clock rates may be possible, although at reduced performance levels. Conversely, some slight performance improvements might be realized by clocking the AD9281 at slower clock rates. The power dissipated by the output buffers is largely proportional to the clock frequency; running at reduced clock rates provides a reduction in power consumption. DIGITAL OUTPUTS Each of the on-chip buffers for the AD9281 output bits (D0-D9) is powered from the DVDD supply pin, separate from AVDD. The output drivers are sized to handle a variety of logic families while minimizing the amount of glitch energy generated. In all cases, a fan-out of one is recommended to keep the capacitive load on the output data bits below the specified 20 pF level. For DVDD = 5 V, the AD9281 output signal swing is compatible with both high speed CMOS and TTL logic families. For TTL, the AD9281 on-chip, output drivers were designed to support several of the high speed TTL families (F, AS, S). For applications where the clock rate is below 28 MSPS, other TTL families may be appropriate. For interfacing with lower voltage CMOS logic, the AD9281 sustains 28 MSPS operation with DVDD = 3 V. In all cases, check your logic family data sheets for compatibility with the AD9281's Specification table. I PROCESSING I LATCH SELECT DATA DATA OUT DATA Q PROCESSING Q LATCH CLOCK Figure 28. Typical De-Mux Connection APPLICATIONS USING THE AD9281 FOR QAM DEMODULATION QAM is one of the most widely used digital modulation schemes in digital communication systems. This modulation technique can be found in both FDMA as well as spread spectrum (i.e., CDMA) based systems. A QAM signal is a carrier frequency which is both modulated in amplitude (i.e., AM modulation) and in phase (i.e., PM modulation). At the transmitter, it can be generated by independently modulating two carriers of identical frequency but with a 90 phase difference. This results in an inphase (I) carrier component and a quadrature (Q) carrier component at a 90 phase shift with respect to the I component. The I and Q components are then summed to provide a QAM signal at the specified carrier or IF frequency. Figure 29 shows a typical analog implementation of a QAM modulator using a dual 10-bit DAC with 2x interpolation, the AD9761. A QAM signal can also be synthesized in the digital domain thus requiring a single DAC to reconstruct the QAM signal. The AD9853 is an example of a complete (i.e., DAC included) digital QAM modulator. A 2 ns reduction in output delays can be achieved by limiting the logic load to 5 pF per output line. IOUT DSP OR ASIC 10 AD9761 CARRIER FREQUENCY 0 90 TO MIXER QOUT THREE-STATE OUTPUTS The digital outputs of the AD9281 can be placed in a high impedance state by setting the CHIP SELECT pin to HIGH. This feature is provided to facilitate in-circuit testing or evaluation. -12- NYQUIST FILTERS QUADRATURE MODULATOR Figure 29. Typical Analog QAM Modulator Architecture REV. F AD9281 At the receiver, the demodulation of a QAM signal back into its separate I and Q components is essentially the modulation process explain above but in the reverse order. A common and traditional implementation of a QAM demodulator is shown in Figure 30. In this example, the demodulation is performed in the analog domain using a dual, matched ADC and a quadrature demodulator to recover and digitize the I and Q baseband signals. The quadrature demodulator is typically a single IC containing two mixers and the appropriate circuitry to generate the necessary 90 phase shift between the I and Q mixers' local oscillators. Before being digitized by the ADCs, the mixed down baseband I and Q signals are filtered using matched analog filters. These filters, often referred to as Nyquist or PulseShaping filters, remove images-from the mixing process and any out-of-band. The characteristics of the matching Nyquist filters are well defined to provide optimum signal-to-noise (SNR) performance while minimizing intersymbol interference. The ADC's are typically simultaneously sampling their respective inputs at the QAM symbol rate or, most often, at a multiple of it if a digital filter follows the ADC. Oversampling and the use of digital filtering eases the implementation and complexity of the analog filter. It also allows for enhanced digital processing for both carrier and symbol recovery and tuning purposes. The use of a dual ADC such as the AD9281 ensures excellent gain, offset, and phase matching between the I and Q channels. I ADC DSP OR ASIC CARRIER FREQUENCY LO 90C FROM PREVIOUS STAGE Q ADC DUAL MATCHED ADC NYQUIST FILTERS QUADRATURE DEMODULATOR Figure 30. Typical Analog QAM Demodulator GROUNDING AND LAYOUT RULES As is the case for any high performance device, proper grounding and layout techniques are essential in achieving optimal performance. The analog and digital grounds on the AD9281 have been separated to optimize the management of return currents in a system. Grounds should be connected near the ADC. It is recommended that a printed circuit board (PCB) of at least four layers, employing a ground plane and power planes, be used with the AD9281. The use of ground and power planes offers distinct advantages: AVDD A A ADC IC D DIGITAL LOGIC ICs CSTRAY ANALOG CIRCUITS VIN A A IA DIGITAL CIRCUITS B CSTRAY ID AVSS A = ANALOG D = DIGITAL LOGIC SUPPLY DVDD A GND DVSS DV A Figure 31. Ground and Power Consideration These characteristics result in both a reduction of electromagnetic interference (EMI) and an overall improvement in performance. It is important to design a layout that prevents noise from coupling onto the input signal. Digital signals should not be run in parallel with the input signal traces and should be routed away from the input circuitry. Separate analog and digital grounds should be joined together directly under the AD9281 in a solid ground plane. The power and ground return currents must be carefully managed. A general rule of thumb for mixed signal layouts dictates that the return currents from digital circuitry should not pass through critical analog circuitry. Transients between AVSS and DVSS will seriously degrade performance of the ADC. If the user cannot tie analog ground and digital ground together at the ADC, he should consider the configuration in Figure 32. Another input and ground technique is shown in Figure 32. A separate ground plane has been split for RF or hard to manage signals. These signals can be routed to the ADC differentially or single ended (i.e., both can either be connected to the driver or RF ground). The ADC will perform well with several hundred mV of noise or signals between the RF and ADC analog ground. RF GROUND ANALOG GROUND DIGITAL GROUND LOGIC ADC AIN 1. The minimization of the loop area encompassed by a signal and its return path. DATA BIN 2. The minimization of the impedance associated with ground and power paths. 3. The inherent distributed capacitor formed by the power plane, PCB insulation and ground plane. Figure 32. RF Ground Scheme REV. F D -13- AD9281 REVISION HISTORY 1/11--Rev. E to Rev. F Updated Format.................................................................. Universal Changes to Pin Configuration Diagram ........................................ 4 Changes to Pin Function Descriptions Table ................................ 4 Removed Evaluation Boards; Renumbered Sequentially ............................................................................ 14 to 18 Changes to Ordering Guide ...........................................................15 8/99--Rev. D to Rev. E Rev. F | Page 14 of 15 AD9281 OUTLINE DIMENSIONS 10.50 10.20 9.90 15 5.60 5.30 5.00 1 14 8.20 7.80 7.40 0.25 0.09 1.85 1.75 1.65 2.00 MAX 0.05 MIN COPLANARITY 0.10 0.65 BSC 0.38 0.22 SEATING PLANE 8 4 0 COMPLIANT TO JEDEC STANDARDS MO-150-AH 0.95 0.75 0.55 060106-A 28 Figure 33. 28-Lead Shrink Small Outline Package [SSOP] (RS-28) Dimensions shown in millimeters ORDERING GUIDE Model1, 2 AD9281ARS AD9281ARSRL AD9281ARSZ AD9281ARSZRL 1 2 Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C Package Description 28-Lead SSOP 28-Lead SSOP 28-Lead SSOP 28-Lead SSOP Z = RoHS Compliant Part. RS = Shrink Small Outline. (c)1999-2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D00583-0-1/11(F) Rev. F | Page 15 of 15 Package Option RS-28 RS-28 RS-28 RS-28