AD768-EB

a FEATURES 30 MSPS Update Rate 16-Bit Resolution Linearity: 1/2 LSB DNL @ 14 Bits 1 LSB INL @ 14 Bits Fast Settling: 25 ns Full-Scale Settling to 0.025% SFDR @ 1 MHz Output: 86 dBc THD @ 1 MHz Output: 71 dBc Low Glitch Impulse: 35 pV-s Power Dissipation: 465 mW On-Chip 2.5 V Reference Edge-Triggered Latches Multiplying Reference Capability APPLICATIONS Arbitrary Waveform Generation Communications Waveform Reconstruction Vector Stroke Display PRODUCT DESCRIPTION DCOM (MSB) DB15 16-Bit, 30 MSPS D/A Converter AD768 FUNCTIONAL BLOCK DIAGRAM VDD AD768 MSBs: SEGMENTED CURRENT SOURCES AND SWITCHES MSB DECODER AND EDGETRIGGERED BIT LATCHES LSBs: CURRENT SOURCES, SWITCHES, AND 1kΩ R-2R LADDERS IOUTA IOUTB 1k 1k LADCOM DB0 (LSB) CLOCK NC 2.5V BANDGAP REFERENCE CONTROL AMP VEE REFCOM REFOUT IREFIN NR The AD768 is a 16-bit, high speed digital-to-analog converter (DAC) that offers exceptional ac and dc performance. The AD768 is manufactured on ADI’s Advanced Bipolar CMOS (ABCMOS) process, combining the speed of bipolar transistors, the accuracy of laser-trimmable thin film resistors, and the efficiency of CMOS logic. A segmented current source architecture is combined with a proprietary switching technique to reduce glitch energy and maximize dynamic accuracy. Edge triggered input latches and a temperature compensated bandgap reference have been integrated to provide a complete monolithic DAC solution. The AD768 is a current-output DAC with a nominal full-scale output current of 20 mA and a 1 kΩ output impedance. Differential current outputs are provided to support single-ended or differential applications. The current outputs may be tied directly to an output resistor to provide a voltage output, or fed to the summing junction of a high speed amplifier to provide a buffered voltage output. Also, the differential outputs may be interfaced to a transformer or differential amplifier. The on-chip reference and control amplifier are configured for maximum accuracy and flexibility. The AD768 can be driven by the on-chip reference or by a variety of external reference voltages based on the selection of an external resistor. An external capacitor allows the user to optimally trade off reference bandwidth and noise performance. The AD768 operates on ± 5 V supplies, typically consuming 465 mW of power. The AD768 is available in a 28-pin SOIC package and is specified for operation over the industrial temperature range. REV. B 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. PRODUCT HIGHLIGHTS 1. The low glitch and fast settling time provide outstanding dynamic performance for waveform reconstruction or digital synthesis requirements, including communications. 2. The excellent dc accuracy of the AD768 makes it suitable for high speed A/D conversion applications. 3. On-chip, edge-triggered input CMOS latches interface readily to CMOS logic families. The AD768 can support update rates up to 40 MSPS. 4. A temperature compensated, 2.5 V bandgap reference is included on-chip allowing for generation of the reference input current with the use of a single external resistor. An external reference may also be used. 5. The current output(s) of the AD768 may be used singly or differentially, either into a load resistor, external op amp summing junction or transformer. 6. Proper selection of an external resistor and compensation capacitor allow the performance-conscious user to optimize the AD768 reference level and bandwidth for the target application. © Analog Devices, Inc., 1996 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 Fax: 617/326-8703 AD768–SPECIFICATIONS Parameter RESOLUTION DC ACCURACY Linearity Error TA = +25°C TMIN to TMAX Differential Nonlinearity TA = +25°C TMIN to TMAX Monotonicity (13-Bit) ANALOG OUTPUT Offset Error Gain Error Full-Scale Output Current2 Output Compliance Range Output Resistance Output Capacitance REFERENCE OUTPUT Reference Voltage Reference Output Current3 REFERENCE INPUT Reference Input Current Reference Bandwidth4 Small Signal, IREF = 5 mA ± 0.1 mA Large Signal, IREF = 4 mA ± 2 mA TEMPERATURE COEFFICIENTS Unipolar Offset Drift Gain Drift5 Gain Drift6 Reference Voltage Drift DYNAMIC PERFORMANCE7 Maximum Output Update Rate Output Settling Time (tST) (to 0.025%) Output Propagation Delay (tPD) Glitch Impulse Output Rise Time (10% to 90%) Output Fall Time (10% to 90%) Output Noise (DB0–DB15 High, into 50 Ω) Differential Gain Error Differential Phase Error DIGITAL INPUTS Logic “1” Voltage Logic “0” Voltage Logic “1” Current Logic “0” Current Input Capacitance Input Setup Time (tS) Input Hold Time (tH) Latch Pulse Width (tLPW) 1 (TMIN to TMAX , VDD = +5.0 V, VEE = –5.0 V, LADCOM, REFCOM, DCOM = 0 V, IREFIN = 5 mA, CLOCK = 10 MHz, unless otherwise noted) Min 16 Typ Max Units Bits –8 –8 ±4 +8 +8 LSB LSB –6 ±2 +6 LSB –8 +8 LSB GUARANTEED OVER RATED SPECIFICATION TEMPERATURE RANGE –0.2 –1.0 20 –1.2 0.8 1.0 3 2.5 +5.0 5 28 9 –5 –20 –40 –30 30 40 25 10 35 5 5 3 0.01 0.01 +5 +20 +40 +30 +5.0 1.2 +0.2 +1.0 % of FSR % of FSR mA V kΩ pF V mA mA MHz MHz ppm of FSR/oC ppm of FSR/oC ppm of FSR/oC ppm/oC MSPS ns ns pV-s ns ns nV/√Hz % Degree V V µA µA pF ns ns ns 2.475 2.525 +15 7 1 35 3.5 –10 –10 10 10 5 10 1.5 +10 +10 AC LINEARITY7 Spurious-Free Dynamic Range (SFDR Within a Window) FOUT = 1.002 MHz; CLOCK = 10 MHz; 2 MHz Span FOUT = 1.002 MHz; CLOCK = 20 MHz; 2 MHz Span FOUT = 5.002 MHz; CLOCK = 30 MHz; 10 MHz Span Spurious-Free Dynamic Range (SFDR to Nyquist) FOUT = 1.002 MHz; CLOCK = 10 MHz FOUT = 1.002 MHz; CLOCK = 20 MHz FOUT = 5.002 MHz; CLOCK = 30 MHz Total Harmonic Distortion (THD) FOUT = 1.002 MHz; CLOCK = 10 MHz FOUT = 1.002 MHz; CLOCK = 20 MHz FOUT = 5.002 MHz; CLOCK = 30 MHz –2– 86 85 78 74 73 67 –71 –66 –61 79 dB dB dB dB dB dB dB dB dB REV. B 70 –68 AD768 Parameter POWER SUPPLY Positive Voltage Range Negative Voltage Range Positive Supply Current Negative Supply Current Nominal Power Dissipation Power Supply Rejection Ratio (PSRR) OPERATING RANGE Min 4.75 –5.25 Typ 5 –5 30 63 465 Max 5.25 –4.75 40 73 600 +0.2 +85 Units V V mA mA mW % of FSR/V °C –0.2 –40 NOTES 1 Measured at IOUTA, driving a virtual ground. 2 Nominal FS output current is 4 × the current at IREFIN. Therefore, nominal FS current is 20 mA when IREFIN = 5 mA. 3 Output current is defined as total current available for IREFIN and any external load. 4 Reference bandwidth is a function of external cap at NR pin. Refer to compensation section of data sheet for details. 5 Excludes internal reference drift. 6 Includes internal reference drift. 7 Measured as unbuffered voltage output (1 V range) with FS current into 50 Ω load at IOUTB. Specifications subject to change without notice. ABSOLUTE MAXIMUM RATINGS * Parameter Positive Supply Voltage (V DD) Negative Supply Voltage (VEE) Analog-to-Other Grounds (REFCOM) Digital-to-Other Grounds (DCOM) Reference Output (REFOUT) Reference Input Current (IREFIN) Digital Inputs (DB0–DB15, CLOCK) Analog Outputs (IOUTA, IOUTB) Maximum Junction Temperature Storage Temperature Lead Temperature with Respect to DCOM, REFCOM, LADCOM DCOM, REFCOM, LADCOM DCOM, LADCOM LADCOM, REFCOM REFCOM DCOM LADCOM Min –0.5 –6.0 –0.5 –0.5 Max +6.0 +0.5 +0.5 +0.5 VDD + 0.5 +7.5 VDD + 0.5 +5.0 +150 +150 +300 Units V V V V V mA V V °C °C °C –0.5 –2.0 –65 *Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and 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 rating for extended periods may affect device reliability. ORDERING GUIDE DB0–DB15 Model AD768AR AD768ACHIPS AD768-EB Package Description 28-Pin 300 mil SOIC Die AD768 Evaluation Board Package Option R-28 tS CLOCK tH tLPW tPD tST 0.025% IOUTA OR IOUTB 0.025% Timing Diagram 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 AD768 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. WARNING! ESD SENSITIVE DEVICE REV. B –3– AD768 WAFER TEST LIMITS1 Parameter Integral Nonlinearity Differential Nonlinearity2 Offset Error Gain Error Reference Voltage Positive Supply Current Negative Supply Current Power Dissipation 2 (TA = +25 C, VDD = +5.0 V, VEE = –5.0 V, IREFIN = 5 mA, unless otherwise noted) AD768ACHIPS Limit ±8 ±6 ± 0.2 ± 1.0 ± 1.0 40 73 600 Units LSB max LSB max % FSR max % FSR max % of nom. 2.5 V max mA max mA max mW max NOTES 1 Electrical test are performed at wafer probe to the limits shown. Due to variations in assembly methods and normal yield loss, yield after packaging is not guaranteed for standard product dice. 2 Limits extrapolated from testing of individual bit errors. 3 Die offers latch control pad. Edge triggered latches become level triggered when latch control and clock pads are high. 4 Die substrate is connected to V EE. PIN DESCRIPTIONS Pin No. 1 2 3 4 5 6 7 8–14 15 16 17–23 24 25 26 27 28 Symbol IOUTA NR REFOUT NC REFCOM IREFIN DB0 DB1–DB7 DCOM CLOCK DB8–DB14 DB15 VDD VEE IOUTB LADCOM Type AO AI AO NC P AI DI DI P DI DI DI P P AO P Name and Function DAC Current Output. Full-scale current when all data bits are 1s. Noise Reduction Node. Add capacitor for noise reduction. Reference Output Voltage. Nominal value is 2.5 V. No Connect. Reserved for internal use. Reference Ground. Reference Input Current. Nominal is 5 mA. DAC full-scale is 4× this current. Data Bit 0 (LSB). Data Bits 1–7. Digital Ground. Clock Input. Data latched on positive edge of clock. Data Bits 8–14. Data Bit 15 (MSB). Positive Supply Voltage. Nominal is +5 V. Negative Supply Voltage. Nominal is –5 V. Complementary DAC Current Output. Full-scale current when all data bits are 0s. DAC Ladder Common. Type: AI = Analog Input; DI = Digital Input; AO = Analog Output; P = Power. PIN CONFIGURATION VDD VDD DICE CHARACTERISTICS 3, 4 DB15 DB14 DB13 DB12 DB11 DB10 IOUTA 1 NR 2 REFOUT 3 NC 4 REFCOM 5 IREFIN 6 (LSB) DB0 7 28 LADCOM 27 IOUTB 26 VEE (–5V) 25 VDD (+5V) 24 DB15 (MSB) VEE VEE DB9 DB8 AD768 23 DB14 IOUTB LADCOM IOUTA NR REFOUT NC REFCOM IREFIN DB0 DB1 DB2 DB3 DB4 CLOCK LATCH CONTROL DCOM DB7 DB6 DB5 TOP VIEW 22 DB13 (Not to Scale) DB1 8 21 DB12 DB2 9 DB3 10 DB4 11 DB5 12 DB6 13 DB7 14 20 DB11 19 DB10 18 DB9 17 DB8 16 CLOCK 15 DCOM NC = NO CONNECT Die Size: 0.1106 × 0.1417 inch, 15,672 sq. mils (2.81 × 3.60 mm, 10.116 sq. mm) –4– REV. B AD768 DEFINITIONS OF SPECIFICATIONS Linearity Error (Also Called Integral Nonlinearity or INL) Temperature Drift Linearity error is defined as the maximum deviation of the actual analog output from the ideal output, determined by a straight line drawn from zero to full scale. Differential Nonlinearity (or DNL) Temperature drift is specified as the maximum change from the ambient (+25°C) value to the value at either TMIN or TMAX. For offset and gain drift, the drift is reported in ppm of full-scale range (FSR) per degree C. For reference drift, the drift is reported in ppm per degree C. Power Supply Rejection DNL is the measure of the variation in analog value, normalized to full scale, associated with a 1 LSB change in digital input code. Monotonicity A D/A converter is monotonic if the output either increases or remains constant as the digital input increases. Offset Error The maximum change in the full-scale output as the supplies are varied from nominal to minimum and maximum specified voltages. Settling Time The deviation of the output current from the ideal of zero is called offset error. For IOUTA, 0 mA output is expected when the inputs are all 0s. For IOUTB, 0 mA output is expected when all inputs are set to 1s. Gain Error The time required for the output to reach and remain within a specified error band about its final value, measured from the start of the output transition. Spurious-Free Dynamic Range The difference between the actual and ideal output span. The actual span is determined by the output when all inputs are set to 1s minus the output when all inputs are set to 0s. The ideal output current span is 4× the current applied to the IREFIN pin. Output Compliance Range The difference, in dB, between the rms amplitude of the input signal and the peak spurious signal over the specified bandwidth. Total Harmonic Distortion THD is the ratio of the rms sum of the first six harmonic components to the rms value of the measured input signal. It is expressed as a percentage or in decibels (dB). Glitch Impulse The range of allowable voltage at the output of a current-output DAC. Operation beyond the maximum compliance limits may cause either output stage saturation or breakdown, resulting in nonlinear performance. CREFCOMP 1µF NC +5V 1µF 5 25 4 3 6 Asymmetrical switching times in a DAC give rise to undesired output transients which are quantified by a glitch impulse. It is specified as the net area of the glitch in pV-sec. RREF 500Ω REFIN 5mA RLAD 1kΩ IOUTA IOUTB RLAD 1kΩ LADCOM 1 27 REFOUT VDD REFCOM NR VEE DCOM CLOCK +2.5V REF IOUTA IOUTB 50Ω RLOAD 50Ω 1µF 1µF CNR –5V 26 15 2 28 AD768 MSB DECODE & LATCHES 24 23 22 21 20 19 18 SEGMENTED CURRENT SOURCES CURRENT SOURCES AND R-2R LADDER CLOCK 16 LATCHES – LOWER 12 BITS 17 14 13 12 11 10 9 8 7 DB15 DB14 DB13 DB11 DB3 DB7 DB2 DB1 DB10 DB9 DB12 DB8 DB6 DB5 Figure 1. Functional Block Diagram and Basic Hookup FUNCTIONAL DESCRIPTION The AD768 is a current-output DAC with a nominal full-scale current of 20 mA and a 1 kΩ output impedance. Differential outputs are provided to support single-ended or differential applications. The DAC architecture combines segmented current sources for the top four bits (MSBs) and a 1 kΩ R-2R ladder for the lower 12 bits (LSBs). The DAC current sources are implemented with laser-trimmable thin film resistors for excellent dc linearity. A proprietary switching technique is utilized to reduce glitch energy and maximize dynamic accuracy. The digital interface offers CMOS compatible edge-triggered input latches that interface readily to CMOS logic and supports clock rates up to 40 MSPS. A temperature compensated 2.5 V bandgap reference is integrated on-chip to drive the AD768 reference input current with the use of a single external resistor. The functional block diagram in Figure 1 is a simple representation of the internal circuitry to aid the understanding of the AD768’s operation. The DAC transfer function is described, and followed by a detailed description of each key portion of the circuit. Typical circuit configurations are shown in the section APPLYING THE AD768. REV. B –5– DB4 DB0 AD768 DAC TRANSFER FUNCTION 550 The AD768 may be used in either current-output mode with the output connected to a virtual ground, or voltage-output mode with the output connected to a resistive load. In current output mode, POWER – mW 500 IOUT = (DAC CODE/65536) × (IREFIN × 4) In voltage output mode, VOUT = IOUT × RLOAD RLAD where: DAC CODE is the decimal representation of the DAC inputs; an integer between 0 and 65535. IREFIN is the current applied at the IREFIN pin, determined by VREF/RREF. Substituting for IOUT and IREFIN, VOUT = –VREF × (DAC CODE/65536) × 4 × [(RLOAD RLAD)/RREF] These equations clarify an important aspect of the AD768 transfer function; the full-scale current output of the DAC is proportional to a current input. The voltage output is then a function of the ratio of (RLOAD RLAD)/RREF, allowing for cancellation of resistor drift by selection of resistors with matched characteristics. REFERENCE INPUT 450 400 350 300 1.0 2.0 3.0 4.0 IREFIN – mA 5.0 6.0 7.0 Figure 3. Power Dissipation vs. IREFIN Current Note the AD768 is optimized for operation at an input current of 5 mA. Both linearity and dynamic performance at other input currents may be somewhat degraded. Figure 4 shows typical dc linearity over a range of input currents. Figure 5 shows typical SFDR (to Nyquist) performance over a range of input currents and CLOCK input rates for a 1 MHz output frequency. 10 9 8 7 ERROR – LSB The IREFIN pin is a current input node with low impedance to REFCOM. This input current sets the magnitude of the DAC current sources such that the full-scale output current is exactly four times the current applied at IREFIN. For the nominal input current of 5 mA, the nominal full-scale output current is 20 mA. The 5 mA reference input current can be generated from the on-chip 2.5 V reference with an external resistor of 500 Ω from REFOUT to IREFIN. If desired, a variety of external reference voltages may be used based on the selection of an appropriate resistor. However, to maintain stability of the reference amplifier, the external impedance at IREFIN must be kept below 1 kΩ. REFCOM IREFIN 5 6 5 4 3 2 1 0 1.0 2.0 3.0 4.0 IREFIN – mA 5.0 6.0 7.0 DNL INL Figure 4. INL/DNL vs. IREFIN Current 6 –85 IFB 5mA VEE VEE CLOCK = 10 MSPS –80 –75 SFDR – dB Figure 2. Equivalent Reference Input Circuit CLOCK = 20 MSPS The IREFIN current can be varied from 1 mA to 7 mA which subsequently will result in a proportional change in the DAC full-scale. Since the operating currents within the DAC vary with IREFIN, so does the power dissipation. Figure 3 illustrates that relationship. –70 CLOCK = 30 MSPS –65 CLOCK = 40 MSPS –60 –55 1.0 2.0 3.0 4.0 IREFIN – mA 5.0 6.0 7.0 Figure 5. SFDR (to Nyquist) vs. IREFIN @ FOUT = 1 MHz –6– REV. B AD768 REFERENCE OUTPUT 100M The internal 2.5 V bandgap reference is provided for generation of the IREFIN current, and must be compensated externally with a capacitor of 0.1 µF or greater from REFOUT to REFCOM. If an external reference is used, REFOUT should be tied directly to the positive supply voltage, VDD. This effectively turns off the internal reference, eliminating the need for the external capacitor at REFOUT. The reference is specified to drive a nominal load of 5 mA with a maximum of 15 mA. Operation with a heavier load will result in degradation of supply rejection and reference voltage accuracy. Therefore, the reference output should be buffered with an amplifier when additional load current is required. A properly sized pull-up resistor can also be used to source additional current to the load. The resistors value should be selected such that REFOUT will always source a minimum of 5 mA to IREFIN and the additional load. 500Ω 10M BANDWIDTH – Hz 1M 100k 10k 1k 10p 100p 1n 10n 100n 1µ NOISE REDUCTION CAPACITOR – F AD768 6 IREFIN 3 REFOUT Figure 7. External Noise Reduction Capacitor vs. –3 dB Bandwidth CREFCOMP 1µF 5 REFCOM Figure 6. Typical Reference Hookup TEMPERATURE CONSIDERATIONS Note that the reference plays a key role in the overall temperature performance of the AD768. Any drift of IREFIN shows up directly in IOUT. When the output is taken as a current, the drift of IREFIN (which depends on both VREF and RREF) must be minimized. This can be done by using the internal temperature compensated reference for VREF and a low temperature coefficient resistor for RREF. If the output is taken as a voltage, it is a function of a resistor ratio, not an absolute resistor value. By selecting resistors with matched temperature coefficients for RREF and RLOAD, the drift in the resistor values will cancel, providing optimal drift performance. REFERENCE NOISE REDUCTION AND MULTIPLYING BANDWIDTH The sensitivity of the NR node requires that care be taken in capacitor placement. The capacitor should be located as physically close to the package pins as possible and lead lengths should be minimized. For this purpose, the use of a chip capacitor is recommended. For applications that do not require high frequency modulation at IREFIN, it is recommended that a capacitor on the order of 1 µF be connected from NR to VEE. If the reference input is purely dc, noise may be minimized with multiple capacitors, such as 1 µF and 0.1 µF, to more effectively filter both high and low frequency disturbances. ANALOG OUTPUTS The AD768 offers two analog outputs; IOUTA is trimmed for optimal INL and DNL performance and has a full-scale output when all bits are high. For applications that require the specified dc accuracy, IOUTA should be used. IOUTB is the complementary output with full-scale output when all bits are low. Both IOUTA and IOUTB provide similar dynamic performance. Refer to Figures 8 and 9 for typical INL and DNL performance curves. The outputs can also be used differentially. Refer to the section “Applying the AD768” for examples of various output configurations. 8 6 4 INL ERROR – LSB For application flexibility and multiplying capabilities, the reference amplifier is designed to offer adjustable bandwidth that can be reduced by connecting an external capacitor from the NR node to the negative supply pin, VEE. This capacitor limits the bandwidth and acts as a filter to reduce the noise contribution from the reference amplifier. The noise reduction capacitor, CNR, is not required for stability and does not affect the settling time of the DAC output. Without this capacitor, the IREFIN bandwidth is 15 MHz allowing high frequency modulation of the DAC full-scale range through the reference input node. Figure 7 shows the relationship between the external noise reduction capacitor and the –3 dB bandwidth of the reference amplifier. 2 0 –2 –4 –6 –8 0 5 10 15 20 25 30 35 40 45 DIGITAL INPUT CODE – k 50 55 60 65 Figure 8. Typical INL Performance REV. B –7– AD768 8 6 4 DNL ERROR – LSB IOUTA 1 1kΩ 3pF IOUT LADCOM 28 1kΩ 3pF IOUTB 27 2 0 –2 –4 –6 –8 IREFIN x2.75 IOUT 26 VEE Figure 10. Equivalent Analog Output Circuit 0 5 10 15 20 25 30 35 40 45 DIGITAL INPUT CODE – k 50 55 60 65 DIGITAL INPUTS Figure 9. Typical DNL Performance The outputs have a compliance range of –1.2 V to +5.0 V with respect to LADCOM. The current steering output stages will remain functional over this range. Operation beyond the maximum compliance limits may cause either output stage saturation or breakdown, resulting in nonlinear performance. The rated dc and ac performance specifications are for an output voltage of 0 V to –1 V. The current in LADCOM is proportional to IREFIN and has been carefully configured to be independent of digital code when the output is connected to a virtual ground. This minimizes any detrimental effects of ladder ground resistance on linearity. For optimal dc linearity, IOUTA should be connected directly to a virtual ground, and IOUTB should be grounded. An example of this configuration is provided in the section “Buffered Voltage Output.” If IOUTA is driving a resistive load directly, then IOUTB should be terminated with an equal impedance. This will ensure the current in LADCOM remains constant with digital code, and is recommended for improved dc linearity in the unbuffered voltage output configuration. As shown in Figure 10, there is an equivalent output impedance of 1 kΩ in parallel with 3 pF at each output terminal. If the output voltage deviates from the ladder common voltage, an error current flows through this 1 kΩ impedance. This is a linear effect which does not change with input code, so it appears as a gain error. With 50 Ω output termination, the resulting gain error is approximately –5%. An example of this configuration is provided in the section Unbuffered Voltage Output. The AD768 digital inputs consist of 16 data input pins and a clock pin. The 16-bit parallel data inputs follow standard positive binary coding, where DB15 is the most significant bit (MSB) and DB0 is the least significant bit (LSB). IOUTA produces full-scale output current when all data bits are at logic 1. IOUTB is the complementary output, with full-scale when all data bits are at logic 0. The full-scale current is split between the two outputs as a function of the input code. The digital interface is implemented using an edge-triggered master slave latch. The DAC output is updated following the rising edge of the clock, and is designed to support a clock rate as high as 40 MSPS. The clock can be operated at any duty cycle that meets the specified minimum latch pulse width. The setup and hold times can also be varied within the clock cycle as long as the specified minimums are met, although the location of these transition edges may affect digital feedthrough. The digital inputs are CMOS compatible with logic thresholds set to approximately half the positive supply voltage. The small input current requirements allow for easy interfacing to unbuffered CMOS logic. Figure 11 shows the equivalent digital input circuit. VCC VCC DIGITAL INPUT VEE DCOM Figure 11. Equivalent Digital Input Circuit Digital input signals to the DAC should be isolated from the analog output as much as possible. Interconnect distances to the DAC inputs should be kept as short as possible. Termination resistors may improve performance if the digital lines become too long. To minimize digital feedthrough, the inputs should be free from glitches and ringing, and may be further improved with a reduction of edge speed. –8– REV. B Typical Performance Curves––AD768 AD768 –0.9405 –0.9415 –0.9424 –0.9433 OUTPUT – dB 1.00 0.90 RL = 50Ω VOUT = 0 TO –1V –0.470 –0.472 RL = 50Ω VOUT = 0 TO –1V 0.2 0.0 RL = 50Ω VOUT = 0 TO –1V 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.05 0.025 ERROR BAND – % –0.474 OUTPUT – V VOLTS –0.2 –0.4 –0.6 –0.9443 –0.9453 –0.9462 –0.9472 –0.9481 –0.9498 –0.476 –0.478 –0.8 –0.480 –1.0 –0.9500 12 14 16 18 20 22 24 26 28 30 32 34 36 TIME – ns –0.482 TIME – 2ns/Div TIME – 5ns/Div Figure 12. Settling Time Figure 13. Glitch Impulse at Major Carry Figure 14. Rise and Fall Characteristics –80 RL = 50Ω VOUT = 0 TO –1V 0 CLOCK = 10MHz FOUT = 1.002MHz AMPLITUDE = 0dBm OUTPUT IOUTB RL = 50Ω 0 CLOCK = 10MHz FOUT = 1.002MHz AMPLITUDE = 0dBm –75 –20 –20 OUTPUT – dB OUTPUT – dB –70 THD – dB –40 –40 –65 –60 –60 –60 –55 –80 –80 –50 4 5 6 7 8 9 10 20 FREQUENCY – MHz 30 40 –100 0.1 1 2 3 FREQUENCY – MHz 4 5 –100 10k 1.0M FREQUENCY – Hz 2.0M Figure 15. THD vs. Clock Frequency at FOUT = 1 MHz Figure 16. Typical Spectral Performance Figure 17. Typical SFDR (With a Window) –86 CLOCK = 30MHz –82 –80 CLOCK = 1MHz –75 CLOCK = 10MHz 0 CLOCK = 10 MHz FOUT1 = 240 kHz FOUT2 = 260 kHz AMPLITUDE = –6.0 dB FS PER TONE OUTPUT IOUTB RL = 50Ω –78 OUTPUT – dB –20 –74 THD – dB –70 CLOCK = 10MHz SFDR –70 –66 –62 –58 –54 –50 1 FREQUENCY – MHz 10 CLOCK = 20MHz CLOCK = 20MHz –65 CLOCK = 30MHz –40 –60 –60 CLOCK = 40MHz –55 –50 0.01 –80 0.1 1 FREQUENCY – MHz 10 –100 150 190 230 270 FREQUENCY – Hz 310 350 Figure 18. SFDR (Within a Window) vs. FOUT Figure 19. THD vs. FOUT Figure 20. Intermodulation Distortion REV. B –9– AD768 APPLYING THE AD768 OUTPUT CONFIGURATIONS The following sections illustrate some typical output configurations for the AD768. While most figures take the output at IOUTA, IOUTB can be interchanged in all cases. Unless otherwise noted, it is assumed that IREFIN and full-scale currents are set to nominal values. For application that require the specified dc accuracies, proper resistor selection is required. In addition to absolute resistor tolerances, resistor self-heating can result in unexpected errors. For optimal INL, the buffered voltage output is recommended as shown in Figure 23. In this configuration, self-heating of RFB may cause a change in gain, producing a bow in the INL curve. This effect can be minimized by selection of a low temperature coefficient resistor. UNBUFFERED VOLTAGE OUTPUT CONFIGURATIONS DAC output is the parallel combination of the AD768’s output impedance, RL, and bias resistor RB. The nominal output swing with the values given in Figure 22 is ± 0.5 V assuming RB >> RL. The gain of the circuit will be a function of the tolerances of the impedances RLAD, RB, and RL. Choosing the value of RB and C will depend primarily on the desired –3 dB high pass cutoff frequency and the bias current, IB, of the subsequent stage connected to RB. The –3 dB frequency can be approximated by the equation, f–3 dB = 1/[2 × π × (RB + RL RLAD) × C]. The dc offset of the output is a function of the bias current of the subsequent stage and the value of RB. For example, if C = 390 pF, RB = 20 kΩ, and IB = 1.0 µA, the –3 dB frequency is approximately 20.4 kHz and the dc offset would be 20 mV. AD768 IOUTA 1 RL 49.9Ω RB C IB Figure 21 shows the AD768 configured to provide a unipolar output range of approximately 0 V to –1 V. The nominal fullscale current of 20 mA flows through the parallel combination of the 50 Ω RL resistor and the 1 kΩ DAC output resistance (from the R-2R ladder), for a combined 47.6 Ω. This produces an ideal full-scale voltage of –0.952 V with respect to LADCOM. In addition, the 1 kΩ DAC output resistance has a tolerance of ± 20% which may vary the full-scale gain by ± 1%. This linear variation results in a gain error which can be easily compensated for by adjusting IREFIN. AD768 IOUTA 1 RL 49.9Ω LADCOM 28 RL 49.9Ω IOUTB 27 VB VA IOUTB 27 RL 49.9Ω LADCOM 28 Figure 22. 0.5 V to –0.5 V Unbuffered AC-Coupled Output BUFFERED VOLTAGE OUTPUT CONFIGURATIONS Unipolar Configuration Figure 21. 0 V to –1 V Unbuffered Voltage Output In this configuration, it is important to note the restrictions from the output compliance limits. The maximum negative voltage compliance is –1.2 V, prohibiting use of a 100 Ω load to produce a 0 V to –2 V output swing. One additional consideration for operation in this mode is integral nonlinearity. As the voltage at the output node changes, the finite output impedance of the DAC current steering switches gives rise to small changes in the output current that vary with output voltage, producing a bow (up to 8 LSBs) in the INL. For optimal INL performance, the buffered voltage output mode is recommended. The INL is also slightly dependent on the termination of the unused output (IOUTB) as described in the ANALOG OUTPUT section. To eliminate this effect, IOUTB should be terminated with the same impedance as IOUTA, so both outputs see the same resistive divider to ground. This will keep the current in LADCOM constant, minimizing any code-dependent IR drops within the DAC ladder that may give rise to additional nonlinearities. AC-Coupled Output For positive output voltages, or voltage ranges greater than allowed by output compliance limits, some type of external buffer is needed. A wide variety of amplifiers may be selected based on considerations such as speed, accuracy and cost. The AD9631 is an excellent choice when dynamic performance is important, offering low distortion up to 10 MHz. Figure 23 shows the implementation of 0 V to +2 V full-scale unipolar buffered voltage output. The amplifier establishes a summing node at ground for the DAC output. The buffered output voltage results from the DAC output current flowing through the amplifier’s feedback resistor, RFB. In this case, the 20 mA fullscale current across RFB (100 Ω) produces an output voltage range of 0 V through +2 V. The same configuration using a precision amplifier such as the AD845 is recommended for optimal dc linearity. RFB 100Ω AD768 IOUTA 1 A1 IOUTB 27 LADCOM 28 Figure 23. Unipolar 0 V to +2 V Buffered Voltage Output Buffered Output Using a Current Divider Configuring the output as shown in Figure 22 provides a bipolar output signal from the AD768 without requiring the use of a summing amplifier. The ac load impedance presented to the The configuration shown in Figure 23 may not be possible in cases where the amplifier cannot supply the requisite 20 mA feedback current. As an alternative, Figure 24 shows amplifier A1 in conjunction with a resistive current divider. The values of RFF and RL are chosen to limit the current, I3, which must be supplied by A1. Current, I2, is shunted to ground through resistor, RL. The parallel combination of RFF and RL should not exceed 60 Ω to avoid exceeding the specified compliance voltage. –10– REV. B AD768 For the values given in Figure 24, I3 equals 4 mA, which results in a nominal unipolar output swing of 0 V to 2 V. Note, since A1 has an inverting gain of approximately –4 and a noise gain of +5, A1’s distortion and noise performance should be considered. RFB 500Ω primary side is multiplied by a factor of 4 (i.e., in this case 200 Ω). To avoid dc current from flowing into the R-2R ladder of the DAC, the center tap of the transformer should be connected to LADCOM. In order to comply with the minimum voltage compliance of –1.2 V, the maximum differential resistance seen between IOUTA and IOUTB should not exceed 240 Ω. Note that the differential resistance consists of the load RL, referred to the primary side of the transformer in parallel with any added differential resistance, RDIFF, across the two outputs. RDIFF is typically added to the primary side of the transformer to match the effective primary source impedance to the load (i.e., in this case 200 Ω). AD768 IOUTA 1 RDIFF 200Ω IOUTB 27 LADCOM 28 T1 RL 50Ω AD768 IOUTA 1 I1 I3 RFF 100Ω I2 IOUTB 27 RP 20Ω LADCOM 28 RL 24.9Ω A1 Figure 24. 0 V to 2 V Buffered Unipolar Output Using a Current Divider Bipolar Configuration Bipolar mode is accomplished by providing an offset current, IBIPOLAR, to the I/V amplifier’s (A1) summing junction. By setting IBIPOLAR to exactly half the full-scale current flowing through RFB, the resulting output voltage will be symmetrical about the summing junction voltage, typically ground. Figure 25 shows the implementation for a bipolar ± 2.5 V buffered voltage output. The resistor divider sets the full-scale current for IDAC to 5 mA. The internal 2.5 V reference generates a 2.5 mA IBIPOLAR current across RBIP. An output voltage of 0 V is produced when the DAC is set to half scale (100. . .0) such that the 2.5 mA current, IDAC, is exactly offset by IBIPOLAR. As the DAC is varied from zero to full-scale, the output voltage swings from –2.5 V to +2.5 V. Note, in configurations that require more than 15 mA of total current from REFOUT, an external buffer is required. Op amps such as the AD811, AD8001, and AD9631 are good selections for superior dynamic performance. In dc applications, op amps such as the AD845 or AD797 may be more appropriate. AD768 REFOUT 3 C IBIPOLAR 75Ω IOUTA 1 24.9Ω RP 20Ω IDAC RBIP 1kΩ RFB 1kΩ 4:1 IMPEDANCE RATIO T1 = MINI-CIRCUITS T4-6T Figure 26. Differential Output Using a Transformer DC COUPLING VIA AN AMPLIFIER A dc differential to single-ended conversion can be easily accomplished using the circuit shown in Figure 27. This circuit will attenuate both ac and dc common-mode error sources due to the differential nature of the circuit. Thus, common-mode noise (i.e., clock feedthrough) as well as dc unipolar offset errors will be significantly reduced. Also, excellent temperature stability can be obtained by using temperature tracking, thin film resistors for R and RREF. The design equations for the circuit are provided such that the voltage output swing and IREF can be optimized for a given application. R* AD768 IOUTA 1 A1 A1 IOUTB 27 R* LADCOM 28 REFIN 6 RREF* REFOUT 3 IREF VOUT = ±4 IREF R VOUT WHERE IREF = 2.5V RREF IOUTB 27 LADCOM 28 VOUT ± 2V R = 200Ω RREF = 5 x 200Ω Figure 25. Bipolar ± 2.5 V Buffered Voltage Output DIFFERENTIAL OUTPUT CONFIGURATIONS AC Coupling via a Transformer *OHMTEK TDP-1403 Figure 27. DC Differential to Single-Ended Conversion POWER AND GROUNDING CONSIDERATIONS Applications that do not require baseband operation typically use transformer coupling. Transformer coupling the complementary outputs of the AD768 to a load has the inherent benefit of providing electrical isolation while consuming no additional power. Also, a properly applied transformer should not degrade the AD768’s output signal with respect to noise and distortion, since the transformer is a passive device. Figure 26 shows a center-tapped output transformer that provides the necessary dc load conditions at the outputs IOUTA and IOUTB to drive a ± 0.5 V signal into a 50 Ω load. In this particular circuit, the center-tapped transformer has an impedance ratio of 4 that corresponds to a turns ratio of 2. Hence, any load, RL, referred to the In systems seeking to simultaneously achieve high speed and high accuracy, the implementation and construction of the printed circuit board design is often as important as the circuit design. Proper RF techniques must be used in device selection, placement and routing, and supply bypassing and grounding. Maintaining low noise on power supplies and ground is critical to obtaining optimum results from the AD768. Figure 28 provides an illustration of the recommended printed circuit board ground plane layout which is implemented on the AD768 evaluation board. REV. B –11– AD768 Figure 28. Printed Circuit Board Ground Plane Layout Figure 29. Printed Circuit Board Power Plane Layout –12– REV. B AD768 If properly implemented, ground planes can perform a host of functions on high speed circuit boards: bypassing, shielding, current transport, etc. In mixed signal design, the analog and digital portions of the board should be distinct from each other, with the analog ground plane confined to the areas covering analog signal traces and the digital ground plane confined to areas covering the digital interconnects. All analog ground pins of the DAC, reference, and other analog output components, should be tied directly to the analog ground plane. The two ground planes should be connected by a path 1/4 to 1/2 inch wide underneath or within 1/2 inch of the DAC as shown in Figure 28. Care should be taken to ensure that the ground plane is uninterrupted over crucial signal paths. On the digital side, this includes the digital input lines running to the DAC as well as any clock signals. On the analog side, this includes the DAC output signal, reference signal, and the supply feeders. The use of wide runs or planes in the routing of power lines is also recommended. This serves the dual role of providing a low series impedance power supply to the part, as well as, providing some “free” capacitive decoupling to the appropriate ground plane. Figure 29 illustrates the power plane layout used in the AD768 evaluation board. The AD768 evaluation board uses a four layer P.C. board which illustrates good layout practices as discussed above. It is essential that care be taken in the layout of signal and power ground interconnects to avoid inducing extraneous voltage drops in the signal ground paths. It is recommended that all connections be short, direct and as physically close to the package as possible, in order to minimize the sharing of conduction paths between different currents. When runs exceed an inch in length, some type of termination resistor should be considered. The necessity and value of this resistor will be dependent upon the logic family used. For maximum ac performance, the DAC should be mounted directly to the circuit board; sockets should be avoided since they introduce unwanted capacitive coupling between adjacent pins of the device. POWER SUPPLY AND DECOUPLING A clean digital supply may be generated using the circuit shown in Figure 30. The circuit consists of a differential LC filter with separate power supply and return lines. Lower noise can be attained using low ESR (Equivalent Series Resistance) type electrolytic and tantalum capacitors. TTL/CMOS LOGIC CIRCUITS FERRITE BEADS VDD 100µF ELECT. 10–20µF TANT. 0.1µF CER. DCOM +5V DGND +5V POWER SUPPLY Figure 30. Differential LC Filter for Single +5 V Applications APPLICATIONS USING THE AD768 AS A MULTIPLYING DAC The AD768 can be easily configured as a multiplying DAC since IREFIN can be modulated from 1 mA to 7 mA. The reference amplifier sets the maximum multiplying bandwidth to 15 MHz, while any external capacitor to the NR node serves to limit the bandwidth according to Figure 7. IREFIN can be easily modulated by properly scaling and summing into the IREFIN node the modulating signal. Figure 31 demonstrates how the modulating signal VMOD can be properly scaled and converted to a current via RREFMOD such that its peak current does not exceed 3.0 mA. Figure 32 shows the AD768’s typical distortion versus the reference channel frequency. AD768 6 RREF 625Ω 3 1µF VMOD ≤ ±3.0mA RREFMOD REFOUT IREFIN RREFMOD VMOD THD – dB One of the most important external components associated with high speed designs are the capacitors used to bypass the power supplies. Both selection and placement of these capacitors can be critical and, to a large extent, dependent upon the specifics of the system configuration. The dominant consideration in the selection of bypass capacitors for the AD768 is the minimization of the series resistance and inductance. Many capacitors will begin to look inductive at 20 MHz and above. Ceramic and film type capacitors generally feature lower series inductance than tantalum or electrolytic types. It is recommended that each power supply to the AD768 be decoupled by a 0.1 µF capacitor located as close to the device pins as possible. Surface-mount chip capacitors, by virtue of their low parasitic inductance, are preferable to through-hole types. Some series inductance between the DAC supply pins and the power supply plane may help to provide additional filtering of high frequency power supply noise. This inductance can be generated by using small ferrite beads. Figure 31. Typical Multiplying DAC Application –75 –70 –65 IREF = 4.0+/–2 mA IREF = 5.0+/–1 mA –60 –55 IREF = 4.0+/–3 mA –50 –45 –40 250 500 750 1000 1250 1500 1750 FREQUENCY – kHz 2000 2250 2500 Figure 32. Reference Channel Distortion vs. Frequency REV. B –13– AD768 AD768 IN MULTITONE TRANSMITTERS (FOR ADSL) 1.5 Communications applications frequently require aspects of component performance that differ significantly from the simple, single tone signals used in typical SNR and THD tests. This is particularly true for spread-spectrum and frequency division multiplexed (FDM) type signals, where information content is held in a number of small signal components spread across the frequency band. In these applications, a combination of wide dynamic range, good fine-scale linearity, and low intermodulation distortion is required. Unfortunately, a part’s full scale SNR and THD performance may not be a reliable indicator of how it will perform in these multitone applications. One example of an FDM communications system is the DMT (discrete multitone) ADSL (Asymmetrical Digital Subscriber Line) standard currently being considered by ANSI. Figure 33 shows a block diagram of a transmitter function. The digital bits are used to QAM modulate each of approximately 200 discrete tones. An inverse FFT turns this modulated frequency domain information into 512 time points at a 2.2 MSPS sample rate. These time points are then put through an FIR interpolation filter to upsample (in this case to 4.4 MSPS). The bit stream is run through the AD768, which is followed by a 4th order analog smoothing filter, then run to the line-driving circuitry BIT STREAM 1024 TIME POINTS @ 4.4MSPS 2X INVERSE QAM INTERPOLATOR FFT ENCODER FREQUENCY @ 2.2MSPS FIR BINS 256 MODULATED 512 TIME POINTS 1.0 OUTPUT – Volts 0.5 0 –0.5 –1.0 –1.5 TIME – 25µs/DIV Figure 34b. Time Domain Output Signal of ADSL Test Vector Table I and II show the available SNR and THD at the output of the filter vs. frequency bin for the ADSL application. The AD768’s combination of 16-bit dynamic range and 14-bit linearity provides excellent performance for the DMT signal. Its fast input rate would support even faster rates of oversampling, if one were interested in trading off digital filter complexity in the interpolator for a simplified analog filter. Table I. SNR vs. Frequency AD768 +BUFFER 4TH ORDER SMOOTHING FILTER Frequency TO TRANSMITTER SNR 70.1 dB 69.7 dB 69.4 dB 69.8 dB Figure 33. Typical DMT ADSL Transmit Chain 151 kHz 349 kHz 500 kHz 1 MHz Figure 34a shows a frequency domain representation of a test vector run through this system, while 34b shows the time domain representation. (Clearly the frequency domain picture is more informative.) We wish to optimize the SINAD of each 4 kHz frequency band: this is a function of both noise (wideband and quantization) and distortion (simple harmonic and intermod). 0 Table II. THD vs. Frequency Frequency 160 kHz 418 kHz 640 kHz 893 kHz THD –68.9 dBc –64.0 dBc –64.3 dBc –63.8 dBc –20 OUTPUT – dB –40 –60 –80 0 FREQUENCY – Hz 1.1M Figure 34a. Output Spectrum of ADSL Test Vector –14– REV. B AD768 AD768 EVALUATION BOARD GENERAL DESCRIPTION The AD768-EB is an evaluation board for the AD768 16-bit 30 Msps D/A converter. Careful attention to layout and circuit design combined with analog and digital prototyping areas allows the user to easily and effectively evaluate the AD768 in any application where high resolution, high speed conversion is required. The digital inputs to the AD768-EB may be driven directly using the standard 40-pin IDC connector. An external clock is also required. These signals may be applied from a user’s bench, or they can be generated from a circuit built on the prototyping area. The analog outputs from the AD768-EB are available on BNC connectors. These outputs may be configured to use either resistors, op amps, or a transformer. OPERATING PROCEDURE AND FUNCTIONAL DESCRIPTION Power JP1. Buffered op amp output “A”. Jumper JP1 should be installed if the buffered op amp output is desired. When JP1 is installed, JP2 and JP3 must be removed for proper operation. The output, available on the “A” connector, has a nominal voltage swing of 0 V to 2 V and is in-phase with the digital input. This is the factory default setting. JP2. Bipolar 50 Ω transformer output. If jumper JP2 is installed, a transformer coupled output is available on the “A” connector. When JP2 is installed, JP1 and JP3 must be removed for proper operation. The transformer acts both as a differential-to-single-ended converter and as an impedance transformer. For proper operation, the transformer must be terminated with a 50 Ω resistor. R2 must be replaced with the 100 Ω resistor, R7. An additional 100 Ω resistor and the transformer are included with the AD768-EB. The additional 100 Ω resistor must be soldered into the appropriate position labeled “R3” and the transformer must be inserted into the socket labeled “T1.” The nominal output voltage into a 50 Ω load is 1 V p-p centered on a common-mode voltage of 0 V. JP3. Resistor output “A.” JP3 is used to connect the resistor R2 to the “A” output. U2 should be removed from its socket. Using a 24.9 Ω resistor for R2, the output is an unbuffered 0 V to –0.5 V output that is out of phase with the digital input. Resistor R2 may be replaced with other values, but careful attention to the recommended output compliance range should be observed. When JP3 is installed, JP1 and JP2 must be removed for proper operation. JP4. Resistor output “B.” JP4 is used to connect the resistor R3 to the “A” output. U3 should be removed from its socket. The AD768-EB is shipped from the factory with resistor R3 shorted to ground. A different value selected by the user can be installed for R3 to generate an unbuffered output that is inphase with the digital input. Careful attention to the recommended output compliance range should be observed when selecting the value of R3. When JP4 is installed, JP5 must be removed for proper operation. JP5. Buffered op amp output “B.” Jumper JP5 should be installed if the buffered op amp output is desired. When JP5 is installed, JP4 must be removed for proper operation. The output is available on the “B” connector and has a nominal voltage swing determined by the combination of resistors R3, R9, and R10. This op amp is not provided with the AD768-EB. Reference Power may be supplied to the AD768-EB by applying either wires or banana plugs to the metal binding posts included on the printed circuit board. DGND. Digital Ground. The digital ground and the analog ground are connected together underneath the AD768. Optimal performance can be obtained with separate analog and digital supplies. For evaluation purposes, a single-supply which makes a second analog and digital ground connection at the supply is acceptable. +5D. The +5 V (± 5%) digital supply should be capable of supplying 50 mA. –5A. The –5 V (± 5%) analog supply should be capable of supplying –75 mA. AGND. Analog ground. The analog ground and the digital ground are connected together underneath the AD768. Optimal performance can be obtained with separate analog and digital supplies. For evaluation purposes, a single-supply which makes a second analog and digital ground connection at the supply is acceptable. –VEE. Negative analog supply; typically –5 V to –15 V. This supply is used as the negative supply rail for the external op amps. For the AD811 supplied with the AD768-EB, a supply capable of supplying –20 mA (excluding external load requirements) is required. +VCC. Positive analog supply; typically +5 V to +15 V. This supply is used as the positive supply rail for the external op amps. For the AD811 supplied with the AD768-EB, a supply capable of supplying +20 mA (excluding external load requirements) is required. Analog Outputs Either the internal reference of the AD768 or an external reference may be selected on the AD768-EB. R12 is used to adjust the full-scale output current of the AD768. SW2. Internal/External reference select switch. When SW2 is in position 1, the internal reference of the AD768 is selected. When SW2 is in position 2, an external reference must be provided by the user. Level-Shifting the Analog Output The analog output(s) from the AD768-EB are available on BNC jacks “A” and “B.” The complementary current outputs from the AD768 can be configured using either resistors, op amps, or a transformer. Only the “A” portion of the AD768-EB is populated and shipped from the factory. The “B” side, or complementary output, may be populated by the user if so desired. Resistor sockets R8 and R6 can be populated with an appropriately valued resistor to add dc offset current to an output which uses the op amp configuration. As an example, to generate a bipolar output signal, a 1.25 kΩ resistor installed into the “R8” socket level-shifts the normally unipolar output by –1 V. The factory defaults for R8 and R6 are open circuits. REV. B –15– AD768 Clock Input Table III. Summary of Jumper Functionality An external sample clock must be provided to either the BNC connector labeled “CLOCK” or on Pin 33 of the IDC connector. This clock must comply with the logic levels outlined in the AD768 data sheet. The “CLOCK” input is terminated with a removable 51 Ω resistor. The IDC connector clock connection is unterminated. SW1. Clock source select switch. When SW1 is in position 1, Pin 33 of the IDC connected is applied to the CLOCK input of the AD768. When SW2 is in position 2, the “CLOCK” BNC connector is applied to the CLOCK input of the AD768. Digital Inputs Installed Jumper JP1 JP2 JP3 (STBY) JP4 JP5 Jumper Function Buffered Output A 50 Ω Transformer Output Unbuffered Output A Unbuffered Output B Buffered Output B Table IV. AD768-EB Parts List Reference Value / Part Type Package Qty/Bd The digital inputs of the AD768, DB0–DB15, are available via J1, a 40-pin IDC connector. These inputs should comply with the specifications given in the AD768 data sheet. Layout Considerations U1 U2 T1 AD768 AD811 Mini-Circuits T4–6T 28-Pin SOIC 8-Pin DIP Not Installed Small, Vertical 2-Pin 0.1" × 0.3" R.A., Male, w/ Latches 1/4 W, 0.01%, Vishay 1/4 W, 0.01%, Vishay 1 1 1 3 5 2 1 1 1 Figures 28 and 29 show the AD768-EB ground and power plane layouts. Figures 35–38 show the schematic diagram, trace routing, silk screening, and component layout for the AD768 4 layer evaluation board. Separate ground and power planes have several advantages for high speed layouts. (For further information outlining these advantages, see the application note “Design and Layout of a Video Graphics System for Reduced EMI” [E1309] available from Analog Devices [(617) 461-3392].) A solid ground plane can be used if the digital return current can be routed such that it does not modulate the analog ground plane. If this is not possible, it may be necessary to split the ground plane in order to force currents to flow in a controlled direction. This type of grounding scheme is shown in the Figure 28. The ground plane is separated into analog and digital planes that are joined together under the AD768. In any case, the AD768 should be treated as an analog component and a common ground connection should be made underneath the AD768 despite some pins being labeled “digital” ground and some as “analog” ground. A complete parts list for the AD768 evaluation board is given in Table IV. A, B, CLOCK BNC JACKs, Small JP1–5 SW1, 2 J1 R1 R2 Header SPDT, Secme 40-Pin IDC Connector 500 Ω 25 Ω R3, R13–21, & R23–29 Wire Jumpers R5 R7 R11 R12 C1–4 C5–8, C10, 12, 14, & C16–19 500 Ω 100 Ω 51 Ω 10 kΩ Pot. 1/4 W, 0.01%, Vishay 1/4 W, 0.01%, Vishay 17 1 1 1/8 W, 5%, Carbon 1 3266 W 1 4 11 4 1 µF Ceram. Cap. Leaded 0.1 µF Chip Cap, 1206 C9, 11, 13, 15 22 µF Tant. Cap., Teardrop, 25 V 0.1" Spacing –16– REV. B AD768 R5 499 +VCC U2 2 3 7 AD811 4 A –VEE A 3 6 C6 0.1µF T1 4:1 A 1 C5 0.1µF 50 2 1 2 B JP5 R10 499 +VCC U3 2 7 AD811 4 A –VEE C9 47µF, 25V R9 100 R6 50 A JP1 JP2 C7 0.1µF 6 C8 0.1µF A 1 A 3 4 R7 100 R8 R2 24.9 5 6 R3 24.9 JP3 JP4 A –VEE C10 0.1µF ∞ A ∞ +VCC C11 47µF, 25V A C12 0.1µF –5A R12 10k A C2 1µF C18 0.1µF 1 2 A 1 SW2 TP1 J1 J1 J1 J1 J1 J1 J1 J1 31 29 27 25 23 21 19 17 2 R13 R14 R15 R16 R17 R18 R19 R20 0 0 0 0 0 0 0 0 R1 499 C1 1µF 3 4 5 6 7 8 9 10 11 12 13 14 IOUTA NR REFOUT NC REFCOM IREFIN DB0 (LSB) DB1 DB2 DB3 DB4 DB5 DB6 DB7 U1 C3 1µF 28 27 26 25 24 23 22 21 20 19 18 17 16 15 A C4 1µF C19 0.1µF C15 47µF, 25V R21 R23 R24 R25 R26 R27 R28 R29 C17 0.1µF C13 47µF, 25V C14 0.1µF AD768 LADCOM IOUTB (–5V) VEE (+5V) VDD (MSB) DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 CLOCK DCOM A +5D C16 0.1µF 0 0 0 0 0 0 0 0 1 3 5 7 9 11 13 15 J1 J1 J1 J1 J1 J1 J1 J1 J1 33 CLOCK 1 2 SW1 1 2 R11 50 J1 J1 J1 J1 J1 J1 J1 2 4 6 8 10 12 14 J1 J1 J1 J1 J1 J1 J1 16 18 20 22 24 26 28 J1 J1 J1 J1 J1 J1 30 32 34 36 38 40 A AGND CONNECTED TO DGND ON GND PLANE UNDER U1 IN BETWEEN PINS 5 AND 25 AGND DGND Figure 35. AD768 Evaluation Board Schematic REV. B –17– AD768 Figure 36. Silkscreen Layer (Not to Scale) –18– REV. B AD768 Figure 37. Component Side PCB Layout (Not to Scale) Figure 38. Solder Side PCB Layout (Not to Scale) REV. B –19– AD768 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). R-28 300 Mil 28-Pin SOIC C1941a–5–6/96 0.0291 (0.74) x 45° 0.0098 (0.25) 0.0500 (1.27) 0.0157 (0.40) 0.7125 (18.10) 0.6969 (17.70) 28 15 0.2992 (7.60) 0.2914 (7.40) 0.4193 (10.65) 0.3937 (10.00) 1 14 PIN 1 0.1043 (2.65) 0.0926 (2.35) 0.0118 (0.30) 0.0040 (0.10) 0.0500 (1.27) BSC 8° 0.0192 (0.49) 0° SEATING 0.0125 (0.32) 0.0138 (0.35) PLANE 0.0091 (0.23) –20– REV. B PRINTED IN U.S.A.