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FEATURES 5 ns Settling Time 100 MSPS Update Rate 20 mA Output Current ECL-Compatible 40 MHz Multiplying Mode APPLICATIONS Raster Scan & Vector Graphic Displays High Speed Waveform Generation Digital VCOs Ultrafast Digital Attenuators GENERAL DESCRIPTION
Ultrahigh Speed IC D/A Converter AD9768
FUNCTIONAL BLOCK DIAGRAM
The Analog Devices AD9768SD D/A converter is a monolithic current-output converter which can accept 8 bits of ECL-level digital input voltages and convert them into analog signals at update rates as high as 100 MSPS. In addition to its use as a standard D/A converter, it can also be utilized as a two-quadrant multiplying D/A at multiplying bandwidths as high as 40 MHz. An inherently low glitch design is used, and the complementary current outputs are suitable for driving transmission lines directly. Nominal full-scale output is 20 mA, which corresponds to a 1 volt drop across a 50 load, or 1 volt across 100 returned to +1 volt. The actual output current is determined by the on-chip reference voltage (VREF -1.26 V) and an external current setting resistor, RSET. Full-scale output current IOUT with digital "1" at all inputs is calculated with the equation:
The reference voltage source is a modified bandgap type and is nominally -1.26 volts. This reference supply requires no external regulation. To reduce the possibility of noise generation and/or instability, Pin 15 (REFERENCE OUT) can be decoupled using a high-quality ceramic chip capacitor. Stabilization of the internal loop amplifier is by a single capacitor connected from Pin 17 (COMPENSATION) to ground. The minimum value for this capacitor is 3900 pF, although a 0.01 F ceramic chip capacitor is recommended. The incredible speed characteristics of the AD9768SD D/A converter make it attractive for a wide range of high speed applications. The ability of the unit to operate as a twoquadrant multiplying D/A converter adds another dimension to its usefulness and makes the AD9768SD a truly versatile device.
AD9768SE PIN CONNECTIONS
IOUT = 4 x
VRET -VREF RSET
The setting resistor RSET and the output load resistor should both have low temperature coefficients. A complementary IOUT is also provided.
AD9768JD/SD PIN CONNECTIONS
REV. A
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.
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AD9768-SPECIFICATIONS (typical @ +25 C under following conditions unless otherwise noted;=nominal digital input levels; nominal power supplies; R = 50 ; R = 220 ; V 0 V)
L SET RET
Parameter RESOLUTION(FS = FULL SCALE) LSB WEIGHT (CURRENT) ACCURACY1 Differential Nonlinearity Integral Nonlinearity Monotonicity Zero Offset (lnitial) TEMPERATURE COEFFICIENTS Zero Offset Reference Voltage (-1.26 V) DIGITAL DATA INPUTS Logic Compatibility Logic Voltage Levels "l" = "0" = Coding OUTPUT Current (Unipolar) FS IOUT (@ Pin 13) All Digital "1" Input All Digital "0" Input IOUT(@ Pin 14) All Digital "l" Input All Digital "0" Input Compliance Impedance SPEED PERFORMANCE Settling Time (to 0.2% FS)2 Slew Rate Update Rate Rise Time Glitch Energy REFERENCE Internal, Monolithic3 External, Variable4 Voltage-Multiplying Mode Current-Multiplying Mode
4
Unit Bits A % FS % FS A ppm/C ppm/C
AD9768SJD/SD/SE 8 78 0.2 0.2 Guaranteed 60 1.5 70
Parameter CURRENT-MULTIPLYING MODE (See Figure 4) IM Range (at Pins 17 & 18) Resistance (at Pin 18) Transfer Function -
Unit
AD9768SJD/SD/SE
ECL V -0.9 V -1.7 Binary (BIN) = Unipolar Out Offset Binary (OBN) = Bipolar Out mA (max) mA mA mA mA V (Pin 13) V (Pin 14) ( 15%) ns V/s MSPS ns pV-sec V V (max) mA (max) 2 to 20 (30) 20 0 0 20 -0.7 to +3.0 -1.1 to +3.0 750 5 400 100 1.8 200 -1.26 0 to -1.1 (-2) 0 to -5 (-7.5 )
Large Signal Bandwidth (-3dB Point) POWER REQUIREMENTS -5.2 V 0.25 +5.0 V 0.25 Power Dissipation Power Supply Sensitivity5 TEMPERATURE RANGES6 Operating AD9768JD AD9768SD/SE Storage THERMAL RESlSTANCE 7 Junction to Air, JA (Free Air) Junction to Case, JA PACKAGE OPTION8 Ceramic (D-18) LCC (E-20A)
mA 0 to 5 160 Measured at Pin 13; Digital "0" Applied to Bits 1-8: 1 mA IM Input = 0 mA IOUT 5 mA IM Input = 0 mA IOUT Measured at Pin 13; Digital "1" Applied to Bits 1-8: 1 mA IM Input = 4 mA IOUT 5 mA IM Input = 20 mA IOUT MHz 40 mA (max) mA (max) mW (max) %/% 66(70) 14(15) 410(430) 0.07
C C C C/W C/W AD9768JD AD9768SD AD9768SE
0 to +70 -55 to +125 -55 to +150 90 20
VOLTAGE-MULTIPLYING MODE (See Figure 2) VM Range (at Pin 16) V 0.5 VM Center V -0.6 Resistance (at Pin 16) k 800 Transfer Function - Measured at Pin 13; Digital "0" Applied to Bits 1-8: -0.1 VM Input = 0 mA IOUT -1.1 VM Input = 0 mA IOUT Measured at Pin 13; Digital "1" Applied to Bits 1-8: -0.1 VM Input = 1 mA IOUT -1.1 VM Input = 20 mA IOUT Large Signal Bandwidth (-3 dB Point) kHz 250
NOTES 1 Relative to FS, including linearity (within voltage compliance limits). 2 Worst case settling time; includes FS and Most Significant Bit (MSB) transitions. 3 Applies when operating AD9768 as standard D/A. 4 Based on RL = 50 ohms; RSET = 220 ohms; V RET = 0 V. 5 1% change in either power supply voltage causes 0.07% change in analog output. 6 Case temperature. 7 Maximum junction temperature 125C. 8 D = Ceramic DIP, E = Leadless Ceramic Chip Carrier. Specifications subject to change without notice.
AD9768SD D/A Schematic
-2-
REV. A
AD9768
THEORY OF OPERATION
Refer to the AD9768SD schematic. The transistors pictured on the bottom of the diagram, connected to paired transistors in the middle of the schematic, are current sources which are always "on". The paired transistors are differential current switches, designed to steer current from the current sources to either Pin 13 (IO) or Pin 14 ( IO ). Digital inputs applied to Pins 1-8 determine which transistors will be operating in each pair and establish what current will flow at Pins 13 and 14. The transistor on the extreme left of the schematic is a base reference for the paired current switches and is used to assure the switches will be centered around an ECL voltage swing. The diodes connected to the base of this transistor are temperature compensation devices for the base reference circuit. There are three different current sources in the AD9768 D/A. The eight transistors shown on the bottom of the schematic are structured as two identical groups of four current sources, each of which is binarily weighted. The MSB group, comprised of the four on the right, is connected to the LSB group through a 15:1 current divider made up of two 50 and two 750 resistor networks. The geometry of the AD9768 guarantees the binary weighing ratios among the 100, 200, 400 and 800 resistors in each emitter circuit are correct. The resistor values which are shown indicate the ratios among the resistors, and not their nominal values. The third current source is a single transistor, pictured in the lower left portion of the schematic with its collector connected to Pin 18 RSET. Its function is to help establish the base voltage on the eight current sources; it works in conjunction with the external RSET resistor selected by the user of the AD9768, and the reference amplifier. Current flowing through this transistor is referred to as IM in the figures and text. When the AD9768 is operating as a conventional current-output D/A converter, IM develops a voltage across RSET which is one of the inputs to the on-board reference amplifier shown in the schematic. The other input to this amplifier is the on-chip reference voltage of -1.26 volts. The output of the reference amplifier adjusts the current-source base reference voltage at Pin 17; this, in turn, adjusts the value of IM in the single-transistor current source and causes it to develop a voltage across RSET which maintains Pin 18 at the -1.26 volts of the on-chip reference supply. To maintain good stability in the internal loop reference amplifier, a ceramic chip capacitor with a nominal value of 0.01 F should be connected to Pin 17 COMPENSATION; minimum recommended value for this capacitor is 3900 pF. The temperature coefficient of the load resistor (RL) can affect the performance of the AD9768 D/A converter, as it can with any current-output converter. The design and use of the AD9768 and its dependence on an external RSET resistor, however, make it sensitive also to the tempco of RSET. The user is cautioned to select RL and RSET resistors which have low temperature coefficients. DIGITAL GROUND (Pin 11) and ANALOG RETURN (Pin 12) are normally connected together; this connection should be made as close as possible to the device case to minimize possible noise problems. The AD9768 D/A is similar to any other highREV. A -3-
speed, high performance device: optimum use requires careful attention to all design details, including the layout of the circuit in which the converter is used.
CONVENTIONAL AD9768SD
Refer to Figure 1, Conventional AD9768SD. The output current of the AD9768 appears at Pin 13 (IO) and develops a voltage across the load resistor RL which is based on: A. IM (the current flowing through the single-transistor source discussed above) B. Value of RL
Figure 1. Conventional AD9768SD
IM is a function of the return voltage (VRET), the reference voltage (VREF), and the value of RSET; all of these are selected by the user for his application. The necessary equations for calculating precise values for each are part of Figure 1. As indicated, the voltage drop across RL is added to the return voltage; the resulting voltage is the total VOUT of the converter.
VOLTAGE MULTIPLYING MODE
In addition to its use as an ultra-high speed current output D/A converter, the AD9768 can also be used as a two-quadrant multiplying D/A in either a voltage mode or a current mode. Refer to Figure 2, Multiplying AD9768 (Voltage Mode). When operating in this mode, the analog output of the AD9768 is influenced by the digital inputs and an external multiplying voltage (VM) applied to Pin 16 REFERENCE IN, which takes the place of the internal reference used when the D/A is operating in a conventional manner.
Figure 2. Multiplying AD9768 (Voltage Mode)
The value of IM flowing through RSET is set by the voltage of VRET minus the multiplying voltage (VM), divided by RSET; the amount of this current is part of the equation which establishes the analog output (VOUT) of the AD9768 and is chosen by the user for his application. As it is when operating the D/A in a conventional fashion, VRET can be any value between 0 volts and +3 volts. VM (for purposes of discussion here) is some negative voltage and can be varied over a range which is approximately 1 volt peak-to-peak.
AD9768
If the load resistor (RL) has a value of 50 ohms, if RSET has a value of 220 ohms, and if VRET is 0 V, the center of the VM voltage will be -0.6 V; and it can vary from -0.1 V to -1.1 V. Typically, the frequency of these variations has an upper limit of 250 kHz when operating in the voltage multiplying mode; that frequency is the 3 dB point of the bandwidth of the internal reference amplifier. The combined effects of variations in VM and changes in digital input values are shown in Figure 3, IOUT vs. Multiplying Voltage. In this illustration, the ordinate of the graph is expressed in terms of milliamps of IOUT current at Pin 13. VOUT, of course, will be a function of the value of RL chosen by the user. VIN is some voltage chosen by the user for his particular application; the value of this voltage is based in part on the size of the load resistor and the 0 mA to 5 mA range of IM. VIN can have frequency components as high as 40 MHz. VADJ and RADJ provide an offset adjustment to compensate for the dc component of VIN to assure IM is always a unipolar current between 0 mA and 5 mA. The values of the required voltages and resistors can be calculated using the equations which are part of Figure 4. Refer to Figure 5, IOUT vs. Multiplying Current.
Figure 5. IOUT vs. Multiplying Current
Figure 3. IOUT vs. Multiplying Voltage
The negative value of VM on the horizontal axis is shown starting at approximately -0.1 V, rather than 0 V, because the AD9768 must have some small value of voltage applied to perform a multiplying function. For the conditions shown in the figure, output current starts to become nonlinear at approximately 20 mA because of the maximum 30 mA output drive capabilities of the device. Different values for RSET and RL would alter the point where limiting first appears.
CURRENT MULTIPLYING MODE
As shown, IM can vary over the range of 0 mA to 5 mA; a value of approximately 0.3 mA may be the practical lower limit because of nonlinearities at extremely small current levels. These changes in IM are combined with variations in digital inputs, producing complex changes in the output current (at pin 13) and in VOUT. The "rounding" of the current curve in the graph is the result of IOUT approaching the 30 mA maximum drive capabilities of the AD9768 and needs to be taken into account to assure optimum performance in the selected application.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
The AD9768 D/A converter can be operated at markedly higher multiplying rates when operated in a current-multiplying mode, as contrasted with the voltage multiplying mode. Refer to Figure 4, Multiplying AD9768SD (Current Mode).
Ceramic (D-18)
LCC (E-20A)
Figure 4. Multiplying AD9768SD (Current Mode)
In this mode, the internal reference amplifier and its inherent frequency limitations are replaced by a current source comprised of U1 and associated circuits. These circuits supply a unipolar current IM which is one-fourth the full-scale output current (with digital "1" applied to all inputs) and set current flow through the load resistor. -4- REV. A
PRINTED IN U.S.A.
C753c-5-12/89

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