DAC122S085 DAC122S085 12-Bit Micro Power DUAL Digital-to-Analog Converter with Rail-to-Rail Output Literature Number: SNAS361D DAC122S085 12-Bit Micro Power DUAL Digital-to-Analog Converter with Rail-to-Rail Output General Description Features The DAC122S085 is a full-featured, general purpose DUAL 12-bit voltage-output digital-to-analog converter (DAC) that can operate from a single +2.7V to 5.5V supply and consumes 0.6 mW at 3V and 1.6 mW at 5V. The DAC122S085 is packaged in 10-lead LLP and MSOP packages. The 10-lead LLP package makes the DAC122S085 the smallest DUAL DAC in its class. The on-chip output amplifier allows rail-to-rail output swing and the three wire serial interface operates at clock rates up to 40 MHz over the entire supply voltage range. Competitive devices are limited to 25 MHz clock rates at supply voltages in the 2.7V to 3.6V range. The serial interface is compatible with standard SPITM, QSPI, MICROWIRE and DSP interfaces. The reference for the DAC122S085 serves all four channels and can vary in voltage between 1V and VA, providing the widest possible output dynamic range. The DAC122S085 has a 16-bit input shift register that controls the outputs to be updated, the mode of operation, the powerdown condition, and the binary input data. Both outputs can be updated simultaneously or individually depending on the setting of the two mode of operation bits. A power-on reset circuit ensures that the DAC output powers up to zero volts and remains there until there is a valid write to the device. A power-down feature reduces power consumption to less than a microWatt with three different termination options. The low power consumption and small packages of the DAC122S085 make it an excellent choice for use in battery operated equipment. The DAC122S085 is one of a family of pin compatible DACs, including the 8-bit DAC082S085 and the 10-bit DAC102S085. The DAC122S085 operates over the extended industrial temperature range of -40C to +105C. Guaranteed Monotonicity Low Power Operation Rail-to-Rail Voltage Output Power-on Reset to 0V Simultaneous Output Updating Wide power supply range (+2.7V to +5.5V) Industry's Smallest Package Power Down Modes Key Specifications Resolution INL DNL Settling Time Zero Code Error Full-Scale Error Supply Power -- Normal -- Power Down 12 bits 8 LSB (max) +0.7 / -0.5 LSB (max) 8.5 s (max) +15 mV (max) -0.75 %FS (max) 0.6 mW (3V) / 1.6 mW (5V) typ 0.3 W (3V) / 0.8 W (5V) typ Applications Battery-Powered Instruments Digital Gain and Offset Adjustment Programmable Voltage & Current Sources Programmable Attenuators Pin Configuration 20195201 20195202 SPITM is a trademark of Motorola, Inc. (c) 2008 National Semiconductor Corporation 201952 www.national.com DAC122S085 12-Bit Micro Power DUAL Digital-to-Analog Converter with Rail-to-Rail Output November 20, 2008 DAC122S085 Ordering Information Order Numbers Temperature Range Package Top Mark DAC122S085CISD -40C TA +105C LLP X73C DAC122S085CISDX -40C TA +105C LLP Tape-and-Reel X73C DAC122S085CIMM -40C TA +105C MSOP X72C DAC122S085CIMMX -40C TA +105C MSOP Tape-and-Reel X72C DAC122S085EVAL Evaluation Board (MSOP) Block Diagram 20195203 www.national.com 2 DAC122S085 Pin Descriptions LLP MSOP Pin No. Symbol Type 1 VA Supply 2 VOUTA Analog Output Channel A Analog Output Voltage. 3 VOUTB Analog Output Channel B Analog Output Voltage. 4 NC 5 NC 6 GND Ground 7 VREFIN Analog Input Unbuffered reference voltage shared by both channels. Must be decoupled to GND. 8 DIN Digital Input Serial Data Input. Data is clocked into the 16-bit shift register on the falling edges of SCLK after the fall of SYNC. Description Power supply input. Must be decoupled to GND. Not Connected Not Connected Ground reference for all on-chip circuitry. 9 SYNC Digital Input Frame synchronization input for the data input. When this pin goes low, it enables the input shift register and data is transferred on the falling edges of SCLK. The DAC is updated on the 16th clock cycle unless SYNC is brought high before the 16th clock, in which case the rising edge of SYNC acts as an interrupt and the write sequence is ignored by the DAC. 10 SCLK Digital Input Serial Clock Input. Data is clocked into the input shift register on the falling edges of this pin. 11 PAD (LLP only) Ground Exposed die attach pad can be connected to ground or left floating. Soldering the pad to the PCB offers optimal thermal performance and enhances package self-alignment during reflow. 3 www.national.com DAC122S085 Operating Ratings (Notes 1, 2) Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage, VA Voltage on any Input Pin Input Current at Any Pin (Note 3) Package Input Current (Note 3) Power Consumption at TA = 25C ESD Susceptibility (Note 5) Human Body Model Machine Model Junction Temperature Storage Temperature Operating Temperature Range Supply Voltage, VA Reference Voltage, VREFIN Digital Input Voltage (Note 7) Output Load SCLK Frequency 6.5V -0.3V to 6.5V 10 mA 20 mA See (Note 4) -40C TA +105C +2.7V to 5.5V +1.0V to VA 0.0V to 5.5V 0 to 1500 pF Up to 40 MHz Package Thermal Resistances 2500V 250V +150C -65C to +150C Package JA 10-Lead MSOP 10-Lead LLP 240C/W 250C/W Soldering process must comply with National Semiconductor's Reflow Temperature Profile specifications. Refer to www.national.com/packaging. (Note 6) Electrical Characteristics The following specifications apply for VA = +2.7V to +5.5V, VREFIN = VA, CL = 200 pF to GND, fSCLK = 30 MHz, input code range 48 to 4047. Boldface limits apply for TMIN TA TMAX and all other limits are at TA = 25C, unless otherwise specified. Limits (Note 9) Units (Limits) Resolution 12 Bits (min) Monotonicity 12 Bits (min) 2.4 8 LSB (max) +0.2 +0.7 LSB (max) -0.1 -0.5 LSB (min) 0.15 0.5 LSB (max) Symbol Parameter Conditions Typical (Note 9) STATIC PERFORMANCE INL Integral Non-Linearity DNL Differential Non-Linearity VA = 2.7V to 5.5V VA = 4.5V to 5.5V (Note 10) ZE Zero Code Error IOUT = 0 +4 +15 mV (max) FSE Full-Scale Error IOUT = 0 -0.1 -0.75 %FSR (max) GE Gain Error All ones Loaded to DAC register -0.2 -1.0 %FSR ZCED TC GE Zero Code Error Drift Gain Error Tempco -20 V/C VA = 3V -0.7 ppm/C VA = 5V -1.0 ppm/C OUTPUT CHARACTERISTICS Output Voltage Range IOZ ZCO FSO IOS (Note 10) High-Impedance Output Leakage Current (Note 10) Zero Code Output Full Scale Output Output Short Circuit Current (source) www.national.com 0 VREFIN V (min) V (max) 1 A (max) VA = 3V, IOUT = 200 A 1.3 mV VA = 3V, IOUT = 1 mA 6.0 mV VA = 5V, IOUT = 200 A 7.0 mV VA = 5V, IOUT = 1 mA 10.0 mV VA = 3V, IOUT = 200 A 2.984 V VA = 3V, IOUT = 1 mA 2.934 V VA = 5V, IOUT = 200 A 4.989 V VA = 5V, IOUT = 1 mA 4.958 V VA = 3V, VOUT = 0V, Input Code = FFFh -56 mA VA = 5V, VOUT = 0V, Input Code = FFFh -69 mA 4 IOS Parameter Output Short Circuit Current (sink) IO Continuous Output Current (Note 10) CL Maximum Load Capacitance ZOUT Conditions Typical (Note 9) Limits (Note 9) Units (Limits) VA = 3V, VOUT = 3V, Input Code = 000h 52 mA VA = 5V, VOUT = 5V, Input Code = 000h 75 mA Available on each DAC output 11 mA (max) RL = 1500 pF RL = 2k 1500 pF 7.5 DC Output Impedance REFERENCE INPUT CHARACTERISTICS Input Range Minimum VREFIN 0.2 Input Range Maximum Input Impedance 1.0 V (min) VA V (max) 60 k LOGIC INPUT CHARACTERISTICS IIN VIL Input Current (Note 10) Input Low Voltage (Note 10) VIH Input High Voltage (Note 10) CIN Input Capacitance (Note 10) 1 A (max) VA = 3V 0.9 0.6 V (max) VA = 5V 1.5 0.8 V (max) VA = 3V 1.4 2.1 V (min) VA = 5V 2.1 2.4 V (min) 3 pF (max) POWER REQUIREMENTS VA Supply Voltage Minimum 2.7 V (min) Supply Voltage Maximum 5.5 V (max) fSCLK = 30 MHz IN Normal Supply Current (output unloaded) fSCLK = 0 IPD Power Down Supply Current (output All PD Modes, unloaded, SYNC = DIN = 0V after (Note 10) PD mode loaded) fSCLK = 30 MHz PN Normal Supply Power (output unloaded) fSCLK = 0 PPD Power Down Supply Power (output All PD Modes, unloaded, SYNC = DIN = 0V after (Note 10) PD mode loaded) 5 VA = 2.7V to 3.6V 210 270 A (max) VA = 4.5V to 5.5V 320 410 A (max) VA = 2.7V to 3.6V 190 A VA = 4.5V to 5.5V 290 A VA = 2.7V to 3.6V 0.1 1.0 A (max) VA = 4.5V to 5.5V 0.15 1.0 A (max) VA = 2.7V to 3.6V 0.6 1.0 mW (max) VA = 4.5V to 5.5V 1.6 2.3 mW (max) VA = 2.7V to 3.6V 0.6 mW VA = 4.5V to 5.5V 1.5 mW VA = 2.7V to 3.6V 0.3 3.6 W (max) VA = 4.5V to 5.5V 0.8 5.5 W (max) www.national.com DAC122S085 Symbol DAC122S085 A.C. and Timing Characteristics Values shown in this table are design targets and are subject to change before product release. The following specifications apply for VA = +2.7V to +5.5V, VREFIN = VA, CL = 200 pF to GND, fSCLK = 30 MHz, input code range 48 to 4047. Boldface limits apply for TMIN TA TMAX and all other limits are at TA = 25C, unless otherwise specified. Symbol fSCLK Parameter Conductions SCLK Frequency ts Output Voltage Settling Time (Note 10) SR Output Slew Rate Glitch Impulse Typical (Note 9) Limits (Note 9) Units (Limits) 40 30 MHz (max) 400h to C00h code change RL = 2 k, CL = 200 pF 6 8.5 s (max) 1 V/s Code change from 800h to 7FFh 12 nV-sec Digital Feedthrough 0.5 nV-sec Digital Crosstalk 1 nV-sec DAC-to-DAC Crosstalk 3 nV-sec Multiplying Bandwidth VREFIN = 2.5V 0.1Vpp 160 kHz Total Harmonic Distortion VREFIN = 2.5V 1Vpp input frequency = 10kHz 70 dB VA = 3V 6 sec VA = 5V 39 tWU Wake-Up Time 1/fSCLK SCLK Cycle Time 25 33 ns (min) tCH SCLK High time 7 10 ns (min) tCL SCLK Low Time 7 10 ns (min) tSS SYNC Set-up Time prior to SCLK Falling Edge 4 10 ns (min) tDS Data Set-Up Time prior to SCLK Falling Edge 1.5 3.5 ns (min) tDH Data Hold Time after SCLK Falling Edge 1.5 3.5 ns (min) tCFSR SCLK fall prior to rise of SYNC 0 3 ns (min) tSYNC SYNC High Time 6 10 ns (min) sec Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Operation of the device beyond the maximum Operating Ratings is not recommended. Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified. Note 3: When the input voltage at any pin exceeds 5.5V or is less than GND, the current at that pin should be limited to 10 mA. The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 10 mA to two. Note 4: The absolute maximum junction temperature (TJmax) for this device is 150C. The maximum allowable power dissipation is dictated by TJmax, the junction-to-ambient thermal resistance (JA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax - TA) / JA. The values for maximum power dissipation will be reached only when the device is operated in a severe fault condition (e.g., when input or output pins are driven beyond the operating ratings, or the power supply polarity is reversed). Note 5: Human body model is 100 pF capacitor discharged through a 1.5 k resistor. Machine model is 220 pF discharged through ZERO Ohms. Note 6: Reflow temperature profiles are different for lead-free packages. Note 7: The inputs are protected as shown below. Input voltage magnitudes up to 5.5V, regardless of VA, will not cause errors in the conversion result. For example, if VA is 3V, the digital input pins can be driven with a 5V logic device. 20195204 www.national.com 6 Note 9: Typical figures are at TJ = 25C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing Quality Level). Note 10: This parameter is guaranteed by design and/or characterization and is not tested in production. LSB = VREF / 2n Specification Definitions where VREF is the supply voltage for this product, and "n" is the DAC resolution in bits, which is 12 for the DAC122S085. MAXIMUM LOAD CAPACITANCE is the maximum capacitance that can be driven by the DAC with output stability maintained. MONOTONICITY is the condition of being monotonic, where the DAC has an output that never decreases when the input code increases. MOST SIGNIFICANT BIT (MSB) is the bit that has the largest value or weight of all bits in a word. Its value is 1/2 of VA. MULTIPLYING BANDWIDTH is the frequency at which the output amplitude falls 3dB below the input sine wave on VREFIN with a full-scale code loaded into the DAC. POWER EFFICIENCY is the ratio of the output current to the total supply current. The output current comes from the power supply. The difference between the supply and output currents is the power consumed by the device without a load. SETTLING TIME is the time for the output to settle to within 1/2 LSB of the final value after the input code is updated. TOTAL HARMONIC DISTORTION (THD) is the measure of the harmonics present at the output of the DACs with an ideal sine wave applied to VREFIN. THD is measured in dB. WAKE-UP TIME is the time for the output to exit power-down mode. This is the time from the falling edge of the 16th SCLK pulse to when the output voltage deviates from the powerdown voltage of 0V. ZERO CODE ERROR is the output error, or voltage, present at the DAC output after a code of 000h has been entered. DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB, which is VREF / 4096 = VA / 4096. DAC-to-DAC CROSSTALK is the glitch impulse transferred to a DAC output in response to a full-scale change in the output of another DAC. DIGITAL CROSSTALK is the glitch impulse transferred to a DAC output at mid-scale in response to a full-scale change in the input register of another DAC. DIGITAL FEEDTHROUGH is a measure of the energy injected into the analog output of the DAC from the digital inputs when the DAC outputs are not updated. It is measured with a full-scale code change on the data bus. FULL-SCALE ERROR is the difference between the actual output voltage with a full scale code (FFFh) loaded into the DAC and the value of VA x 4095 / 4096. GAIN ERROR is the deviation from the ideal slope of the transfer function. It can be calculated from Zero and FullScale Errors as GE = FSE - ZE, where GE is Gain error, FSE is Full-Scale Error and ZE is Zero Error. GLITCH IMPULSE is the energy injected into the analog output when the input code to the DAC register changes. It is specified as the area of the glitch in nanovolt-seconds. INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a straight line through the input to output transfer function. The deviation of any given code from this straight line is measured from the center of that code value. The end point method is used. INL for this product is specified over a limited range, per the Electrical Tables. LEAST SIGNIFICANT BIT (LSB) is the bit that has the smallest value or weight of all bits in a word. This value is 7 www.national.com DAC122S085 Note 8: To guarantee accuracy, it is required that VA and VREFIN be well bypassed. DAC122S085 Transfer Characteristic 20195205 FIGURE 1. Input / Output Transfer Characteristic Timing Diagrams 20195206 FIGURE 2. Serial Timing Diagram www.national.com 8 VREF = VA, fSCLK = 30 MHz, TA = 25C, Input Code Range 48 to INL at VA = 3.0V INL at VA = 5.0V 20195252 20195253 DNL at VA = 3.0V DNL at VA = 5.0V 20195254 20195255 INL/DNL vs VREFIN at VA = 3.0V INL/DNL vs VREFIN at VA = 5.0V 20195256 20195257 9 www.national.com DAC122S085 Typical Performance Characteristics 4047, unless otherwise stated DAC122S085 INL/DNL vs fSCLK at VA = 2.7V INL/DNL vs VA 20195250 20195222 INL/DNL vs Clock Duty Cycle at VA = 3.0V INL/DNL vs Clock Duty Cycle at VA = 5.0V 20195224 20195225 INL/DNL vs Temperature at VA = 3.0V INL/DNL vs Temperature at VA = 5.0V 20195226 www.national.com 20195227 10 DAC122S085 Zero Code Error vs. VA Zero Code Error vs. VREFIN 20195230 20195231 Zero Code Error vs. fSCLK Zero Code Error vs. Clock Duty Cycle 20195235 20195234 Zero Code Error vs. Temperature Full-Scale Error vs. VA 20195236 20195237 11 www.national.com DAC122S085 Full-Scale Error vs. VREFIN Full-Scale Error vs. fSCLK 20195232 20195233 Full-Scale Error vs. Clock Duty Cycle Full-Scale Error vs. Temperature 20195238 20195239 Supply Current vs. VA Supply Current vs. Temperature 20195245 20195244 www.national.com 12 DAC122S085 5V Glitch Response Power-On Reset 20195247 20195246 13 www.national.com DAC122S085 reason, linearity is specified over less than the full output range of the DAC. However, if the reference is less than VA, there is only a loss in linearity in the lowest codes. The output capabilities of the amplifier are described in the Electrical Tables. The output amplifiers are capable of driving a load of 2 k in parallel with 1500 pF to ground or to VA. The zero-code and full-scale outputs for given load currents are available in the Electrical Characterisics Table. 1.0 Functional Description 1.1 DAC SECTION The DAC122S085 is fabricated on a CMOS process with an architecture that consists of switches and resistor strings that are followed by an output buffer. The reference voltage is externally applied at VREFIN and is shared by all four DACs. For simplicity, a single resistor string is shown in Figure 3. This string consists of 4096 equal valued resistors with a switch at each junction of two resistors, plus a switch to ground. The code loaded into the DAC register determines which switch is closed, connecting the proper node to the amplifier. The input coding is straight binary with an ideal output voltage of: 1.3 RERENCE VOLTAGE The DAC122S085 uses a single external reference that is shared by both channels. The reference pin, VREFIN, is not buffered and has an input impedance of 60 k. It is recommended that VREFIN be driven by a voltage source with low output impedance. The reference voltage range is 1.0V to VA, providing the widest possible output dynamic range. VOUTA,B = VREFIN x (D / 4096) where D is the decimal equivalent of the binary code that is loaded into the DAC register. D can take on any value between 0 and 4095. This configuration guarantees that the DAC is monotonic. 1.4 SERIAL INTERFACE The three-wire interface is compatible with SPI, QSPI and MICROWIRE, as well as most DSPs and operates at clock rates up to 40 MHz. See the Timing Diagram for information on a write sequence. A write sequence begins by bringing the SYNC line low. Once SYNC is low, the data on the DIN line is clocked into the 16bit serial input register on the falling edges of SCLK. To avoid misclocking data into the shift register, it is critical that SYNC not be brought low simultaneously with a falling edge of SCLK (see Serial Timing Diagram, Figure 2). On the 16th falling clock edge, the last data bit is clocked in and the programmed function (a change in the DAC channel address, mode of operation and/or register contents) is executed. At this point the SYNC line may be kept low or brought high. Any data and clock pusles after the 16th falling clock edge will be ignored. In either case, SYNC must be brought high for the minimum specified time before the next write sequence is initiated with a falling edge of SYNC. Since the SYNC and DIN buffers draw more current when they are high, they should be idled low between write sequences to minimize power consumption. 1.5 INPUT SHIFT REGISTER The input shift register, Figure 4, has sixteen bits. The first bit must be set to "0" and the second bit is an address bit. The address bit determines whether the register data is for DAC A or DAC B. This bit is followed by two bits that determine the mode of operation (writing to a DAC register without updating the outputs of both DACs, writing to a DAC register and updating the outputs of both DACs, writing to the register of both DACs and updating their outputs, or powering down both outputs). The final twelve bits of the shift register are the data bits. The data format is straight binary (MSB first, LSB last), with all 0's corresponding to an output of 0V and all 1's corresponding to a full-scale output of VREFIN - 1 LSB. The contents of the serial input register are transferred to the DAC register on the sixteenth falling edge of SCLK. See Timing Diagram, Figure 2. 20195207 FIGURE 3. DAC Resistor String 1.2 OUTPUT AMPLIFIERS The output amplifiers are rail-to-rail, providing an output voltage range of 0V to VA when the reference is VA. All amplifiers, even rail-to-rail types, exhibit a loss of linearity as the output approaches the supply rails (0V and VA, in this case). For this www.national.com 14 DAC122S085 20195208 FIGURE 4. Input Register Contents down modes. However, the contents of the DAC registers are unaffected when in power-down. Each DAC register maintains its value prior to the DAC122S085 being powered down unless it is changed during the write sequence which instructed it to recover from power down. Minimum power consumption is achieved in the power-down mode with SYNC and DIN idled low and SCLK disabled. The time to exit power-down (Wake-Up Time) is typically tWU sec as stated in the A.C. and Timing Characteristics table. Normally, the SYNC line is kept low for at least 16 falling edges of SCLK and the DAC is updated on the 16th SCLK falling edge. However, if SYNC is brought high before the 16th falling edge, the data transfer to the shift register is aborted and the write sequence is invalid. Under this condition, the DAC register is not updated and there is no change in the mode of operation or in the DAC output voltages. 1.6 POWER-ON RESET The power-on reset circuit controls the output voltages both DACs during power-up. Upon application of power, the DAC registers are filled with zeros and the output voltages are 0V. The outputs remain at 0V until a valid write sequence is made to the DAC. 2.0 Applications Information 2.1 USING REFERENCES AS POWER SUPPLIES While the simplicity of the DAC122S085 implies ease of use, it is important to recognize that the path from the reference input (VREFIN) to the VOUTs will have essentially zero Power Supply Rejection Ratio (PSRR). Therefore, it is necessary to provide a noise-free supply voltage to VREFIN. In order to utilize the full dynamic range of the DAC122S085, the supply pin (VA) and VREFIN can be connected together and share the same supply voltage. Since the DAC122S085 consumes very little power, a reference source may be used as the reference input and/or the supply voltage. The advantages of using a reference source over a voltage regulator are accuracy and stability. Some low noise regulators can also be used. Listed below are a few reference and power supply options for the DAC122S085. 1.7 POWER-DOWN MODES The DAC122S085 has four power-down modes, two of which are identical. In power-down mode, the supply current drops to 20 A at 3V and 30 A at 5V. The DAC122S085 is set in power-down mode by setting OP1 and OP0 to 11. Since this mode powers down both DACs, the first two bits of the shift register are used to select different output terminations for the DAC outputs. Setting A1 and A0 to 00 or 11 causes the outputs to be tri-stated (a high impedance state). While setting A1 and A0 to 01 or 10 causes the outputs to be terminated by 2.5 k or 100 k to ground respectively (see Table 1). TABLE 1. Power-Down Modes A1 A0 OP1 OP0 0 0 1 1 0 1 1 1 2.5 k to GND 1 0 1 1 100 k to GND 1 1 1 1 High-Z outputs 2.1.1 LM4130 The LM4130, with its 0.05% accuracy over temperature, is a good choice as a reference source for the DAC122S085. The 4.096V version is useful if a 0 to 4.095V output range is desirable or acceptable. Bypassing the LM4130 VIN pin with a 0.1F capacitor and the VOUT pin with a 2.2F capacitor will improve stability and reduce output noise. The LM4130 comes in a space-saving 5-pin SOT23. Operating Mode High-Z outputs The bias generator, output amplifiers, resistor strings, and other linear circuitry are all shut down in any of the power- 15 www.national.com DAC122S085 20195213 FIGURE 5. The LM4130 as a power supply LM4050 voltage at its minimum, and the DAC122S085 drawing zero current. The maximum resistor value must allow the LM4050 to draw more than its minimum current for regulation plus the maximum DAC122S085 current in full operation. The conditions for minimum current include the input voltage at its minimum, the LM4050 voltage at its maximum, the resistor value at its maximum due to tolerance, and the DAC122S085 draws its maximum current. These conditions can be summarized as 2.1.2 LM4050 Available with accuracy of 0.44%, the LM4050 shunt reference is also a good choice as a reference for the DAC122S085. It is available in 4.096V and 5V versions and comes in a space-saving 3-pin SOT23. R(min) = ( VIN(max) - VZ(min) ) /IZ(max) and R(max) = ( VIN(min) - VZ(max) ) / ( (IDAC(max) + IZ(min) ) where V Z(min) and VZ(max) are the nominal LM4050 output voltages the LM4050 output tolerance over temperature, IZ (max) is the maximum allowable current through the LM4050, IZ(min) is the minimum current required by the LM4050 for proper regulation, and IDAC(max) is the maximum DAC122S085 supply current. 2.1.3 LP3985 The LP3985 is a low noise, ultra low dropout voltage regulator with a 3% accuracy over temperature. It is a good choice for applications that do not require a precision reference for the DAC122S085. It comes in 3.0V, 3.3V and 5V versions, among others, and sports a low 30 V noise specification at low frequencies. Since low frequency noise is relatively difficult to filter, this specification could be important for some applications. The LP3985 comes in a space-saving 5-pin SOT23 and 5-bump micro SMD packages. 20195214 FIGURE 6. The LM4050 as a power supply The minimum resistor value in the circuit of Figure 6 must be chosen such that the maximum current through the LM4050 does not exceed its 15 mA rating. The conditions for maximum current include the input voltage at its maximum, the 20195215 FIGURE 7. Using the LP3985 regulator www.national.com 16 2.1.4 LP2980 The LP2980 is an ultra low dropout regulator with a 0.5% or 1.0% accuracy over temperature, depending upon grade. It is available in 3.0V, 3.3V and 5V versions, among others. 20195216 FIGURE 8. Using the LP2980 regulator Like any low dropout regulator, the LP2980 requires an output capacitor for loop stability. This output capacitor must be at least 1.0F over temperature, but values of 2.2F or more will provide even better performance. The ESR of this capacitor should be within the range specified in the LP2980 data sheet. Surface-mount solid tantalum capacitors offer a good combination of small size and ESR. Ceramic capacitors are attractive due to their small size but generally have ESR values that are too low for use with the LP2980. Aluminum electrolytic capacitors are typically not a good choice due to their large size and have ESR values that may be too high at low temperatures. 2.2 BIPOLAR OPERATION The DAC122S085 is designed for single supply operation and thus has a unipolar output. However, a bipolar output may be obtained with the circuit in Figure 9. This circuit will provide an output voltage range of 5 Volts. A rail-to-rail amplifier should be used if the amplifier supplies are limited to 5V. 20195217 FIGURE 9. Bipolar Operation The output voltage of this circuit for any code is found to be TABLE 2. Some Rail-to-Rail Amplifiers VO = (VA x (D / 4096) x ((R1 + R2) / R1) - VA x R2 / R1) where D is the input code in decimal form. With VA = 5V and R1 = R2, VO = (10 x D / 4096) - 5V A list of rail-to-rail amplifiers suitable for this application are indicated in Table 2. 17 Typ ISUPPLY AMP PKGS LMC7111 DIP-8 SOT23-5 0.9 mV 25 A LM7301 SO-8 SOT23-5 0.03 mV 620 A LM8261 SOT23-5 0.7 mV 1 mA Typ VOS www.national.com DAC122S085 An input capacitance of 1.0F without any ESR requirement is required at the LP3985 input, while a 1.0F ceramic capacitor with an ESR requirement of 5m to 500m is required at the output. Careful interpretation and understanding of the capacitor specification is required to ensure correct device operation. DAC122S085 mits data in 8-bit bytes with eight falling clock edges. Data is transmitted with the MSB first. PC7 must remain low after the first eight bits are transferred. A second write cycle is initiated to transmit the second byte of data to the DAC, after which PC7 should be raised to end the write sequence. 2.3 DSP/MICROPROCESSOR INTERFACING Interfacing the DAC122S085 to microprocessors and DSPs is quite simple. The following guidelines are offered to hasten the design process. 2.3.1 ADSP-2101/ADSP2103 Interfacing Figure 10 shows a serial interface between the DAC122S085 and the ADSP-2101/ADSP2103. The DSP should be set to operate in the SPORT Transmit Alternate Framing Mode. It is programmed through the SPORT control register and should be configured for Internal Clock Operation, Active Low Framing and 16-bit Word Length. Transmission is started by writing a word to the Tx register after the SPORT mode has been enabled. 20195211 FIGURE 12. 68HC11 Interface 2.3.4 Microwire Interface Figure 13 shows an interface between a Microwire compatible device and the DAC122S085. Data is clocked out on the rising edges of the SK signal. As a result, the SK of the Microwire device needs to be inverted before driving the SCLK of the DAC122S085. 20195209 FIGURE 10. ADSP-2101/2103 Interface 2.3.2 80C51/80L51 Interface A serial interface between the DAC122S085 and the 80C51/80L51 microcontroller is shown in Figure 11. The SYNC signal comes from a bit-programmable pin on the microcontroller. The example shown here uses port line P3.3. This line is taken low when data is transmitted to the DAC122S085. Since the 80C51/80L51 transmits 8-bit bytes, only eight falling clock edges occur in the transmit cycle. To load data into the DAC, the P3.3 line must be left low after the first eight bits are transmitted. A second write cycle is initiated to transmit the second byte of data, after which port line P3.3 is brought high. The 80C51/80L51 transmit routine must recognize that the 80C51/80L51 transmits data with the LSB first while the DAC122S085 requires data with the MSB first. 20195212 FIGURE 13. Microwire Interface 2.4 LAYOUT, GROUNDING, AND BYPASSING For best accuracy and minimum noise, the printed circuit board containing the DAC122S085 should have separate analog and digital areas. The areas are defined by the locations of the analog and digital power planes. Both of these planes should be located in the same board layer. There should be a single ground plane. A single ground plane is preferred if digital return current does not flow through the analog ground area. Frequently a single ground plane design will utilize a "fencing" technique to prevent the mixing of analog and digital ground current. Separate ground planes should only be utilized when the fencing technique is inadequate. The separate ground planes must be connected in one place, preferably near the DAC122S085. Special care is required to guarantee that digital signals with fast edge rates do not pass over split ground planes. They must always have a continuous return path below their traces. The DAC122S085 power supply should be bypassed with a 10F and a 0.1F capacitor as close as possible to the device with the 0.1F right at the device supply pin. The 10F capacitor should be a tantalum type and the 0.1F capacitor should be a low ESL, low ESR type. The power supply for the DAC122S085 should only be used for analog circuits. Avoid crossover of analog and digital signals and keep the clock and data lines on the component side of the board. The clock and data lines should have controlled impedances. 20195210 FIGURE 11. 80C51/80L51 Interface 2.3.3 68HC11 Interface A serial interface between the DAC122S085 and the 68HC11 microcontroller is shown in Figure 12. The SYNC line of the DAC122S085 is driven from a port line (PC7 in the figure), similar to the 80C51/80L51. The 68HC11 should be configured with its CPOL bit as a zero and its CPHA bit as a one. This configuration causes data on the MOSI output to be valid on the falling edge of SCLK. PC7 is taken low to transmit data to the DAC. The 68HC11 trans- www.national.com 18 DAC122S085 Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead MSOP Order Numbers DAC122S085CIMM NS Package Number MUB10A 10-Lead LLP Order Numbers DAC122S085CISD NS Package Number SDA10A 19 www.national.com DAC122S085 12-Bit Micro Power DUAL Digital-to-Analog Converter with Rail-to-Rail Output Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH(R) Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy PowerWise(R) Solutions www.national.com/powerwise Solutions www.national.com/solutions Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero Temperature Sensors www.national.com/tempsensors Solar Magic(R) www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless Analog University(R) www.national.com/AU THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION ("NATIONAL") PRODUCTS. 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