® DAC1221 DAC 122 1 For most current data sheet and other product information, visit www.burr-brown.com 16-Bit Low Power DIGITAL-TO-ANALOG CONVERTER FEATURES APPLICATIONS ● 16-BIT MONOTONICITY GUARANTEED OVER –40°C TO +85°C ● LOW POWER: 1.2mW ● PROCESS CONTROL ● ATE PIN ELECTRONICS ● VOLTAGE OUTPUT ● SMART TRANSMITTERS ● SETTLING TIME: 2ms to 0.012% ● PORTABLE INSTRUMENTS ● MAX LINEARITY ERROR: 30ppm ● VCO CONTROL ● CLOSED-LOOP SERVO-CONTROL ● ON-CHIP CALIBRATION DESCRIPTION The DAC1221 features a synchronous serial interface. In single converter applications, the serial interface can be accomplished with just two wires, allowing lowcost isolation. For multiple converters, a CS signal allows for selection of the appropriate D/A converter. The DAC1221 is a Digital-to-Analog (D/A) converter offering 16-bit monotonic performance over the specified temperature range. It utilizes delta-sigma technology to achieve inherently linear performance in a small package at very low power. The output range is two times the external reference voltage. On-chip calibration circuitry dramatically reduces offset and gain errors. XIN XOUT The DAC1221 has been designed for closed-loop control applications in the industrial process control market, and high resolution applications in the test and measurement market. It is also ideal for remote applications, battery-powered instruments, and isolated systems. The DAC1221 is available in a SSOP-16 package. C1 C2A C2B C3 VREF Clock Generator Microcontroller Instruction Register Command Register Data Register Offset Register Full-Scale Register SDIO SCLK Second-Order ∆∑ Modulator Second-Order Continuous Time Post Filter VOUT Modulator Control Serial Interface CS First-Order Switched Capacitor Filter DVDD DGND AVDD AGND International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 ® © 1999 Burr-Brown Corporation PDS-1519B 1 Printed in U.S.A. May, 2000 DAC1221 SPECIFICATIONS All specifications TMIN to TMAX, AVDD = DVDD = +3V, fXIN = 2.5MHz, VREF = +1.25V, C1 = 2.2nF, C2 = 150pF, C3 = 6.8nF, unless otherwise noted. DAC1221E PARAMETER CONDITIONS ACCURACY Monotonicity Linearity Error(1) Offset Error (2) Offset Error Drift(3) Midscale Error(2) Midscale Error Drift(3) Gain Error(2) Gain Error Drift(3) Power-Supply Rejection Ratio MIN TYP 16 ±30 ±190 VOUT = 20mV, CALPIN = 1(6) 50 ±20 50 VOUT = VREF, CALPIN = 1(6) CALPIN = 1(6) 0.015 3 57 at DC, dB = –20log(∆VOUT/∆VDD) ANALOG OUTPUT Output Voltage(4) Output Current(1) Capacitive Load Short-Circuit Current Short-Circuit Duration GND or V DD ±10 Indefinite DYNAMIC PERFORMANCE Settling Time(1,5) Output-Noise Voltage To ±0.012% 1Hz to 2kHz 0 REFERENCE INPUT Input Voltage Input Impedance DIGITAL INPUT/OUTPUT Logic Family Logic Levels (all except XIN) VIH VIL VOH VOL Input-Leakage Current XIN Frequency Range (fXIN) Data Format POWER SUPPLY REQUIREMENTS Power-Supply Voltage Supply Current Analog Current Digital Current Power Dissipation MAX 1.125 UNITS Bits ppm of FSR µV µV/°C µV µV/°C % ppm/°C dB 2 • VREF ±0.25 500 V mA pF mA 1.8 45 2 ms µVrms 1.25 1 1.375 V MΩ DVDD + 0.3 0.8 0.4 ±10 2.5 V V V V µA MHz 3.3 V TTL-Compatible CMOS 2.0 –0.3 2.4 IOH = –0.8mA IOL = 1.6mA 1.0 User Programmable Offset Two’s Complement or Straight Binary 2.7 320 70 1.2 0.25 Normal Mode Sleep Mode TEMPERATURE RANGE Specified Performance –40 1.6 +85 µA µA mW mW °C NOTES: (1) Valid from AGND + 20mV to 2 • V REF. (2) Applies after calibration. (3) Recalibration can remove these errors. (4) Ideal output voltage. (5) Using external low-pass filter with 2kHz corner frequency. (6) See Command Register for description of CALPIN. ® DAC1221 2 PIN CONFIGURATION PIN DESCRIPTIONS PIN NAME DESCRIPTION 1 DVDD Digital Supply, +3V nominal 2 XOUT Digital, System Clock Output 3 XIN 4 DGND 5 AVDD 6 DNC 7 C3 Analog, Filter Capacitor 8 C2B Analog, Filter Capacitor 9 C1 Analog, Filter Capacitor 10 C2A Analog, Filter Capacitor 11 VOUT Analog Output Voltage C2A 12 VREF Analog, Reference Input C1 13 AGND 14 CS Top View SSOP DVDD 1 16 SCLK XOUT 2 15 SDIO XIN 3 14 CS DGND 4 13 AGND 12 VREF DAC1221E AVDD DNC C3 C2B 5 6 11 7 10 8 9 VOUT Digital, System Clock Input Digital Ground Analog Supply, +3V nominal Do Not Connect Analog Ground Digital, Chip Select Input 15 SDIO Digital, Serial Data Input/Output 16 SCLK Digital, Clock Input for Serial Data Transfer ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) AVDD to DVDD ................................................................................... ±0.3V AVDD to AGND ........................................................................ –0.3V to 4V DVDD to DGND ....................................................................... –0.3V to 4V AGND to DGND ............................................................................... ±0.3V VREF Voltage to AGND .......................................................... 1.0V to 1.5V Digital Input Voltage to DGND .............................. –0.3V to DVDD + 0.3V Digital Output Voltage to DGND ........................... –0.3V to DVDD + 0.3V Package Power Dissipation ............................................. (TJMAX – TA) /θJA Maximum Junction Temperature (TJMAX) ..................................... +150°C Thermal Resistance, θJA SSOP-16 ............................................................................... 200°C/W Lead Temperature (soldering, 10s) ............................................... +300°C This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. Exposure to absolute maximum conditions for extended periods may affect device reliability. PACKAGE/ORDERING INFORMATION PRODUCT PACKAGE PACKAGE DRAWING NUMBER DAC1221E " SSOP-16 " 322 " SPECIFIED TEMPERATURE RANGE PACKAGE MARKING ORDERING NUMBER(1) TRANSPORT MEDIA –40°C to +85°C " DAC1221E " DAC1221E DAC1221E/2K5 Rails Tape and Reel NOTE: (1) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “DAC1221E/2K5” will get a single 2500-piece Tape and Reel. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. ® 3 DAC1221 TYPICAL PERFORMANCE CURVES At TA = +25°C, AVDD = DVDD = +3.0V, fXIN = 2.5MHz, VREF = 1.25V, C1 = 2.2nF, C2 = 150pF and C3 = 6.8nF. FULL SCALE OUTPUT SWING POWER SUPPLY REJECTION RATIO vs FREQUENCY 70 3.0 60 2.5 2.0 Output (V) PSRR (dB) 50 40 30 1.5 1.0 20 0.5 10 0 0 10 100 1000 10000 0 100000 1 2 SETTLING TIME: 20mV to FS 4 SETTLING TIME: FS to 20mV 300 1500 0 1200 ∆ Around 20mV (µV) ∆ Around FS (µV) 3 Time (ms) Frequency (Hz) –300 –600 –900 900 600 300 –1200 0 –1500 –300 0 2 4 6 8 10 0 2 4 Time (ms) 6 8 10 Time (ms) OFFSET vs TEMPERATURE OUTPUT NOISE VOLTAGE vs FREQUENCY 4 10000 2 Offset (mV) Noise (nv/√Hz) 1000 100 10 0 (can be corrected with calibration) –2 –4 –6 1 10 100 1000 10000 –50 10000 0 25 50 Temperature (°C) Frequency (Hz) ® DAC1221 –25 4 75 100 TYPICAL PERFORMANCE CURVES At TA = +25°C, AVDD = DVDD = +3.0V, fXIN = 2.5MHz, VREF = 1.25V, C1 = 2.2nF, C2 = 150pF and C3 = 6.8nF. LINEARITY ERROR vs CODE GAIN ERROR vs TEMPERATURE 0.020 25 Linearity Error (ppm of FSR) 0.015 (can be corrected with calibration) Gain Error (%) 0.010 0.005 0.000 –0.005 –0.010 –0.015 20 15 10 5 0 –5 –50 –25 0 25 50 75 100 0 Temperature (°C) 0.2 0.4 0.6 0.8 1 16-Bit Input Code Normalized ® 5 DAC1221 THEORY OF OPERATION ANALOG OPERATION The DAC1221 is a precision, high dynamic range, selfcalibrating, 16-bit, delta-sigma digital-to-analog converter. It contains a second-order delta-sigma modulator, a firstorder switched-capacitor filter, a second-order continuoustime post filter, a microcontroller including the Instruction, Command and Calibration registers, a serial interface, and a clock generator circuit. The system clock is divided down to provide the sample clock for the modulator. The sample clock is used by the modulator to convert the multi-bit digital input into a 1-bit digital output stream. The use of a 1-bit DAC provides inherent linearity. The digital output stream is then converted into an analog signal via the 1-bit DAC and then filtered by the 1st-order switched-capacitor filter. The design topology provides low system noise and good power-supply rejection. The modulator frequency of the delta-sigma D/A converter is controlled by the system clock. The output of the switched-capacitor filter feeds into the continuous time filter. The continuous time filter uses external capacitors, C1 and C2, to adjust the settling time. The connections for capacitors are shown in Figure 1. C1 connects to VREF. C2 connects between the C2 pins. C3 is connected between C3 and VREF, and is used for calibration. The DAC1221 also includes complete onboard calibration that can correct for internal offset and gain errors. The calibration registers are fully readable and writable. This feature allows for system calibration. The various settings, modes, and registers of the DAC1221 are read or written via a synchronous serial interface. This interface operates as an externally clocked interface. DEFINITION OF TERMS Differential Nonlinearity Error—The differential nonlinearity error is the difference between an actual step width and the ideal value of 1 LSB. If the step width is exactly 1 LSB, the differential nonlinearity error is zero. A differential nonlinearity specification of less than 1 LSB guarantees monotonicity. DAC1221 VREF 12 11 7 C3 C2A 10 8 C2B C1 9 C1 2.2nF C3 6.8nF Drift—The drift is the change in a parameter over temperature. Full-Scale Range (FSR)—This is the magnitude of the typical analog output voltage range which is 2 • VREF. For example, when the converter is configured with a 1.25V reference, the full-scale range is 2.5V. C2 150pF NOTE: C1 and C2 should be NPO type capacitors. Gain Error—This error represents the difference in the slope between the actual and ideal transfer functions. Linearity Error—The linearity error is the deviation of the actual transfer function from an ideal straight line between the data end points. FIGURE 1. Capacitor Connections. Least Significant Bit (LSB) Weight—This is the ideal change in voltage that the analog output will change with a change in the digital input code of 1 LSB. CALIBRATION Offset Error—The offset error is the difference between the expected and actual output, when the output is zero. The value is calculated from measurements made when VOUT = 20mV. The DAC1221 offers a self-calibration mode which automatically calibrates the output offset and gain. The calibration is performed once and then normal operation is resumed. In general, calibration is recommended immediately after power-on and whenever there is a “significant” change in the operating environment. The amount of change which should cause re-calibration is dependent on the application. Where high accuracy is important, re-calibration should be done on changes in temperature and power supply. Settling Time—The settling time is the time it takes the output to settle to its new value after the digital code has been changed. After a calibration has been accomplished, the Offset Calibration Register (OCR) and the Full-Scale Calibration Register (FCR) contain the results of the calibration. fXIN—The frequency of the crystal oscillator or CMOScompatible input signal at the XIN input of the DAC1221. Note that the values in the calibration registers will vary from configuration to configuration and from part to part. Monotonicity—Monotonicity assures that the analog output will increase or stay the same for increasing digital input codes. ® DAC1221 6 Self Calibration A self-calibration is performed after the bits “01” have been written to the Command Register Operation Mode bits (MD1 and MD0). This initiates a self-calibration on the next clock cycle. The offset correction code is determined by a repeated sequence of auto-zeroing the calibration comparator to the offset reference and then comparing the DAC output to the offset reference value. The end result is the averaged, Offset Two’s Complement adjusted, and placed in the OCR. The gain correction is done in a similar fashion, except the correction is done against VREF to eliminate common-mode errors. The FCR result represents the gain code and is not Offset Two’s Complement adjusted. REFERENCE INPUT The reference input voltage of 1.25V can be directly connected to VREF pin. The recommended reference circuit for the DAC1221 is shown in Figure 2. DIGITAL OPERATION SYSTEM CONFIGURATION The DAC1221 is controlled by 8-bit instruction codes (INSR) and 16-bit command codes (CMR) via the serial interface, which is externally clocked. The calibration function takes between 300ms and 500ms (for fXIN = 2.5MHz) to complete. Once calibration is initiated, further writing of register bits is disabled until calibration completes. The status of calibration can be verified by reading the status of the Command Register Operation Mode bits (MD1 and MD0). These bits will return to normal mode “00” when calibration is complete. The DAC1221 Microcontroller (MC) consists of an ALU and a register bank. The MC has three states: power-on reset, calibration, and normal operation. In the power-on reset state, the MC resets all the registers to their default states. In the calibration state, the MC performs offset and gain self-calibration. In the normal state, the MC performs D/A conversions. It is recommended that the output be connected during calibration. The output isolation is controlled by the CALPIN bit in the CMR register. Setting the CALPIN bit will connect the output and clearing the bit will disconnect and isolate the output. Although it is recommended to connect the output during calibration, the load impedance should be such that the DAC1221 is not required to sink any current, but is able to source up to the specified maximum. The DAC1221 has five internal registers, as shown in Table I. Two of these, the Instruction Register (INSR) and the Command Register (CMR), control the operation of the converter. The Instruction Register utilizes an 8-bit instruction code to control the serial interface to determine whether the next operation is either a read or a write, to control the word length, and to select the appropriate register to read/write. Communication with the DAC1221 is controlled via the INSR. Under normal operation, the INSR is written as the first part of each serial communication. The instruction that is sent determines what type of communication will occur next. It is not possible to read the INSR. The Command Register has a 16-bit command code to set up the Output Mode The output of the DAC1221 can be synchronously reset. By setting the CLR bit in the CMR, the data input register is cleared to zero. This will result in an output of 0V when DF = 1, or VREF when DF = 0. The settling time is determined by the DISF and ADPT bits of the command register. The default state of DISF = 0 and ADPT = 0 enables fast settling, unless the output step is small (≈ 40mV). However, the DAC1221 can be forced to always use fast settling if the ADPT bit is set to 1. If DISF is set to 1, all fast settling is disabled. The CRST bit of the CMR can be used to reset the offset and calibration registers. By setting the CRST bit, the contents of the calibration registers are reset to 0. INSR Instruction Register 8 Bits DIR Data Input Register 16 Bits CMR Command Register 16 Bits OCR Offset Calibration Register 24 Bits FCR Full-Scale Calibration Register 24 Bits TABLE I. DAC1221 Registers. +3V +3V 0.10µF 7 87.6kΩ 2 6 1 20kΩ 3 + REF1004-1.2 10µF 0.10µF 100Ω To VREF Pin OPA336 + 10µF 0.1µF 4 FIGURE 2. Recommended External Voltage Reference Circuit for Best Low Noise Operation with the DAC1221. ® 7 DAC1221 DAC1221 operation mode, settling mode and data format. The Data Input Register (DIR) contains the value for the next conversion. The Offset and Full-Scale Calibration Registers (OCR and FCR) contain data used for correcting the internal conversion value after it is placed into the DIR. The data in these two registers may be the result of a calibration routine, or they may be values which have been written directly via the serial interface. or the write operation for the CMR register. If the next location is reserved in Table III, the results are unknown. Reading or writing continues until the number of bytes specified by MB1 and MB0 have been transferred. INSTRUCTION REGISTER (INSR) Each serial communication starts with the 8 bits of INSR being sent to the DAC1221. The read/write bit, the number of bytes (n), and the starting register address are defined in Table II. When the n bytes have been transferred, the instruction is complete. A new communication cycle is initiated by sending a new INSR (under restrictions outlined in the Interfacing section). MSB R/W LSB MB1 MB0 0 A3 A3 A1 A0 NOTE: INSR is a write-only register with the MSB (Most Significant Byte and Bit) written first, independent of the BD bit. A3 A2 A1 A0 0 0 0 0 Data Input Register Byte 1 MSB 0 0 0 1 Data Input Register Byte 0 LSB 0 0 1 0 Reserved 0 0 1 1 Reserved 0 1 0 0 Command Register Byte 1 MSB 0 1 0 1 Command Register Byte 0 LSB 0 1 1 0 Reserved 0 1 1 1 Reserved 1 0 0 0 Offset Cal Register Byte 2 MSB 1 0 0 1 Offset Cal Register Byte 1 1 0 1 0 Offset Cal Register Byte 0 LSB 1 0 1 1 Reserved 1 1 0 0 Full-Scale Cal Register Byte 2 MSB 1 1 0 1 Full-Scale Cal Register Byte 1 1 1 1 0 Full-Scale Cal Register Byte 0 LSB 1 1 1 1 Reserved TABLE III. A3 - A0 Addressing. TABLE II. Instruction Register. COMMAND REGISTER (CMR) The CMR controls all of the functionality of the DAC1221. The new configuration is latched in on the negative transition of SCLK for the last bit of the last byte of data being written to the command register. The organization of the CMR is comprised of 16 bits of information in 2 bytes of 8 bits each. R/W (Read/Write) Bit—For a write operation to occur, this bit of the INSR must be 0. For a read, this bit must be 1, as shown: R/W 0 Write 1 Read MSB Byte 1 ADPT CALPIN 1 0 CLR DF 0 1 0 CRST 0 BD MSB MD1 MD0 Byte 0 MB1, MB0 (Multiple Bytes) Bits—These two bits are used to control the word length (number of bytes) of the read or write operation, as shown: MB1 MB0 0 0 1 Byte 0 1 2 Bytes 1 0 3 Bytes TABLE IV. Command Register. ADPT (Adaptive Filter Disable) Bit—The ADPT bit determines if the adaptive filter is enabled or disabled. When the Adaptive Filter is enabled, the DAC1221 does fast settling only when there is an output step of larger than ≈ 40mV. For small changes in the data, fast settling is not necessary. When ADPT = 1, the Adaptive Filter is disabled and the DAC1221 will not look at the size of a step to determine the necessity of using fast settling. In either case, fast settling can be defeated if DISF = 1. A3 – A0 (Address) Bits—These four bits select the beginning register location that will be read from or written to, as shown in Table III. Each subsequent byte will be read from or written to the next higher location (increment address). If the BD bit in the Command register is set, each subsequent byte will be read from or written to the next lower location (decrement address). This bit does not affect INSR register ADPT ® DAC1221 LSB DISF 8 0 Enabled (default) 1 Disabled CALPIN (Calibration Pin) Bit—The CALPIN bit determines if the output is isolated or connected during calibration. Care must be observed in reading the Command Register if the state of the BD bit is unknown. If a two byte read is started at address 0100 with BD = 0, it will read 0100, then 0101. However, if BD = 1, it will read 0100, then 0011. If the BD bit is unknown, all reads of the command register are best performed as read commands of one byte. CALPIN 0 Output Isolated (default) 1 Output Connected MSB (Bit Order) Bit—The MSB bit controls the order in which bits within a byte of data are read or written (either most significant bit first or least significant bit first), as follows: CRST (Calibration Reset) Bit—The CRST bit resets the offset and full-scale calibration registers, as shown: MSB CRST 0 OFF (default) 1 Reset DF (Data Format) Bit—The DF bit controls the format of the input data, shown in hexadecimal (either Offset Two’s Complement or Straight Binary), as shown: DF = 0 DF = 1 MD0 0 0 1 1 0 1 0 1 8000 0000 0 0000 8000 VREF 7FFF FFFF 2 • VREF Offset Calibration Register (OCR) The OCR is a 24-bit register containing the offset correction factor that is used to apply a correction to the digital input before it is transferred to the modulator. The results of the self-calibration process will be written to this register. The OCR is both readable and writable via the serial interface. For applications requiring a more accurate calibration, a calibration can be performed, the results averaged, and a more precise offset calibration value written back to the OCR. DISF (Disable Fast Settling) Bit—The DISF bit disables the fast settling option. If this bit is zero the fast settling performance is determined by the ADPT bit. DISF 0 Fast Settling (default) 1 Disable Fast Settling The actual OCR value will change from part to part and with configuration, temperature, and power supply. In addition, be aware that the contents of the OCR are not used to directly correct the digital input. Rather, the correction is a function of the OCR value. This function is linear and two known points can be used as a basis for interpolating intermediate values for the OCR. BD (Byte Order) Bit—The BD bit controls the order in which bytes of data are transferred (either most significant byte first (MSBF) or least significant byte first (LSBF)), as shown: 0 (default) register 1 0 (default) read 1 write only MSBF MSBF CMR MSBF LSBF MSBF MSBF DIR MSBF LSBF MSBF LSBF FCR MSBF MSB OCR23 write only MSBF The results of calibration are averaged, Offset Two's Complement adjusted, and placed in the OCR. write INSR OCR LSBF LSBF Normal Mode Self-Cal Sleep (default) Reserved VOUT (default) BD bit: LSB First MD1 Input Code Straight Binary MSB First (default) 1 MD1 – MD0 (Operating Mode) Bits—The Operating Mode bits control the calibration functions of the DAC1221. The Normal Mode is used to perform conversions. The SelfCalibration Mode is a one-step calibration sequence that calibrates both the offset and full scale. CLR (Clear) Bit—The CLR bit synchronously resets the data input register to zero. The analog output will be based on the DF bit—if 1, the output will be 0V; if 0, the output will be VREF. Offset Two's Complement 0 MSBF MSBF Byte 2 OCR22 OCR21 OCR20 OCR19 OCR18 OCR17 OCR16 OCR11 OCR10 OCR9 OCR8 OCR3 OCR2 OCR1 OCR0 Byte 1 OCR15 OCR14 OCR13 OCR12 Byte 0 LSBF OCR7 LSBF OCR6 OCR5 OCR4 LSB TABLE V. Offset Calibration Register. ® 9 DAC1221 Full-Scale Calibration Register (FCR) The FCR is a 24-bit register which contains the full-scale correction factor that is applied to the digital input before it is transferred to the modulator. The contents of this register will be the result of a self-calibration, or written to by the user. SLEEP MODE The Sleep Mode is entered after the bit combination 10 has been written to the CMR Operation Mode bits (MD1 and MD0). This mode ends when these bits are changed to a value other than 10. Communication with the DAC1221 can continue during Sleep Mode. When a new mode (other than Sleep) has been entered, the DAC1221 will execute a very brief internal power-up sequence of the analog and digital circuitry. In addition, the settling of the external VREF and other circuitry must be taken into account to determine the amount of time required to resume normal operation. The FCR is both readable and writable via the serial interface. For applications requiring a more accurate calibration, a calibration can be performed, the results averaged, and a more precise value written back to the FCR. The actual FCR value will change from part to part and with configuration, temperature, and power supply. Once serial communication is resumed, the Sleep Mode is exited by changing the MD1 - MD0 bits to any other mode. When a new mode (other than Sleep) has been entered, the DAC1221 will execute a very brief internal power-up sequence of the analog and digital circuitry. In addition, the settling of the external VREF and other circuitry must be taken into account to determine the amount of time required to resume normal operation. In addition, be aware that the contents of the FCR are not used to directly correct the digital input. Rather, the correction is a function of the FCR value. This function is linear and two known points can be used as a basis of interpolating intermediate values for the FCR. The contents of the FCR are in unsigned binary format. This is not affected by the DF bit in the Command Register. MSB Byte 2 FCR23 FCR22 FCR21 FCR20 FCR15 FCR14 FCR13 FCR12 FCR7 FCR6 FCR5 FCR19 FCR18 FCR17 FCR16 FCR11 FCR10 FCR9 FCR8 FCR3 FCR2 FCR1 FCR0 SERIAL INTERFACE Byte 1 Byte 0 FCR4 The DAC1221 includes a flexible serial interface which can be connected to microcontrollers and digital signal processors in a variety of ways. Along with this flexibility, there is also a good deal of complexity. This section describes the trade-offs between the different types of interfacing methods in a top-down approach—starting with the overall flow and control of serial data, moving to specific interface examples, and then providing information on various issues related to the serial interface. LSB TABLE VI. Full-Scale Calibration Register. Data Input Register (DIR) The DIR is a 16-bit register which contains the digital input value (see Table VII). The register is latched on the falling edge of the last bit of the last byte sent. The contents of the DIR are then loaded into the modulator. This means that the DIR register can be updated after sending 1 or 2 bytes, which is determined by the MB1 and MB0 bits in the Instruction Register. The contents of the DIR can be Offset Two’s Complement or Straight Binary. MSB DIR15 If this requirement cannot be met or if the circuit has brownout considerations, the timing diagram of Figure 3 can be used to reset the DAC1221. This accomplishes the reset by controlling the duty cycle of the SCLK input. Byte 1 DIR14 DIR13 DIR12 DIR11 DIR10 DIR9 Byte 0 DIR7 Reset, Power-On Reset and Brown-Out The DAC1221 contains an internal power-on reset circuit. If the power supply ramp rate is greater than 50mV/ms, this circuit will be adequate to ensure the device powers up correctly. Due to oscillator settling considerations, communication to and from the DAC1221 should not occur for at least 25ms after power is stable. DIR6 DIR5 DIR4 DIR8 LSB DIR3 DIR2 DIR1 DIR0 Sleep mode is the default state after power on or reset. The output is high impedance during sleep mode. TABLE VII. Data Input Register. ® DAC1221 10 I/O Recovery If serial communication stops during an instruction or data transfer for longer than 100ms (for fXIN = 2.5MHz), the DAC1221 will reset its serial interface. This will not affect the internal registers. The main controller must not continue the transfer after this event, but must restart the transfer from the beginning. This feature is very useful if the main controller can be reset at any point. After reset, simply wait 200ms (for fXIN = 2.5MHz) before starting serial communication. condition. They only become active when serial data is being transmitted from the DAC1221. If the DAC1221 is in the middle of a serial transfer and the SDIO is an output, taking CS HIGH will not tri-state the output signal. If there are multiple serial peripherals utilizing the same serial I/O lines and communication may occur with any peripheral at any time, the CS signal must be used. The CS signal is then used to enable communication with the DAC1221. Isolation The serial interface of the DAC1221 provides for simple isolation methods. An example of an isolated two-wire interface is shown in Figure 4. TIMING The maximum serial clock frequency cannot exceed the DAC1221 XIN frequency divided by 10. Table VIII and Figures 5 through 9 define the basic digital timing characteristics of the DAC1221. Figure 5 and the associated timing symbols apply to the XIN input signal. Figures 6 through 9 and associated timing symbols apply to the serial interface signals (SCLK, SDIO, and CS). The serial interface is discussed in detail in the Serial Interface section. Using CS The serial interface may make use of the CS signal, or this input may simply be tied LOW. There are several issues associated with choosing to do one or the other. The CS signal does not directly control the tri-state condition of the SDIO output. These signals are normally in the tri-state t1: > 512 • tXIN < 800 • tXIN Reset On Falling Edge t2 t2: > 10 • tXIN t2 t3: > 1024 • tXIN < 1800 • tXIN SCLK t1 t3 t4: ≥ 2048 • tXIN < 2400 • tXIN t4 FIGURE 3. Resetting the DAC1221. Isolated Power DVDD DAC1221 C1X 5.6pF 1 DVDD SCLK 16 2 XOUT SDIO 15 3 XIN CS 14 4 DGND AGND 13 5 AVDD VREF 12 6 DNC VOUT 11 XTAL C2X 5.6pF AVDD Opto Coupler P1.1 Opto Coupler P1.0 8051 VREF = Isolated C1 7 C3 C2A 10 8 C2B C1 9 C3 C2 = DGND = AGND FIGURE 4. Isolation for Two-Wire Interface. ® 11 DAC1221 SYMBOL DESCRIPTION MIN MAX UNITS fXIN XIN Clock Frequency 1 NOM 2.5 MHz tXIN XIN Clock Period 400 1000 t1 XIN Clock High 0.4 • tXIN ns t2 XIN Clock LOW 0.4 • tXIN ns t3 SCLK HIGH 5 • tXIN ns t4 SCLK LOW 5 • tXIN ns t5 Data In Valid to SCLK Falling Edge (Setup) 40 ns t6 SCLK Falling Edge to Data In Not Valid (Hold) 20 ns t7 Data Out Valid After Rising Edge of SCLK (Hold) 0 t8 SCLK Rising Edge to New Data Out Valid (Delay)(1) t9 Falling Edge of Last SCLK for INSR to Rising Edge of First SCLK for Register Data 13 • tXIN t10 Falling Edge of CS to Rising Edge of SCLK 11 • tXIN t11 Falling Edge of Last SCLK for INSR to SDIO as Output 8 • tXIN t12 SDIO as Output to Rising Edge of First SCLK for Register Data t13 Falling Edge of Last SCLK for Register Data to SDIO Tri-State 4 • tXIN t14 Falling Edge of Last SCLK for Register Data to Rising Edge of First SCLK of next INSR (CS Tied LOW) 41 • tXIN ns t15 Rising Edge of CS to Falling Edge of CS (Using CS) 22 • tXIN ns ns ns 50 ns ns ns ns 10 • tXIN ns 4 • tXIN ns 6 • tXIN ns NOTE: (1) With 10pF load. TABLE VIII. Digital Timing Characteristics. t3 t4 tXIN t1 t5 SCLK t2 t6 XIN t7 SDIO t8 FIGURE 5. XIN Clock Timing. FIGURE 6. Serial Input/Output Timing. t9 t14 SCLK SDIO IN7 IN1 IN0 INM IN1 IN0 IN7 OUT1 OUT0 IN7 Write Register Data SDIO IN7 IN1 IN0 OUTM Read Register Data FIGURE 7. Serial Interface Timing (CS always LOW). t15 CS t10 t10 t9 SCLK IN7 SDIO IN1 IN0 INM IN1 IN0 IN7 OUTM OUT1 OUT0 IN7 Write Register Data IN7 SDIO IN1 IN0 Read Register Data FIGURE 8. Serial Interface Timing (using CS). ® DAC1221 12 CS SCLK t11 t10 t12 t13 IN7 SDIO IN0 OUT MSB OUT0 t9 SDIO is an input SDIO is an output FIGURE 9. SDIO Input to Output Transition Timing. From Read flowchart To Write flowchart Start Writing Start Reading CS taken HIGH for t15 periods minimum (or CS tied LOW) CS taken HIGH for t15 periods minimum (or CS tied LOW) CS state CS state HIGH LOW CS state External device generates 8 serial clock cycles and transmits instruction register data via SDIO LOW HIGH No End CS state External device generates 8 serial clock cycles and transmits instruction register data via SDIO LOW External device generates n serial clock cycles and transmits specified register data via SDIO More instructions? HIGH HIGH LOW SDIO input to output transition External device generates n serial clock cycles and receives specified register data via SDIO Yes Is next instruction a read? No SDIO transitions to tri-state condition Yes To Read flowchart More instructions? No End Yes Is next instruction a Write? No Yes To Write flowchart FIGURE 10. Flowchart for Writing and Reading Register Data. ® 13 DAC1221 LAYOUT GROUNDING The analog and digital sections of the design should be carefully and cleanly partitioned. Each section should have its own ground plane with no overlap between them. AGND should be connected to the analog ground plane, as well as all other analog grounds. DGND should be connected to the digital ground plane, and all digital signals referenced to this plane. POWER SUPPLIES The DAC1221 requires the digital supply (DVDD) to be no greater than the analog supply (AVDD) +0.3V. In the majority of systems, this means that the analog supply must come up first, followed by the digital supply and VREF. Failure to observe this condition could cause permanent damage to the DAC1221. The DAC1221 pinout is such that the converter is cleanly separated into an analog and digital portion. This should allow simple layout of the analog and digital sections of the design. Inputs to the DAC1221, such as SDIO or VREF, should not be present before the analog and digital supplies are on. Violating this condition could cause latch-up. If these signals are present before the supplies are on, series resistors should be used to limit the input current. For a single converter system, AGND and DGND of the DAC1221 should be connected together, underneath the converter. Do not join the ground planes. Instead, connect the two with a moderate signal trace. For multiple converters, connect the two ground planes at one location, as central to all of the converters as possible. In some cases, experimentation may be required to find the best point to connect the two planes together. The printed circuit board can be designed to provide different analog/digital ground connections via short jumpers. The initial prototype can be used to establish which connection works best. The best scheme is to power the analog section of the design and AVDD of the DAC1221 from one +3V supply, and the digital section (and DVDD) from a separate +3V supply. The analog supply should come up first. This will ensure that SCLK, SDIO, CS and VREF do not exceed AVDD, that the digital inputs are present only after AVDD has been established, and that they do not exceed DVDD. The analog supply should be well regulated and low noise. For designs requiring very high resolution from the DAC1221, power supply rejection will be a concern. See the “PSRR vs Frequency” curve in the Typical Performance Curves section of this data sheet for more information. DECOUPLING Good decoupling practices should be used for the DAC1221 and for all components in the design. All decoupling capacitors, and specifically the 0.1µF ceramic capacitors, should be placed as close as possible to the pin being decoupled. A 1µF to 10µF capacitor, in parallel with a 0.1µF ceramic capacitor, should be used to decouple AVDD to AGND. At a minimum, a 0.1µF ceramic capacitor should be used to decouple DVDD to DGND, as well as for the digital supply on each digital component. The requirements for the digital supply are not as strict. However, high frequency noise on DVDD can capacitively couple into the analog portion of the DAC1221. This noise can originate from switching power supplies, very fast microprocessors, or digital signal processors. If one supply must be used to power the DAC1221, the AVDD supply should be used to power DVDD. This connection can be made via a 10Ω resistor which, along with the decoupling capacitors, will provide some filtering between DVDD and AVDD. In some systems, a direct connection can be made. Experimentation may be the best way to determine the appropriate connection between AVDD and DVDD. ® DAC1221 14