ADC12081CIVT

ADC12081 12-Bit, 5 MHz Self-Calibrating, Pipelined A/D Converter with Internal Sample & Hold November 2002 ADC12081 12-Bit, 5 MHz Self-Calibrating, Pipelined A/D Converter with Internal Sample & Hold General Description The ADC12081 is a monolithic CMOS analog-to-digital converter capable of converting analog input signals into 12-bit digital words at 5 megasamples per second (MSPS). The ADC12081 utilizes an innovative pipeline architecture to minimize die size and power consumption. The ADC12081 uses self-calibration and error correction to maintain accuracy and performance over temperature. The ADC12081 converter operates on a 5V power supply and can digitize analog input signals in the range of 0 to 2V. A single convert clock controls the conversion operation. All digital I/O is TTL compatible. The ADC12081 is designed to minimize external components necessary for the analog input interface. An internal sample-and-hold circuit samples the analog input and an internal amplifier buffers the reference voltage input. The ADC12081 is available in the 32-lead LQFP package and is designed to operate over the extended commercial temperature range of -40˚C to +85˚C. n Internal Sample-and-hold n Internal Reference buffer amplifier n Low power consumption Key Specifications n n n n n n n n Resolution Conversion Rate DNL SNR ENOB Analog Input Range Supply Voltage Power Consumption, 5 MHz 12 Bits 5 Msps (min) ± 0.35 LSB (typ) 68 dB (typ) 10.9 Bits (typ) 2 Vpp (min) +5V ± 5% 105 mW (typ) Applications n n n n n Image processing front end PC-based data acquisition Scanners Fax machines Waveform digitizer Features n Single 5V power supply n Simple analog input interface Connection Diagram 10015001 © 2002 National Semiconductor Corporation DS100150 www.national.com ADC12081 Ordering Information Industrial (−40˚C ≤ TA ≤ +85˚C) ADC12081CIVT ADC12181 EVAL Package 32 pin LQFP Evaluation Board Simplified Block Diagram 10015002 www.national.com 2 ADC12081 Pin Descriptions and Equivalent Circuits #2 No. 2 Symbol VIN Equivalent Circuit Description Analog signal input. With a 2.0V reference voltage, input signal voltages in the range of 0 to 2.0 Volts will be converted. See section 1.2. Reference voltage input. This pin should be driven from an accurate, stable reference source in the range of 1.8 to 2.2V and bypassed to a low-noise analog ground with a monolithic ceramic capacitor, nominally 0.01µF. See section 1.1. Positive reference bypass pin. Bypass with a 0.1µF capacitor. Do not connect anything else to this pin. See section 3.1 1 VREF 32 VRP 31 VRM Reference midpoint bypass pin. Bypass with a 0.1µF capacitor. Do not connect anything else to this pin. See section 3.1 30 VRN Negative reverence bypass pin. Bypass with a 0.1µF capacitor. Do not connect anything else to this pin. See section 3.1 Sample Clock input, TTL compatible. Maximum amplitude should not exceed 3V. Calibration request, active High. Calibration cycle starts when CAL returns to logic low. CAL is ignored during power-down mode. See section 2.2. Power-down, active High, ignored during calibration cycle. See paragraph 2.4 Output enable control, active low. When this pin is high the data outputs are in Tri-state (high-impedance) mode. 10 CLOCK 8 CAL 7 PD 11 OE 28 OR Over range indicator. This pin is at a logic High for VIN < 0 or for VIN > VREF. 29 READY Device ready indicator, active High. This pin is at a logic Low during a calibration cycle and while the device is in the power down mode. 3 www.national.com ADC12081 Pin Descriptions and Equivalent Circuits #2 No. Symbol Equivalent Circuit (Continued) Description 14-19, 22-27 D0 - D11 Digital output word, CMOS compatible. D0 (pin 14) is LSB, D11 (pin 27) is MSB. Load with no more than 50pF. 3 VIN com Analog input common. Connect to a quiet point in analog ground near the driving device. See section 1.2. Positive analog supply pin. Connect to a clean, quiet voltage source of +5V. VA and VD should have a common supply and be separately bypassed with a 5µF to 10µF capacitor and a 0.1µF chip capacitor. The ground return for the analog supply. AGND and DGND should be connected together close to the ADC12081 package. See section 5.0. Positive analog supply pin. Connect to a clean, quiet voltage source of +5V. VA and VD should have a common supply and be separately bypassed with a 5µF to 10µF capacitor and a 0.1 µF chip capacitor. The ground return for the analog supply. AGND and DGND should be connected together close to the ADC12081 package. See section 5.0 The digital output driver supply pin. This pin can be operated from a supply voltage of 3V to 5V, but the voltage on this pin should never exceed the VD supply pin voltage. The ground return for the output drivers. This pin should be returned to a point in the digital ground that is removed from the other ground pins of the ADC12081. 5 VA 4, 6 AGND 13 VD 9, 12 DGND 21 VD I/O 20 DGND I/O www.national.com 4 ADC12081 Absolute Maximum Ratings 2) (Notes 1, Storage Temp. Maximum Junction Temp. −65˚C to +150˚C 150˚C If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Voltage on Any Output Input Current at Any Pin (Note 3) Package Input Current (Note 3) Package Dissipation ESD Susceptibility Human Body Model Machine Model Soldering Temp., Infrared, 10 sec.(Note 6) 1500V 150V 300˚C 6.5V −0.3V to V+ +0.3V Operating Ratings Operating Temp. Range Supply Voltage VD I/O VREF Input CLOCK, CAL, PD, OE |AGND −DGND| −40˚C ≤ TA ≤ +85˚C +4.75V to +5.25V +2.7V to VD 1.8V to 2.2V −0.05V to VD + 0.05V ≤100mV ± 25mA ± 50mA See (Note 4) Converter Electrical Characteristics The following specifications apply for AGND = DGND = DGND I/O = 0V, VA = VD = VD I/O = +5V, PD = +5V, VREF = +2.0V, fCLK = 5MHz, CL = 50 pF/pin. After Auto-Cal at Temperature. Boldface limits apply for TA = TJ to TMIN to TMAX: all other limits TA = TJ = 25˚C (Notes 7, 8) and (Note 9) Symbol Parameter Conditions Typical (Note 10) Limits (Note 11) 12 Units (Limits) Static Converter Characteristics Resolution with No Missing Codes INL DNL Integral Non Linearity Differential Non Linearity Full-Scale Error Zero Error Dynamic Converter Characteristics BW SNR SINAD ENOB THD SFDR Full Power Bandwidth Signal-to-Noise Ratio Signal-to-Noise & Distortion Effective Number of Bits Total Hamonic Distortion Spurious Free Dynamic Range fin = 2.5 MHz, VIN = 2.0VP-P fin = 2.5 MHz, VIN = 2.0VP-P fin = 2.5 MHz, VIN = 2.0VP-P fin = 2.5 MHz, VIN = 2.0VP-P fin = 2.5 MHz, VIN = 2.0VP-P 100 68 67.6 10.9 79 79 0 VREF (CLK LOW) (CLK HIGH) 10 15 2.00 10 1 1.8 2.2 65 64.5 10.4 MHz dB dB Bits dB dB V(min) V(max) pF pF V(min) V(max) µA MΩ(min) Bits(min) LSB( max) LSB( max) %FS(max) %FS(max) ± 0.6 ± 0.35 ± 0.05 ± 0.15 ± 1.7 ± 0.75 ± 0.1 ± 0.24 Reference and Analog Input Characteristics VIN CIN VREF Input Voltage Range VIN Input Capacitance Reference Voltage (Note 14) Reference Input Leakage Current Reference Input Resistance VREF = 2.0V VIN = 1.0Vdc + 0.7Vrms 5 www.national.com ADC12081 DC and Logic Electrical Characteristics The following specifications apply for AGND = DGND = DGND I/O = 0V, VA = VD = VD I/O = +5V, PD = +5V, VREF = +2.0V, fCLK = 50MHz, CL = 50 pF/pin. After Auto-Cal at Temperature. Boldface limits apply for TA = TMIN to TMAX; all other limits TA = TJ = 25˚C (Note 7) (Note 8) and (Note 9) Symbol Parameter Conditions Typical (Note 10) Limits (Note 11) 2.0 0.8 5 −5 8 IOUT = −1mA IOUT = 1.6mA VOUT = 3V or 5V VOUT = 0V +ISC −ISC Output Short Circuit Source Current Output Short Circuit Sink Current VDDO= 3V, VOUT = 0V VDDO= 3V, VOUT = VO PD = VDDO PD = DGND PD = VDDO PD = DGND PD = VDDO PD = DGND 10 −10 −14 16 2.5 20 0.5 1 15 105 4 26 2 2 30 140 4 0.4 Units (Limits) CLK, OE Digital Input Characteristics VIH VIL IIH IIL CIN VOH VOL IOZ Logical "1" Input Voltage Logical "0" Input Voltage Logical "1" Input Current Logical "0" Input Current VIN Input Capacitance Logical "1" Output Voltage Logical "0" Output Voltage TRI-STATE ® Output Current V+ = 5.25V V+ = 4.75V VIN = 5.0V VIN = 0V V(min) V(min) µA µA pF V (min) V (max) µA µA mA(min) mA(min) mA(max) mA(max) mA(max) mA(max) mW(max) mW(max) D0 - D11 Digital Output Characteristics Power Supply Characteristics IA ID Analog Supply Current Digital Supply Current Total Power Consumption AC Electrical Characteristics The following specifications apply for AGND = DGND = DGND I/O = 0V, VA = VD = VD I/O = +5V, PD = +5V, VREF = +2.0V, fCLK = 5 MHz, CL = 50 pF/pin. After Auto-Cal at Temperature. Boldface limits apply for TA = TMIN to TMAX; all other limits TA = TJ = 25˚C (Note 7) (Note 8) and (Note 10) Symbol Parameter Conditions Typical (Note 10) 0.5 5 50 10.25 3.5 VD I/O = 3V VD I/O = 5V 44 40 21 21 3 3 4000 3 3 4000 Limits (Note 11) Units (Limits) MHz(min) MHz(max) % Clock Cycles ns ns nA (max) ns (max) Tclk(min) Tclk Tclk Tclk(min) Tclk Tclk fCLK Clock Frequency Clock Duty Cycle tCONV tAD tOD tDIS tEN tWCAL tRDYC tCAL tWPD tRDYPD tPD Conversion Latency Aperture Delay Time Data output delay after rising clk edge Data outputs into Tristate mode Data outputs active after Tristate Calibration request pulse width Ready Low after CAL request Calibration cycle Power-down pulse width Ready Low after PD request Power down mode exit cycle www.national.com 6 ADC12081 AC Electrical Characteristics (Continued) 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. Note 2: All voltages are measured with respect to GND = AGND = DGND = 0V, unless otherwise specified. Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND, or VIN > VA, VD or VD I/O), the current at that pin should be limited to 25 mA. The 50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 25 mA to two. Note 4: The absolute maximum junction temperatures (TJmax) for this device is 150˚C. The maximum allowable power consumption is dictated by TJmax, the junction-to-ambient thermal resistance (θJA), and the ambient temperature, (TA), and can be calculated using the formula PDMAX = (TJmax - TA )/θJA. In the 32-pin TQFP, θJA is 74˚C/W, so PDMAX = 1,689 mW at 25˚C and 1,013 mW at the maximum operating ambient temperature of 75˚C. Note that the power consumption of this device under normal operation will typically be about 125 mW (typical power consumption + 20 mW TTL output loading). The values for maximum power consumption listed above will be reached only when the ADC12081 is operated in a severe fault condition (e.g. when input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided. Note 5: Human body model is 100 pF capacitor discharged through a 1.5kΩ resistor. Machine model is 220 pf discharged through ZERO Ohms. Note 6: See AN450, "Surface Mounting Methods and Their Effect on Product Reliability", or the section entitled "Surface Mount" found in any post 1986 National Semiconductor Linear Data Book, for other methods of soldering surface mount devices. Note 7: The inputs are protected as shown below. Input voltage magnitudes up to 5V above VA or to 5V below GND will not damage this device, provided current is limited per Note 3. However, errors in the A/D conversion can occur if the input goes above VA or below GND by more than 100 mV. As an example, if VA is 4.75V, the full-scale input voltage must be ≤4.85V to ensure accurate conversions. 10015008 Note 8: To guarantee accuracy, it is required that |VA - VD| ≤ 100mV and separate bypassed capacitors are used at each power supply pin. Note 9: With the test condition for VREF = +2.0V, the 12-bit LSB is 488µV. Note 10: Typical figures are at TA = TJ = 25˚C, and represent most likely parametric norms. Note 11: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 12: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive full-scall and zero. Note 13: Timing specifications are tested at the TTL logic levels, VIL = 0.4V for a falling edge and VIH = 2.4V for a rising edge. TRI-STATE output voltage is forced to 1.4V. Note 14: Optimum SNR performance will be obtained by keeping the reference input in the 1.8V to 2.2V range. The LM4041CIM3-ADJ (SOT-23 package), the LM4041CIZ-ADJ (TO-92 package), or the LM4041CIM-ADJ (SOT-8 package) bandgap voltage reference is recommended for this application. 7 www.national.com ADC12081 10015009 FIGURE 1. Transfer Characteristic 10015010 FIGURE 2. Errors Minimized by the Auto-Cal Cycle Typical Performance Characteristics INL vs Temperature DNL vs Temperature 10015011 10015012 www.national.com 8 ADC12081 Typical Performance Characteristics SNR vs Temperature (Continued) SINAD vs Temperature 10015013 10015014 THD vs Temperature 10015015 9 www.national.com ADC12081 Specification Definitions APERTURE JITTER is the variation in aperture delay from sample to sample. Aperture jitter shows up as input noise. APERTURE DELAY See Sampling Delay. CLOCK DUTY CYCLE is the ratio of the time that the clock waveform is high to the total time for one clock cycle. DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB. EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE BITS) is another method of specifying Signal-to-Noise and Distortion Ratio, or SINAD. ENOB is defined as (SINAD 1.76) / 6.02 and says that the converter is equivalent to a perfect ADC of this (ENOB) number of bits. FULL POWER BANDWIDTH is the frequency at which the reconstructed output fundamental drops 3 dB below its low frequency value for a full scale input. FULL SCALE ERROR is the difference between the input voltage just causing a transition to positive full scale and VREF -1.5 LSB. INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from negative full scale (1⁄2 LSB below the first code transition) through positive full scale (11⁄2 LSB above the last code transition). The deviation of any given code from this straight line is measured from the center of that code value. The end point test method is used. INL is commonly measured at rated clock frequency with a ramp input. INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the power in the intermodulation products to the total power in the original frequencies. IMD is usually expressed in dB. PIPELINE DELAY (LATENCY) is the number of clock cycles between initiation of conversion and the availability of that conversion result at the output. New data is available at every clock cycle, but the data lags the conversion by the pipeline delay plus the Output Delay. SAMPLING (APERTURE) DELAY is the time after the edge of the clock to when the input signal is acquired or held for conversion. SIGNAL TO NOISE PLUS DISTORTION (S/(N+D) or SINAD) is the ratio expressed in dB, of the rms value of the input signal to the rms value of all of the other spectral components below half the clock frequency, including harmonics but excluding dc. SIGNAL TO NOISE RATIO (SNR) is the ratio of the rms value of the input signal to the rms value of the other spectral components below one-half the sampling frequency, not including harmonics or dc. SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the input signal and the peak spurious signal, where a spurious signal is any signal present in the output spectrum that is not present at the input. TOTAL HARMONIC DISTORTION (THD) is the ratio of the rms total of the first six harmonic components, to the rms value of the input signal. ZERO SCALE OFFSET ERROR is the difference between the ideal input voltage (1⁄2 LSB) and the actual input voltage that just causes a transition from an output code of zero to an output code of one. ZERO ERROR see Zero Scale Offset Error. www.national.com 10 ADC12081 Timing Diagrams 10015021 FIGURE 3. Data Output Timing 11 www.national.com ADC12081 Timing Diagrams (Continued) 10015022 FIGURE 4. Reset and Calibration Timing Functional Description The ADC12081 is a monolithic CMOS analog-to-digital converter capable of converting analog input signals into 12-bit digital words at 5 megasamples per second (MSPS). This device utilizes a proprietary pipeline architecture and algorithm to minimize die size and power consumption. The ADC12081 uses self-calibration and digital error correction to maintain accuracy and performance over temperature. The ADC12081 has an input sample-and-hold amplifier and internal reference buffer. The analog input and the reference voltage are converted to differential signals for internal use. Using differential signals in the analog conversion core reduces crosstalk and noise pickup from the digital section and power supply. The pipeline conversion core has 15 sequential signal processing stages. Each stage receives an analog signal from the previous stage (called “residue” ) and produces a 1-bit digital output that is sent to the digital correction module. At each stage the analog signal received from the previous stage is compared to an internally generated reference level. It is then amplified by a factor of 2, and, depending on the output of the comparator, the internal reference signal may be subtracted from the amplifier output. This produces the residue that is passed to the next stage. The calibration module is activated at power-on or by user request. During calibration the conversion core is put into a special mode of operation in order to determine inherent errors in the analog conversion blocks and to determine correction coefficients for each digital output bit from the conversion core and stores these coefficients in RAM. The digital correction module uses the coefficients in RAM to convert the raw data bits from the conversion core into the 12-bit digital output code. www.national.com 12 Applications Information 1.0 ANALOG INPUTS The ADC12081 has two single-ended analog inputs. VREF is the reference input and VIN is the signal input. 1.1 Reference Input The VREF input must be driven from an accurate, stable reference voltage source. of 1.8V to 2.2V, and bypassed to a clean, quiet point in analog ground. 1.2 Analog Signal Input The VIN input must be driven with a low impedance signal source that does not add any distortion to the input signal. The ground reference for the VIN input is the VINCOM pin. The VINCOM pin must be connected to a clean, quiet point in analog ground. 2.0 DIGITAL INPUTS The ADC12081 has four digital inputs. They are CLOCK, CAL, OE and PD. 2.1 CLOCK The CLOCK signal drives an internal phase delay loop to create timing for the ADC. The clock input should be driven with a stable, low phase jitter TTL level clock signal in the range of 0.5 to 5 MHz. The trace carrying the clock signal should be as short as possible. This trace should not cross any other signal line, analog or digital, not even at 90˚. A 100 Ohm resistor should be placed in series with the CLOCK pin, as close to the pin as possible. ADC12081 Applications Information 2.2 CAL (Continued) 3.3 OR (Out of Range) Output The OR output goes high when the analog input is below GND or above VREF. This output is low when the input signal is in the valid range of operation (0V ≤ VIN ≤ VREF). 3.4 Data Outputs The Data Outputs are TTL/CMOS compatible. The output data format is 12 bits straight binary. Minimizing the digital output currents will help to minimize noise due to output switching. This can be done by connecting buffers between the ADC outputs and any other circuitry. Only one buffer input should be connected to each output. Additionally, inserting series resistors of 47 to 56 Ohms right at the digital outputs, close to the ADC pins, will isolate the outputs from other circuitry and limit output currents. 4.0 POWER SUPPLY CONSIDERATIONS Each power pin should be bypassed with a parallel combination of a 10µF capacitor and a 0.1µF ceramic chip capacitor. The chip capacitors should be within 1/2 centimeter of the power pins. Leadless chip capacitors are preferred because they provide low lead inductance. The converter’s digital logic supply (VD) should be well isolated from the supply that is used for other digital circuitry on the board. A common power supply should be used for both VA (analog supply) and VD (digital supply), and each of these supply pins should be separately bypassed with a 0.1µF ceramic capacitor and a low ESR 10µF electrolytic capacitor. A ferrite bead or inductor should be used between VA and VD to prevent noise coupling from the digital supply into the analog circuit. VD I/O is the power pin for the output driver. This pin may be supplied with a potential between 2.7V and VD. This makes it easy to interface the ADC12081 with 3V or 5V logic families. Powering the VD I/O from 3 Volts will also reduce power consumption and noise generation due to output switching. DO NOT operate the VD I/O at a voltage higher than VD or VA! All power supplies connected to the device should be applied simultaneously. As is the case with all high speed converters, the ADC12081 is sensitive to power supply noise. Accordingly, the noise on the analog supply pin should be minimized, keeping it below 100mV P-P. The level sensitive CAL input must be pulsed high for at least three clock cycles to begin ADC calibration. For best performance, calibration should be performed about ten seconds after power up, after resetting the ADC, and after the temperature has changed by more than 50˚C since the last calibration was performed. Calibration should be performed at the same clock frequency that the ADC12081 will be used for conversions to minimize offset errors. Calibration takes 4000 clock cycles. Irrelevant data may appear during CAL. 2.3 OE Pin The OE pin is used to control the state of the outputs. When the OE pin is low, the output buffers go into the active state. When the OE input is high, the output buffers are in the high impedance state. 2.4 PD Pin The PD pin, when high, holds the ADC12081 in a powerdown mode where power consumption is typically less than 15 mW to conserve power when the converter is not being used. The ADC12081 will begin normal operation within tPD after this pin is brought low, provided a valid CLOCK input is present. The data in the pipeline is corrupted while in the power down mode. The ADC12081 should be re-calibrated after a power-down cycle to ensure optimum performance. 3.0 OUTPUTS The ADC12081 has three analog outputs: reference output voltages VRN, VRM , and VRP. There are 14 digital outputs: 12 Data Output pins, Ready and OR (Out of range). 3.1 Reference Output Voltages The reference output voltages are made available only for the purpose of bypassing with capacitors to a clean analog ground. The recommended bypass capacitors are 0.1µF ceramic chip capacitors. Do not load these pins. 3.2 Ready Output The Ready output goes high to indicate that the converter is ready for operation. This signal will go low when the converter is Calibration or Power Down made. 13 www.national.com ADC12081 Applications Information (Continued) 10015023 FIGURE 5. Basic Connections Diagram 5.0 LAYOUT AND GROUNDING Proper grounding and routing of all signals is essential to ensure accurate conversion. Separate analog and digital ground planes that are connected beneath the ADC12081 are required to achieve specified performance. The analog and digital grounds may be in the same layer, but should be separated from each other and should never overlap each other. Separation should be at least 1/8 inch, where possible. The ground return for the output buffer digital supply (DGND I/O) carries the ground current for the output drivers. This pin should be connected to the system digital ground. The current on this pin can exhibit high transients that could add noise to the conversion process. To prevent this from happening, the DGND I/O pin should NOT be connected in close proximity to any of the ADC12081’s other ground pins. See . Capacitive coupling between the typically noisy digital ground plane and the sensitive analog circuitry can lead to poor performance that may seem impossible to isolate and remedy. The solution is to keep the analog circuitry separated from the digital circuitry and from the digital ground plane. Digital circuits create substantial supply and ground current transients. The logic noise thus generated could have significant impact upon system noise performance. The best www.national.com 14 logic family to use in systems with A/D converters is one which employs non-saturating transistor designs, or has low noise characteristics, such as the 74LS, 74HC(T) and 74 AC(T)Q families. The worst noise generators are logic families that draw the largest supply current transients during clock or signal edges, like the 74F and the 74AC(T) families. Since digital switching transients are composed largely of high frequency components, total ground plane copper weight will have little effect upon the logic-generated noise. This is because of the skin effect. Total surface area is more important than is total ground plane volume. An effective way to control ground noise is by connecting the analog and digital ground planes together beneath the ADC with a copper trace that is very narrow compared with the rest of the ground plane. This narrowing beneath the converter provides a fairly high impedance to the high frequency components of the digital switching currents, directing them away from the analog pins. The relatively lower frequency analog ground currents do not create a significant voltage drop across the impedance of this narrow ground connection. To maximize accuracy in high speed, high resolution systems, avoid crossing analog and digital signal traces. It is important to keep any clock lines isolated from ALL other lines. Even the generally accepted 90 degree crossing ADC12081 Applications Information (Continued) should be avoided as even a little coupling can cause problems at high frequencies. This is because other lines can introduce phase noise (jitter) into the clock line, which can lead to degradation of SNR. Best performance at high frequencies and at high resolution is obtained with a straight signal path. That is, the signal path through all components should form a straight line wherever possible. Be especially careful with the layout of inductors. Mutual inductance can change the characteristics of the circuit in which they are used. Inductors should not be placed side by side, even with just a small part of their bodies beside each other. The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input. Any external component (e.g., a filter capacitor) connected between the converter’s input and ground should be connected to a very clean point in the analog ground plane. 10015024 FIGURE 6. Layout example Figure 6 gives an example of a suitable layout. All analog circuitry (input amplifiers, filters, reference components, etc.) should be placed on or over the analog ground plane. All digital circuitry and I/O lines should be placed over the digital ground plane. All ground connections should have a low inductance path to ground. 6.0 LAYOUT AND GROUNDING The ADC12081 can achieve impressive dynamic performance. To achieve the best dynamic performance with the ADC12081, the clock source driving the CLK input must be free of jitter. For best ac performance, isolating the ADC clock from any digital circuitry should be done with adequate buffers, as with a clock tree. See Figure 7. 10015025 FIGURE 7. Isolating the ADC clock from other circuitry with a clock tree. It is good practice to keep the ADC clock line as short as possible and to keep it well away from any other signals. Other signals can introduce phase noise (jitter) into the clock signal, which can lead to increased distortion. Even lines with 90˚ crossings have capacitive coupling, so try to avoid even these 90˚ crossings of the clock line. 15 www.national.com ADC12081 Applications Information 7.0 COMMON APPLICATION PITFALLS (Continued) Driving the inputs (analog or digital) beyond the power supply rails. For proper operation, all inputs should not go more than 300mV beyond the supply rails (more than 300mV below the ground pins or 300mV above the supply pins). Exceeding these limits on even a transient basis may cause faulty or erratic operation. It is not uncommon for high speed digital circuits (e.g., 74F and 74AC devices) to exhibit undershoot that goes more than a volt below ground above the power supply. A resistor of about 50 to 100Ω in series with the offending digital input will eliminate the problem. Care should be taken not to overdrive the inputs of the ADC12081 with a device that is powered from supplies outside the range of the ADC12081 supply. Such practice may lead to conversion inaccuracies and even to device damage. Attempting to drive a high capacitance digital data bus. Capacitive loading on the digital outputs causes instantaneous digital currents to flow from the VD I/O supply into the DGND I/O ground plane. These large charging current spikes can couple into the analog section, degrading dynamic performance. Adequate bypassing and maintaining separate analog and digital ground planes will reduce this problem. The digital data outputs should be buffered (with 74ACQ541, for example). Dynamic performance can also be improved by adding series resistors at each digital output, close to the ADC12081, reducing the energy coupled back into the converter output pins by limiting the output slew rate. A reasonable value for these resistors is 47Ω. Using an inadequate amplifier to drive the analog input. The analog input circuits of the ADC12081 place a switched capacitor load on the input signal source. Therefore the amplifier used to drive the ADC12081 must have a low impedance output and adequate bandwidth to avoid distortion of the input signal. Operating with the reference pins outside of the specified range. As mentioned in section 1.1, VREF should be in the range of 1.8V ≤ VREF ≤ 2.2V. Operating outside of these limits could lead to signal distortion. Using a clock source with excessive jitter, using excessively long clock signal trace, or having other signals coupled to the clock signal trace. This will cause the sampling interval to vary, causing excessive output noise and a reduction in SNR performance. www.national.com 16 ADC12081 12-Bit, 5 MHz Self-Calibrating, Pipelined A/D Converter with Internal Sample & Hold Physical Dimensions unless otherwise noted inches (millimeters) 32-Lead LQFP Package Ordering Number ADC12081CIVT NS Package Number VBE32A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Email: support@nsc.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 www.national.com National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.