ADC14V155CISQ

ADC14V155 14-Bit, 155 MSPS, 1.1 GHz Bandwidth A/D Converter with LVDS Outputs April 28, 2009 ADC14V155 14-Bit, 155 MSPS, 1.1 GHz Bandwidth A/D Converter with LVDS Outputs General Description The ADC14V155 is a high-performance CMOS analog-todigital converter with LVDS outputs. It is capable of converting analog input signals into 14-Bit digital words at rates up to 155 Mega Samples Per Second (MSPS). Data leaves the chip in a DDR (Dual Data rate) format; this allows both edges of the output clock to be utilized while achieving a smaller package size. This converter uses a differential, pipelined architecture with digital error correction and an on-chip sample-and-hold circuit to minimize power consumption and the external component count, while providing excellent dynamic performance. A unique sample-and-hold stage yields a full-power bandwidth of 1.1 GHz. The ADC14V155 operates from dual +3.3V and +1.8V power supplies and consumes 951 mW of power at 155 MSPS. The separate +1.8V supply for the digital output interface allows lower power operation with reduced noise. A powerdown feature reduces the power consumption to 15 mW while still allowing fast wake-up time to full operation. In addition there is a sleep feature which consumes 50 mW of power and has a faster wake-up time. The differential inputs provide a full scale differential input swing equal to 2 times the reference voltage. A stable 1.0V internal voltage reference is provided, or the ADC14V155 can be operated with an external reference. Clock mode (differential versus single-ended) and output data format (offset binary versus 2's complement) are pin-selectable. A duty cycle stabilizer maintains performance over a wide range of input clock duty cycles. The ADC14V155 is pin-compatible with the ADC12V170. It is available in a 48-lead LLP package and operates over the industrial temperature range of −40°C to +85°C. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 1.1 GHz Full Power Bandwidth Internal sample-and-hold circuit Low power consumption Internal precision 1.0V reference Single-ended or Differential clock modes Clock Duty Cycle Stabilizer Dual +3.3V and +1.8V supply operation Power-down and Sleep modes Offset binary or 2's complement output data format Dual Data Rate (DDR) LVDS outputs Pin-compatible: ADC12V170 48-pin LLP package, (7x7x0.8mm, 0.5mm pin-pitch) Key Specifications ■ ■ ■ ■ ■ ■ ■ Resolution Conversion Rate SNR (fIN = 70 MHz) SFDR (fIN = 70 MHz) ENOB (fIN = 70 MHz) Full Power Bandwidth Power Consumption 14 Bits 155 MSPS 71.7 dBFS (typ) 86.9 dBFS (typ) 11.5 bits (typ) 1.1 GHz (typ) 951 mW (typ) Applications ■ ■ ■ ■ ■ ■ ■ High IF Sampling Receivers Wireless Base Station Receivers Power Amplifier Linearization Multi-carrier, Multi-mode Receivers Test and Measurement Equipment Communications Instrumentation Radar Systems Block Diagram 30005202 © 2009 National Semiconductor Corporation 300052 www.national.com ADC14V155 Connection Diagram 30005201 Ordering Information Industrial (−40°C ≤ TA ≤ +85°C) ADC14V155CISQ ADC14V155LFEB ADC14V155HFEB Package 48 Pin LLP Evaluation Board (fIN150 MHz) www.national.com 2 ADC14V155 Pin Descriptions and Equivalent Circuits Pin No. ANALOG I/O 3 VIN− Differential analog input pins. The differential full-scale input signal level is two times the reference voltage with each input pin signal centered on a common mode voltage, VCM. Symbol Equivalent Circuit Description 4 VIN+ 43 45 VRP VRM These pins should each be bypassed to AGND with a low ESL (equivalent series inductance) 0.1 µF capacitor placed very close to the pin to minimize stray inductance. A 0.1 µF capacitor should be placed between VRP and VRN as close to the pins as possible, and a 10 µF capacitor should be placed in parallel. VRP and VRN should not be loaded. VRM may be loaded to 1mA for use as a temperature stable 1.5V reference. It is recommended to use VRM to provide the common mode voltage, VCM, for the differential analog inputs, VIN+ and VIN−. This pin can be used as either the +1.0V internal reference voltage output (internal reference operation) or as the external reference voltage input (external reference operation). To use the internal reference, VREF should be decoupled to AGND with a 0.1 µF, low equivalent series inductance (ESL) capacitor. In this mode, VREF defaults as the output for the internal 1.0V reference. To use an external reference, overdrive this pin with a low noise external reference voltage. The input impedance looking into this pin is 9kΩ. Therefore, to overdrive this pin, the output impedance of the external reference source should be VA), the current at that pin should be limited to ±5 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 ±5 mA to 10. Note 4: The maximum allowable power dissipation is dictated by TJ,max, the junction-to-ambient thermal resistance, (θJA), and the ambient temperature, (TA), and can be calculated using the formula PD,max = (TJ,max - TA )/θJA. The values for maximum power dissipation listed above 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 power supply voltages, or the power supply polarity is reversed). Such conditions should always be avoided. Note 5: Human Body Model is 100 pF discharged through a 1.5 kΩ resistor. Machine Model is 220 pF discharged through 0 Ω Note 6: Reflow temperature profiles are different for lead-free and non-lead-free packages. Note 7: The inputs are protected as shown below. Input voltage magnitudes above VA or 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 2.6V or below GND as described in the Operating Ratings section. 30005211 Note 8: To guarantee accuracy, it is required that |VA–VD| ≤ 100 mV and separate bypass capacitors are used at each power supply pin. Note 9: With the test condition for VREF = +1.0V (2VP-P differential input), the 14-Bit LSB is 122.1 µV. Note 10: Typical figures are at TA = 25°C and represent most likely parametric norms at the time of product characterization. The typical specifications are not guaranteed. Note 11: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive and negative full-scale. Note 12: The input capacitance is the sum of the package/pin capacitance and the sample and hold circuit capacitance. Note 13: Optimum performance will be obtained by keeping the reference input in the 0.9V to 1.1V range. The LM4051CIM3-ADJ (SOT-23 package) is recommended for external reference applications. Note 14: This test parameter is guaranteed by design and characterization. 9 www.national.com ADC14V155 Specification Definitions APERTURE DELAY is the time after the falling edge of the clock to when the input signal is acquired or held for conversion. APERTURE JITTER (APERTURE UNCERTAINTY) is the variation in aperture delay from sample to sample. Aperture jitter manifests itself as noise in the output. CLOCK DUTY CYCLE is the ratio of the time during one cycle that a repetitive digital waveform is high to the total time of one period. The specification here refers to the ADC clock input signal. COMMON MODE VOLTAGE (VCM) is the common DC voltage applied to both input terminals of the ADC. CONVERSION LATENCY is the number of clock cycles between initiation of conversion and when that data is presented to the output driver stage. Data for any given sample is available at the output pins the Pipeline Delay plus the Output Delay after the sample is taken. New data is available at every clock cycle, but the data lags the conversion by the pipeline delay. 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 a measure of the frequency at which the reconstructed output fundamental drops 3 dB below its low frequency value for a full scale input. GAIN ERROR is the deviation from the ideal slope of the transfer function. It can be calculated as: Gain Error = Positive Full Scale Error − Negative Full Scale Error It can also be expressed as Positive Gain Error and Negative Gain Error, which are calculated as: PGE = Positive Full Scale Error - Offset Error NGE = Offset Error - Negative Full Scale Error INTEGRAL NON LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from negative full scale (½ LSB below the first code transition) through positive full scale (½ 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. 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 dBFS. LSB (LEAST SIGNIFICANT BIT) is the bit that has the smallest value or weight of all bits. This value is VFS/2n, where “VFS” is the full scale input voltage and “n” is the ADC resolution in bits. LVDS DIFFERENTIAL OUTPUT VOLTAGE (VOD) is the absolute value of the differnece between VDX+ and VDX- outputs; each measured with respect to Ground. LVDS OUTPUT OFFSET VOLTAGE (VOS) is the midpoint between the DX+ and DX- pins' output voltages; i.e., [VDx+ + VDX-]/2. MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC14V155 is guaranteed not to have any missing codes. MSB (MOST SIGNIFICANT BIT) is the bit that has the largest value or weight. Its value is one half of full scale. NEGATIVE FULL SCALE ERROR is the difference between the actual first code transition and its ideal value of ½ LSB above negative full scale. OFFSET ERROR is the difference between the two input voltages [(VIN+) – (VIN-)] required to cause a transition from code 8191 to 8192. OUTPUT DELAY is the time delay after the falling edge of the clock before the data update is presented at the output pins. PIPELINE DELAY (LATENCY) See CONVERSION LATENCY. POSITIVE FULL SCALE ERROR is the difference between the actual last code transition and its ideal value of 1½ LSB below positive full scale. POWER SUPPLY REJECTION RATIO (PSRR) is a measure of how well the ADC rejects a change in the power supply voltage. PSRR is the ratio of the Full-Scale output of the ADC with the supply at the minimum DC supply limit to the FullScale output of the ADC with the supply at the maximum DC supply limit, expressed in dB. SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms value of the sum of all other spectral components below one-half the sampling frequency, not including harmonics or DC. 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 d.c. 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, expressed in dB, of the rms total of the first nine harmonic levels at the output to the level of the fundamental at the output. THD is calculated as where f1 is the RMS power of the fundamental (output) frequency and f2 through f10 are the RMS power of the first 9 harmonic frequencies in the output spectrum. SECOND HARMONIC DISTORTION (2ND HARM) is the difference expressed in dB, between the RMS power in the input frequency at the output and the power in its 2nd harmonic level at the output. THIRD HARMONIC DISTORTION (3RD HARM) is the difference, expressed in dB, between the RMS power in the input frequency at the output and the power in its 3rd harmonic level at the output. www.national.com 10 ADC14V155 Timing Diagram 30005220 Output Timing 11 www.national.com ADC14V155 Transfer Characteristic 30005210 FIGURE 1. Transfer Characteristic (Offset Binary Format) Typical Performance Characteristics, DNL, INL Unless otherwise specified, the following specifications apply: VIN = -1dBFS, AGND = DGND = DRGND = 0V, VA = VD = +3.3V, VDR = +1.8V, Internal VREF = +1.0V, fCLK = 155 MHz, VCM = VRM, CL = 5 pF/pin, Single-Ended Clock Mode, Offset Binary Format. Typical values are for TA = 25°C. (Notes 7, 8, 9) DNL INL 30005261 30005262 www.national.com 12 ADC14V155 Typical Performance Characteristics, Dynamic Performance Unless otherwise specified, the following specifications apply: VIN = -1dBFS, AGND = DGND = DRGND = 0V, VA = VD = +3.3V, VDR = +1.8V, Internal VREF = +1.0V, fCLK = 155 MHz, fIN = 70 MHz, VCM = VRM, CL = 5 pF/pin, Single-Ended Clock Mode, Offset Binary Format. Typical values are for TA = 25°C. SNR, SINAD, SFDR vs. fIN DISTORTION vs. fIN 30005295 30005283 SNR, SINAD, SFDR vs. VA DISTORTION vs. VA 30005273 30005274 SNR, SINAD, SFDR vs. VDR DISTORTION vs. VDR 30005275 30005276 13 www.national.com ADC14V155 Typical Performance Characteristics, Dynamic Performance Unless otherwise specified, the following specifications apply: VIN = -1dBFS, AGND = DGND = DRGND = 0V, VA = VD = +3.3V, VDR = +1.8V, Internal VREF = +1.0V, fCLK = 155 MHz, fIN = 70 MHz, VCM = VRM, CL = 5 pF/pin, Single-Ended Clock Mode, Offset Binary Format. Typical values are for TA = 25°C. SNR, SINAD, SFDR vs. VREF DISTORTION vs. VREF 30005277 30005278 SNR, SINAD, SFDR vs. Temperature DISTORTION vs. Temperature 30005281 30005282 www.national.com 14 ADC14V155 Typical Performance Characteristics, Dynamic Performance Unless otherwise specified, the following specifications apply: VIN = -1dBFS, AGND = DGND = DRGND = 0V, VA = VD = +3.3V, VDR = +1.8V, Internal VREF = +1.0V, fCLK = 155 MHz, fIN = 70 MHz, VCM = VRM, CL = 5 pF/pin, Single-Ended Clock Mode, Offset Binary Format. Typical values are for TA = 25°C. Spectral Response @ 70 MHz Input Spectral Response @ 169 MHz Input 30005292 30005293 Spectral Response @ 238 MHz Input Spectral Response @ 350 MHz Input 30005294 30005296 Spectral Response @ 400 MHz Input 30005297 15 www.national.com ADC14V155 Functional Description Operating on dual +3.3V and +1.8V supplies, the ADC14V155 digitizes a differential analog input signal to 14 bits, using a differential pipelined architecture with error correction circuitry and an on-chip sample-and-hold circuit to ensure maximum performance. The user has the choice of using an internal 1.0V stable reference, or using an external reference. The ADC14V155 will accept an external reference between 0.9V and 1.1V (1.0V recommended) which is buffered on-chip to ease the task of driving that pin. The +1.8V output driver supply reduces power consumption and decreases the noise at the output of the converter. The quad state function pin CLK_SEL/DF (pin 8) allows the user to choose between using a single-ended or a differential clock input and between offset binary or 2's complement output data format. The digital outputs are LVDS compatible signals that are clocked by a synchronous data ready output signal (DRDY pins 33, 34) at the same rate as the clock input. For the ADC14V155 the clock frequency can be between 5 MSPS and 155 MSPS (typical) with fully specified performance at 155 MSPS. The analog input is acquired at the falling edge of the clock and the digital data for a given sample is output on the falling edge of the DRDY signal and is delayed by the pipeline for 8.5 clock cycles. The data should be captured on the rising edge of the DRDY signal. Power-down is selectable using the PD/Sleep pin (pin 7). A logic high on the PD/Sleep pin disables everything except the voltage reference circuitry and reduces the converter power consumption to 15 mW. When PD/Sleep is biased to VA/2 the the chip enters sleep mode. In sleep mode everything except the voltage reference circuitry and its accompanying on chip buffer is disabled; power consumption is reduced to 50 mW. For normal operation, the PD/Sleep pin should be connected to the analog ground (AGND). A duty cycle stabilizer maintains performance over a wide range of clock duty cycles. 2.0 ANALOG INPUTS 2.1 Signal Inputs 2.1.1 Differential Analog Input Pins The ADC14V155 has one pair of analog signal input pins, VIN+ and VIN−, which form a differential input pair. The input signal, VIN, is defined as VIN = (VIN+) – (VIN−) Figure 2 shows the expected input signal range. Note that the common mode input voltage, VCM, should be 1.5V. Using VRM (pin 45) for VCM will ensure the proper input common mode level for the analog input signal. The peaks of the individual input signals should each never exceed 2.6V. Each analog input pin of the differential pair should have a peak-topeak voltage equal to the reference voltage, VREF, be 180° out of phase with each other and be centered around VCM.The peak-to-peak voltage swing at each analog input pin should not exceed the value of the reference voltage or the output data will be clipped. 30005214 FIGURE 2. Expected Input Signal Range For single frequency sine waves the full scale error in LSB can be described as approximately EFS = 16384 ( 1 - sin (90° + dev)) Where dev is the angular difference in degrees between the two signals having a 180° relative phase relationship to each other (see Figure 3). For single frequency inputs, angular errors result in a reduction of the effective full scale input. For complex waveforms, however, angular errors will result in distortion. Applications Information 1.0 OPERATING CONDITIONS We recommend that the following conditions be observed for operation of the ADC14V155: 3.0V ≤ VA ≤ 3.6V VD = VA VDR = 1.8V 5 MHz ≤ fCLK ≤ 155 MHz 1.0V internal reference 0.9V ≤ VREF ≤ 1.1V (for an external reference) VCM = 1.5V (from VRM) Single Ended Clock Mode 30005216 FIGURE 3. Angular Errors Between the Two Input Signals Will Reduce the Output Level or Cause Distortion It is recommended to drive the analog inputs with a source impedance less than 100Ω. Matching the source impedance for the differential inputs will improve even ordered harmonic performance (particularly second harmonic). Table 1 indicates the input to output relationship of the ADC14V155. www.national.com 16 ADC14V155 TABLE 1. Input to Output Relationship VIN+ VCM − VREF/2 VCM − VREF/4 VCM VCM + VREF/4 VCM + VREF/2 VIN− VCM + VREF/2 VCM + VREF/4 VCM VCM − VREF/4 VCM − VREF/2 Binary Output 00 0000 0000 0000 01 0000 0000 0000 10 0000 0000 0000 11 0000 0000 0000 11 1111 1111 1111 2’s Complement Output 10 0000 0000 0000 11 0000 0000 0000 00 0000 0000 0000 01 0000 0000 0000 01 1111 1111 1111 Positive Full-Scale Mid-Scale Negative Full-Scale 2.1.2 Driving the Analog Inputs The VIN+ and the VIN− inputs of the ADC14V155 have an internal sample-and-hold circuit which consists of an analog switch followed by a switched-capacitor amplifier. The analog inputs are connected to the sampling capacitors through NMOS switches, and each analog input has parasitic capacitances associated with it. When the clock is high, the converter is in the sample phase. The analog inputs are connected to the sampling capacitor through the NMOS switches, which causes the capacitance at the analog input pins to appear as the pin capacitance plus the internal sample and hold circuit capacitance (approximately 9 pF). While the clock level remains high, the sampling capacitor will track the changing analog input voltage. When the clock transitions from high to low, the converter enters the hold phase, during which the analog inputs are disconnected from the sampling capacitor. The last voltage that appeared at the analog input before the clock transition will be held on the sampling capacitor and will be sent to the ADC core. The capacitance seen at the analog input during the hold phase appears as the sum of the pin capacitance and the parasitic capacitances associated with the sample and hold circuit of each analog input (approximately 6 pF). Once the clock signal transitions from low to high, the analog inputs will be reconnected to the sampling capacitor to capture the next sample. Usually, there will be a difference between the held voltage on the sampling capacitor and the new voltage at the analog input. This will cause a charging glitch that is proportional to the voltage difference between the two samples to appear at the analog input pin. The input circuitry must be fast enough to allow the sampling capacitor to settle before the clock signal goes low again, as incomplete settling can degrade the SFDR performance. A single-ended to differential conversion circuit is shown in Figure 4 . A transformer is preferred for high frequency input signals. Terminating the transformer on the secondary side provides two advantages. First, it presents a real broadband impedance to the ADC inputs and second, it provides a common path for the charging glitches from each side of the differential sample-and-hold circuit. One short-coming of using a transformer to achieve the single-ended to differential conversion is that most RF transformers have poor low frequency performance. A differential amplifier can be used to drive the analog inputs for low frequency applications. The amplifier must be fast enough to settle from the charging glitches on the analog input resulting from the sample-and-hold operation before the clock goes high and the sample is passed to the ADC core. The SFDR performance of the converter depends on the external signal conditioning circuity used, as this affects how quickly the sample-and-hold charging glitch will settle. An external resistor and capacitor network as shown in Figure 4 should be used to isolate the charging glitches at the ADC input from the external driving circuit and to filter the wideband noise at the converter input. These components should be 17 placed close to the ADC inputs because the analog input of the ADC is the most sensitive part of the system, and this is the last opportunity to filter that input. For Nyquist applications the RC pole should be at the ADC sample rate. The ADC input capacitance in the sample mode should be considered when setting the RC pole. For wideband undersampling applications, the RC pole should be set at about 1.5 to 2 times the maximum input frequency to maintain a linear delay response. 2.1.3 Input Common Mode Voltage The input common mode voltage, VCM, should be in the range of 1.4V to 1.6V and be a value such that the peak excursions of the analog signal do not go more negative than ground or more positive than 2.6V. It is recommended to use VRM (pin 45) as the input common mode voltage. 2.2 Reference Pins The ADC14V155 is designed to operate with an internal 1.0V reference, or an external 1.0V reference, but performs well with external reference voltages in the range of 0.9V to 1.1V. The internal 1.0 Volt reference is the default condition when no external reference input is applied to the VREF pin. If a voltage in the range of 0.9V to 1.1V is applied to the VREF pin, then that voltage is used for the reference. The VREF pin should always be bypassed to ground with a 0.1 µF capacitor close to the reference input pin. Lower reference voltages will decrease the signal-to-noise ratio (SNR) of the ADC14V155. Increasing the reference voltage (and the input signal swing) beyond 1.1V may degrade THD for a full-scale input, especially at higher input frequencies. It is important that all grounds associated with the reference voltage and the analog input signal make connection to the ground plane at a single, quiet point to minimize the effects of noise currents in the ground path. The Reference Bypass Pins (VRP, VRM, and VRN) are made available for bypass purposes. All these pins should each be bypassed to ground with a 0.1 µF capacitor. A 0.1 µF and a 10 µF capacitor should be placed between the VRP and VRN pins, as shown in Figure 4. This configuration is necessary to avoid reference oscillation, which could result in reduced SFDR and/or SNR. VRM may be loaded to 1mA for use as a temperature stable 1.5V reference. The remaining pins should not be loaded. Smaller capacitor values than those specified will allow faster recovery from the power down and sleep modes, but may result in degraded noise performance. Loading any of these pins, other than VRM, may result in performance degradation. The nominal voltages for the reference bypass pins are as follows: VRM = 1.5 V VRP = VRM + VREF / 2 VRN = VRM − VREF / 2 www.national.com ADC14V155 2.3 Control Inputs 2.3.1 Power-Down & Sleep (PD/Sleep) The power-down and sleep modes can be enabled through this three-state input pin. Table 2 shows how to utilize these options. TABLE 2. Power Down/Sleep Selection Table PD Input Voltage VA VA/2 AGND Power State Power-down Sleep On The power-down and sleep modes allows the user to conserve power when the converter is not being used. In the power-down state all bias currents of the analog circuitry, excluding the reference are shut down which reduces the power consumption to 15 mW with no clock running. In sleep mode some additional buffer circuitry is left on to allow an even faster wake time; power consumption in the sleep mode is 50 mW with no clock running. In both of these modes the output data pins are undefined and the data in the pipeline is corrupted. The Exit Cycle time for both the sleep and power-down mode is determined by the value of the capacitors on the VRP, VRM and VRN reference bypass pins (pins 43, 44 and 45). These capacitors lose their charge when the ADC is not operating and must be recharged by on-chip circuitry before conversions can be accurate. For power-down mode the Exit Cycle time is about 3 ms with the recommended component values. The Exit Cycle time is faster for sleep mode. Smaller capacitor values allow slightly faster recovery from the power down and sleep mode, but can result in a reduction in SNR, SINAD and ENOB performance. 2.3.2 Clock Mode Select/Data Format (CLK_SEL/DF) Single-ended versus differential clock mode and output data format are selectable using this quad-state function pin. Table 3 shows how to select between the clock modes and the output data formats. TABLE 3. Clock Mode and Data Format Selection Table CLK_SEL/DF Input Voltage VA (2/3) * VA (1/3) * VA AGND Clock Mode Differential Differential Single-Ended Single-Ended Output Data Format 2's Complement Offset Binary 2's Complement Offset Binary figure the ADC for either differential or single-ended clock mode (see Section 3.3). In differential clock mode, the two clock signals should be exactly 180° out of phase from each other and of the same amplitude. In the single-ended clock mode, the clock signal should be routed to the CLK+ input and the CLK− input should be tied to AGND in combination with the correct setting from Table 3. To achieve the optimum noise performance, the clock inputs should be driven with a stable, low jitter clock signal in the range indicated in the Electrical Table. The clock input signal should also have a short transition region. This can be achieved by passing a low-jitter sinusoidal clock source through a high speed buffer gate. This configuration is shown in Figure 4. The trace carrying the clock signal should be as short as possible and should not cross any other signal line, analog or digital, not even at 90°. Figure 4 shows the recommended clock input circuit. The clock signal also drives an internal state machine. If the clock is interrupted, or its frequency is too low, the charge on the internal capacitors can dissipate to the point where the accuracy of the output data will degrade. This is what limits the minimum sample rate. The clock line should be terminated at its source in the characteristic impedance of that line. Take care to maintain a constant clock line impedance throughout the length of the line. Refer to Application Note AN-905 for information on setting characteristic impedance. It is highly desirable that the the source driving the ADC clock pins only drive that pin. However, if that source is used to drive other devices, then each driven pin should be AC terminated with a series RC to ground, such that the resistor value is equal to the characteristic impedance of the clock line and the capacitor value is where tPD is the signal propagation rate down the clock line, "L" is the line length and ZO is the characteristic impedance of the clock line. This termination should be as close as possible to the ADC clock pin but beyond it as seen from the clock source. Typical tPD is about 150 ps/inch (60 ps/cm) on FR-4 board material. The units of "L" and tPD should be the same (inches or centimeters). The duty cycle of the clock signal can affect the performance of the A/D Converter. Because achieving a precise duty cycle is difficult, the ADC14V155 has a Duty Cycle Stabilizer. It is designed to maintain performance over a clock duty cycle range of 30% to 70%. 3.0 CLOCK INPUTS The CLK+ and CLK− signals control the timing of the sampling process. The CLK_SEL/DF pin (pin 8) allows the user to con- www.national.com 18 ADC14V155 4.0 DIGITAL OUTPUTS Digital outputs consist of the LVDS signals D0-D13, DRDY and OVR. The ADC14V155 has 16 LVDS compatible data output pins: 14 data output bits corresponding to the converted input value, a data ready (DRDY) signal that should be used to capture the output data and an over-range indicator (OVR) which is set high when the sample amplitude exceeds the 14-Bit conversion range. Valid data is present at these outputs while the PD/Sleep pin is low. The odd data bits should be captured with the falling edge of DRDY and the even data bits should be captured with the rising edge of DRDY. Be very careful when driving a high capacitance bus. The more capacitance the output drivers must charge for each conversion, the more instantaneous digital current flows through VDR and DRGND. These large charging current spikes can cause on-chip ground noise and couple into the analog circuitry, degrading dynamic performance. Adequate bypassing, limiting output capacitance and careful attention to the ground plane will reduce this problem. Additionally, bus capacitance beyond the specified 5 pF/pin will cause tOD to increase, reducing the setup and hold time of the ADC output data. The result could be an apparent reduction in dynamic performance. To minimize noise due to output switching, the load currents at the digital outputs should be minimized. This can be achieved by keeping the PCB traces less than 2 inches long; longer traces are more susceptible to noise. Try to place the 100 ohm termination resistor as close to the receiving circuit as possible. See Figure 4. 19 www.national.com ADC14V155 www.national.com 30005213 20 FIGURE 4. Application Circuit using Transformer Drive Circuit ADC14V155 5.0 POWER SUPPLY CONSIDERATIONS The power supply pins should be bypassed with a 0.1 µF capacitor and with a 0.01 µF ceramic chip capacitor close to each power pin. Leadless chip capacitors are preferred because they have low series inductance. As is the case with all high-speed converters, the ADC14V155 is sensitive to power supply noise. Accordingly, the noise on the analog supply pin should be kept below 100 mVP-P. No pin should ever have a voltage on it that is in excess of the supply voltages, not even on a transient basis. Be especially careful of this during power turn on and turn off. The VDR pin provides power for the output drivers and may be operated from a supply in the range of 1.6V to 2.0V. This enables lower power operation, reduces the noise coupling effects from the digital outputs to the analog circuitry and simplifies interfacing to lower voltage devices and systems. Note, however, that tOD increases with reduced VDR. 6.0 LAYOUT AND GROUNDING Proper grounding and proper routing of all signals are essential to ensure accurate conversion. Maintaining separate analog and digital areas of the board, with the ADC14V155 between these areas, is required to achieve specified performance. The ground return for the data outputs (DRGND) carries the ground current for the output drivers. The output current can exhibit high transients that could add noise to the conversion process. To prevent this from happening, the DRGND pins should NOT be connected to system ground in close proximity to any of the ADC14V155's other ground pins. Capacitive coupling between the typically noisy digital circuitry and the sensitive analog circuitry can lead to poor performance. The solution is to keep the analog circuitry separated from the digital circuitry, and to keep the clock line as short as possible. 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 area. Generally, analog and digital lines should cross each other at 90° to avoid crosstalk. To maximize accuracy in high speed, high resolution systems, however, avoid crossing analog and digital lines altogether. It is important to keep clock lines as short as possible and isolated from ALL other lines, including other digital lines. Even the generally accepted 90° crossing should be avoided with the clock line as even a little coupling can cause problems at high frequencies. This is because other lines can introduce jitter into the clock line, which can lead to degradation of SNR. Also, the high speed clock can introduce noise into the analog chain. 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 and transformers. Mutual inductance can change the characteristics of the circuit in which they are used. Inductors and transformers should not be placed side by side, even with just a small part of their bodies beside each other. For instance, place transformers for the analog input and the clock input at 90° to one another to avoid magnetic coupling. 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 pins and ground or to the reference input pin and ground should be connected to a very clean point in the ground plane. All analog circuitry (input amplifiers, filters, reference components, etc.) should be placed in the analog area of the board. All digital circuitry and dynamic I/O lines should be placed in the digital area of the board. The ADC14V155 should be between these two areas. Furthermore, all components in the reference circuitry and the input signal chain that are connected to ground should be connected together with short traces and enter the ground plane at a single, quiet point. All ground connections should have a low inductance path to ground. 7.0 DYNAMIC PERFORMANCE To achieve the best dynamic performance, the clock source driving the CLK input must have a sharp transition region and be free of jitter. Isolate the ADC clock from any digital circuitry with buffers, as with the clock tree shown in Figure 5 . The gates used in the clock tree must be capable of operating at frequencies much higher than those used if added jitter is to be prevented. Best performance will be obtained with a single-ended drive input drive, compared with a differential clock. As mentioned in Section 6.0, 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 jitter into the clock signal, which can lead to reduced SNR performance, and the clock can introduce noise into other lines. Even lines with 90° crossings have capacitive coupling, so try to avoid even these 90° crossings of the clock line. 30005217 FIGURE 5. Isolating the ADC Clock from other Circuitry with a Clock Tree 21 www.national.com ADC14V155 Physical Dimensions inches (millimeters) unless otherwise noted 48-Lead LLP Package Ordering Number ADC14V155CISQ NS Package Number SQA48A www.national.com 22 ADC14V155 Notes 23 www.national.com ADC14V155 14-Bit, 155 MSPS, 1.1 GHz Bandwidth A/D Converter with LVDS Outputs Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage Reference PowerWise® Solutions Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/AU Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero SolarMagic™ Analog University® THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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