ADC08351 8-Bit, 42 MSPS, 40 mW A/D Converter General Description Features The ADC08351 is an easy to use low power, low cost, small size, 42 MSPS analog-to-digital converter that digitizes signals to 8 bits. The ADC08351 uses an unique architecture that achieves 7.2 Effective Bits with a 4.4 MHz input and 42 MHz clock frequency and 6.8 Effective Bits with a 21 MHz input and 42 MHz clock frequency. Output formatting is straight binary coding. To minimize system cost and power consumption, the ADC08351 requires minimal external components and includes input biasing to allow optional a.c. input signal coupling. The user need only provide a +3V supply and a clock. Many applications require no separate reference or driver components. The excellent dc and ac characteristics of this device, together with its low power consumption and +3V single supply operation, make it ideally suited for many video and imaging applications, including use in portable equipment. Total power consumption is reduced to less than 7 mW in the power-down mode. Furthermore, the ADC08351 is resistant to latch-up and the outputs are short-circuit proof. Fabricated on a 0.35 micron CMOS process, the ADC08351 is offered in TSSOP and is designed to operate over the commercial temperature range of −20˚C to +85˚C. n n n n n Low Input Capacitance Internal Sample-and-Hold Function Single +3V Operation Power Down Feature TRI-STATE ® Outputs Key Specifications n Resolution 8 Bits n Maximum Sampling Frequency n ENOB @ fCLK = 42 MHz, fIN = 4.4 MHz 42 MSPS (min) 7.2 Bits (typ) n Guaranteed No Missing Codes n Power Consumption 40 mW (typ); 48 mW (max) (Excluding Reference Current) Applications n n n n n n n n n Video Digitization Digital Still Cameras Set Top Boxes Digital Camcorders Communications Medical Imaging Personal Computer Video CCD Imaging Electro-Optics Pin Configuration DS100895-1 Ordering Information ADC08351CIMTC TSSOP ADC08351CIMTCX TSSOP (tape & reel) TRI-STATE ® is a registered trademark of National Semiconductor Corporation. © 2000 National Semiconductor Corporation DS100895 www.national.com ADC08351 8-Bit, 42 MSPS, 40 mW A/D Converter March 2000 ADC08351 ADC08351 Block Diagram DS100895-2 Pin Descriptions and Equivalent Circuits Pin No. Symbol 17 VIN Analog signal input. Conversion range is 0.5 VP-P to 0.68 VA. 14 VREF Positive reference voltage input. Operating range of this voltage is 0.75V to VA. This pin should be bypassed with a 10 µF tantalum or aluminum electrolytic capacitor and a 0.1 µF ceramic chip capacitor. 1 OE CMOS/TTL compatible digital input that, when low, enables the digital outputs of the ADC08351. When high, the outputs are in a high impedance state. 12 CLK CMOS/TTL compatible digital clock input. VIN is sampled on the falling edge of CLK input. PD CMOS/TTL compatible digital input that, when high, puts the ADC08351 into the power down mode, where it consumes minimal power. When this pin is low, the ADC08351 is in the normal operating mode. D0–D7 Conversion data digital output pins. D0 is the LSB, D7 is the MSB. Valid data is output just after the rising edge of the CLK input. These pins are enabled by bringing the OE pin low. 15 3 thru 10 www.national.com Equivalent Circuit Description 2 Pin No. Symbol Equivalent Circuit ADC08351 Pin Descriptions and Equivalent Circuits (Continued) Description 11, 13 VD Positive digital supply pin. Connect to a clean, quiet voltage source of +3V. VA and VD should have a common supply and be separately bypassed with a 10 µF tantalum or aluminum electrolytic capacitor and a 0.1 µF ceramic chip capacitor. See Section 3.0 for more information. 2, 20 DGND The ground return for the digital supply. AGND and DGND should be connected together close to the ADC08351. 16 VA Positive analog supply pin. Connected to a clean, quiet voltage source of +3V. VA and VD should have a common supply and be separately bypassed with a 10 µF tantalum or aluminum electrolytic capacitor and a 0.1 µF ceramic chip capacitor. See Section 3.0 for more information. 18, 19 AGND The ground return for the analog supply. AGND and DGND should be connected together close to the ADC08351 package. 3 www.national.com ADC08351 Absolute Maximum Ratings (Notes 1, 2) ESD Susceptibility (Note 5) Human Body Model If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (VA, VD) 4.2V Voltage on Any Input or Output Pin −0.3V to 4.2V Ground Difference (AGND–DGND) Storage Temperature Digital Output Voltage (VOH, VOL) VD to DGND Package Input Current (Note 3) ± 25 mA ± 50 mA Package Dissipation at TA = 25˚C (Note 4) 300˚C −65˚C to +150˚C Operating Ratings (Notes 1, 2) Operating Temperature Range Supply Voltage (VA, VD) −0.5 to (VA + 0.5V) Input Current at Any Pin (Note 3) 200V Soldering Temp., Infrared, 10 sec. (Note 6) ± 100 mV CLK, OE Voltage Range 4000V Machine Model −20˚C TA ≤ +85˚C +2.7V to +3.6V Ground Difference |DGND–AGND| 0V to 100 mV VIN Voltage Range (VP-P) 0.5V to 0.68 VA Converter Electrical Characteristics The following specifications apply for VA = VD = +3.0 VDC, VREF = 2.4V, VIN = 1.63 VP-P, OE = 0V, CL = 20 pF, fCLK = 42 MHz, 50% duty cycle, unless otherwise specified. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C (Notes 7, 8) Symbol Parameter Conditions Typical (Note 9) Limits (Note 9) Units (Limits) ± 0.7 ± 1.4 LSB (max) +1.3 LSB (max) −1.0 LSB (min) 0 (max) DC Accuracy INL Integral Non Linearity Error DNL Differential Non Linearity ± 0.6 Missing Codes EZ Zero Scale Offset Error −17 mV EFS Full Scale Offset Error −7 mV Video Accuracy DP Differential Phase Error fCLK = 20 MHz, Video Ramp Input 1.0 Degree DG Differential Gain Error fCLK = 20 MHz, Video Ramp Input 1.5 % (CLK LOW) 4 pF (CLK HIGH) 11 pF Analog Input and Reference Characteristics CIN VIN Input Capacitance VIN = 1.5V + 0.7 Vrms RIN RIN Input Resistance 7.2 kΩ FPBW Full-Power Bandwidth 120 MHz VREF Reference Input Voltage 0.735 V IREF Reference Input Current At pin 14 VA V 7.7 mA 10.5 mA 1 mA Power Supply Characteristics IA ID Analog Supply Current PD = Low PD = High PD = Low, No Digital Output Load 2.9 mA PD = High 0.5 mA Total Operating Current Excluding Reference Current, VIN = 0 VDC 13.4 16 mA (max) Power Consumption (active) PD = Low (excluding reference current) 40.2 48 mW (max) Power Consumption (power down) PD = High (excluding reference current) Digital Supply Current <7 mW CLK, OE Digital Input Characteristics VIH Logical High Input Voltage VD = VA = 3V 2.0 V (min) VIL Logical Low Input Voltage VD = VA = 3V 1.0 V (max) IIH Logical High Input Current VIH = VD = VA = 3.3V 10 µA IIL Logic Low Input Current VIL = 0V, VD = VA = 3.3V −10 µA CIN Logic Input Capacitance 10 pF www.national.com 4 ADC08351 Converter Electrical Characteristics (Continued) The following specifications apply for VA = VD = +3.0 VDC, VREF = 2.4V, VIN = 1.63 VP-P, OE = 0V, CL = 20 pF, fCLK = 42 MHz, 50% duty cycle, unless otherwise specified. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C (Notes 7, 8) Symbol Parameter Conditions Typical (Note 9) Limits (Note 9) Units (Limits) Digital Output Characteristics IOH High Level Output Current VD = 2.7V, VOH = VD −0.5V −1.1 mA (min) IOL Low Level Output Current VD = 2.7V, OE = DGND, VOL = 0.4V 1.8 mA (min) VOH High Level Output Voltage VD = 2.7V, IOH = −360 µA 2.65 V VOL Low Level Output Voltage VD = 2.7V, IOL = 1.6 mA 0.2 V IOZH, IOZL TRI-STATE Output Current OE = VD = 3.3V, VOH = 3.3V or VOL = 0V ± 10 µA AC Electrical Characteristics fC1 Maximum Conversion Rate fC2 Minimum Conversion Rate tOD Output Delay 42 MHz (min) 19 ns (max) 2 CLK High to Data Valid Pipline Delay (Latency) MHz 2.5 Clock Cycles 2 ns tDS Sampling (Aperture) Delay tOH Output Hold Time CLK High to Data Invalid 9 ns tEN OE Low to Data Valid Loaded as in Figure 2 14 ns tDIS OE High to High Z State Loaded as in Figure 2 10 ns fCLK = 30 MHz, fIN = 1 MHz 7.2 Bits ENOB Effective Number of Bits fCLK = 42 MHz, fIN = 4.4 MHz 7.2 fCLK = 42 MHz, fIN = 21 MHz 6.8 fCLK = 30 MHz, fIN = 1 MHz 45 SINAD SNR THD SFDR Signal-to-Noise & Distortion Signal-to-Noise Ratio Total Harmonic Distortion Spurious Free Dynamic Range CLK Low to Acquisition of Data 14 fCLK = 42 MHz, fIN = 4.4 MHz 45 fCLK = 42 MHz, fIN = 21 MHz 43 fCLK = 30 MHz, fIN = 1 MHz 44 fCLK = 42 MHz, fIN = 4.4 MHz 45 fCLK = 42 MHz, fIN = 21 MHz 44 fCLK = 30 MHz, fIN = 1 MHz −57 fCLK = 42 MHz, fIN = 4.4 MHz −51 fCLK = 42 MHz, fIN = 21 MHz −46 Bits 6.1 Bits (min) dB dB 38.5 dB (min) dB dB 41 dB (min) dB dB −41 dB (min) fCLK = 30 MHz, fIN = 1 MHz 57 dB fCLK = 42 MHz, fIN = 4.4 MHz 54 dB fCLK = 42 MHz, fIN = 21 MHz 49 41 dB (min) 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, less than AGND or DGND, or greater than VA or VD), 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 temperature (TJmax) for this device is 150˚C. The maximum allowable power dissipation 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. For the 20-pin TSSOP, θJA is 135˚C/W, so PDMAX = 926 mW at 25˚C and 481 mW at the maximum operating ambient temperature of 85˚C. Note that the power dissipation of this device under normal operation will typically be about 68 mW (40 mW quiescent power + 23 mW reference ladder power + 5 mW due to 1 TTL loan on each digital output). The values for maximum power dissipation listed above will be reached only when the ADC08351 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 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO Ohms. Note 6: See AN-450, “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: All inputs are protected as shown below. Input voltage magnitudes up to 500 mV above the supply voltage or 500 mV below GND will not damage this device. However, errors in the A/D conversion can occur if the input goes above VA or below AGND by more than 300 mV. As an example, if VA is 3.0 VDC, the full-scale input voltage must be ≤3.3 VDC to ensure accurate conversions. 5 www.national.com ADC08351 Converter Electrical Characteristics (Continued) DS100895-6 Note 8: To guarantee accuracy, it is required that VA and VD be well bypassed. Each VA and VD pin must be decoupled with separate bypass capacitors. Note 9: Typical figures are at TJ = 25˚C, and represent most likely parametric norms. Test limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Typical Performance Characteristics VA = VD = VD I/O = 3V, fCLK = 42 MHz, unless otherwise specified DNL @ 42 MSPS DNL vs Sample Rate DS100895-7 DNL vs Temperature DS100895-8 INL @ 42 MSPS INL vs VA DS100895-9 INL vs Sample Rate DS100895-11 DS100895-10 INL vs Temperature 6 DS100895-12 SINAD and ENOB vs fIN DS100895-14 DS100895-13 www.national.com DNL vs VA DS100895-15 VA = VD = VD I/O = 3V, fCLK = 42 MHz, unless otherwise specified (Continued) SINAD and ENOB vs Clock Duty Cycle SINAD and ENOB vs fCLK SNR vs fIN DS100895-16 DS100895-18 DS100895-17 THD vs fIN (ID) + (IA) vs fCLK tOD vs VD DS100895-19 DS100895-20 DS100895-21 Spectral Response @ 42 MSPS DS100895-22 DIFFERENTIAL PHASE ERROR is the difference in the output phase of a reconstructed small signal sine wave at two different dc input levels. 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. The test is performed with fIN equal to 100KHz plus integer multiples of fCLK The input frequency at which the output is — 3 dB relative to the low frequency input signal is the full power bandwidth. Specification Definitions ANALOG INPUT 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. The test is performed with fIN equal to 100 kHz plus integer multiples of fCLK. The input frequency at which the output is −3 dB relative to the low frequency input signal is the full power bandwidth. DIFFERENTIAL GAIN ERROR is the percentage difference between the output amplitudes of a high frequency reconstructed sine wave at two different dc input levels. DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB. 7 www.national.com ADC08351 Typical Performance Characteristics ADC08351 Specification Definitions 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. (Continued) FULL SCALE OFFSET ERROR is the difference between the analog input voltage that just causes the output code to transition to the full scale code (all 1’s in the case of the ADC08351) and the ideal value of 11⁄2 LSB below the value of VREF. SIGNAL TO NOISE PLUS DISTORTION (S/(N+D) or SINAD) is the ratio 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. 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 analog input voltage that just causes the output code to transition to the first code and the ideal value of 1⁄2 LSB for that transition. INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from zero scale (1⁄2 LSB below the first code transition) through positive full scale (1⁄2 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. OUTPUT DELAY is the time delay after the rising edge of the input clock before the data update is present at the output pins. OUTPUT HOLD TIME is the length of time that the output data is valid after the rise of the input clock. 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. SAMPLING (APERTURE) DELAY is that time required after the fall of the clock input for the sampling switch to open. The sample is effectively taken this amount of time after the fall of the clock input. Timing Diagram DS100895-23 FIGURE 1. ADC08351 Timing Diagram www.national.com 8 ADC08351 Timing Diagram (Continued) DS100895-24 FIGURE 2. tEN, tDIS Test Circuit erably due to the quantization noise. However, the ADC08351 will give adequate results in many applications with signal levels down to about 0.5 VP-P (VREF = 0.735V). Very good performance can be obtained with reference voltages up to the supply voltage (VA = VREF = 3V, 2.04 VP-P). As with all sampling ADCs, the opening and closing of the switches associated with the sampling causes an output of energy from the analog input, VIN. The reference ladder also has switches associated with it, so the reference source must be able to supply sufficient current to hold VREF steady. The analog input of the ADC08351 is self-biased with an 18 kΩ pull-up resistor to VREF and a 12 kΩ pull-down resistor to AGND. This allows for either a.c. or d.c. coupling of the input signal. These two resistors provide a convenient way to ensure a signal that is less than full scale will be centered within the input common mode range of the converter. However, the high values of these resistors and the energy coming from this input means that performance will be improved with d.c. coupling. The driving circuit at the signal input must be able to sink and source sufficient current at the signal frequency to prevent distortion from being introduced at the input. Functional Description The ADC08351 achieves 6.8 effective bits at 21 MHz input frequency with 42 MHz clock frequency digitizing to eight bits the analog signal at VIN that is within the nominal voltage range of 0.5 VP-P to 0.68 VA. Input voltages below 0.0665 times the reference voltage will cause the output word to consist of all zeroes, while input voltages above 3⁄4 of the reference voltage will cause the output word to consist of all ones. For example, with a VREF of 2.4V, input voltages below 160 mV will result in an output word of all zeroes, while input voltages above 1.79V will result in an output word of all ones. The output word rate is the same as the clock frequency. Data is acquired at the falling edge of the clock and the digital equivalent of that data is available at the digital outputs 2.5 clock cycles plus tOD later. The ADC08351 will convert as long as the clock signal is present at pin 12, but the data will not appear at the outputs unless the OE pin 1 is low. The digital outputs are in the high impedance state when the OE pin or when the PD pin is high. Applications Information 2.0 POWER SUPPLY CONSIDERATIONS A tantalum or aluminum electrolytic capacitor of 5 µF to 10 µF should be placed within a centimeter of each of the A/D power pins, with a 0.1 µF ceramic chip capacitor placed within 1⁄2 centimeter of each of the power pins. Leadless chip capacitors are preferred because they provide lower lead inductance than do their leaded counterparts. While a single voltage source should be used for the analog and digital supplies of the ADC08351, these supply pins should be decoupled from each other to prevent any digital noise from being coupled to the analog power pins. A ferrite bead between the analog and digital supply pins would help to isolate the two supplies. The converter digital supply should not be the supply that is used for other digital circuitry on the board. It should be the same supply used for the A/D analog supply, decoupled from the A/D analog supply pin, as described above. A common analog supply should be used for both VA and VD, and each of these pins should be separately bypassed with a 0.1 µF ceramic capacitor and with low ESR a 10 µF capacitor. As is the case with all high speed converters, the ADC08351 is sensitive to power supply noise. Accordingly, the noise on 1.0 THE ADC REFERENCE AND THE ANALOG INPUT The capacitance seen at the input changes with the clock level, appearing as 4 pF when the clock is low, and 11 pF when the clock is high. Since a dynamic capacitance is more difficult to drive than is a fixed capacitance, choose an amplifier that can drive this type of load. The CLC409, CLC440, LM6152, LM6154, LM6181 and LM6182 are good devices for driving analog input of the ADC08351. Do not drive the input beyond the supply rails. The maximum peak-to-peak input level without clipping of the reconstructed output is determined by the values of the resistor string between VREF and AGND. The bottom of the reference ladder has a voltage of 0.0665 times VREF, while the top of the reference ladder has a voltage of 0.7468 times VREF. The maximum peak-to-peak input level works out to be about 68% of the value of VREF. The relationship between the input peak-to-peak voltage and VREF is We do not recommend opertaing with input levels below 1 VP-P because the signal-to-noise ratio will degrade consid9 www.national.com ADC08351 Applications Information ADC08351 are required to meet data sheet limits. The analog and digital grounds may be in the same layer, but should be separated from each other and should never overlap each other. (Continued) the analog supply pin should be minimized, keeping it below 200 mVP-P at 100 kHz. Of course, higher frequency noise on the power supply should be even more severely limited. 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 well separated from the digital circuitry and from the digital ground plane. No pin should ever have a voltage on it that is in excess of the supply voltages. This can be a problem upon application of power to a circuit. Be sure that the supplies to circuits driving the CLK, OE, analog input and reference pins do not come up any faster than does the voltage at the ADC08351 power pins. 3.0 LAYOUT AND GROUNDING Proper grounding and proper routing of all signals is essential to ensure accurate conversion. Separate analog and digital ground planes that are connected beneath the DS100895-25 FIGURE 3. Layout example showing separate analog and digital ground planes connected below the ADC08351. 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. Generally, analog and digital lines should cross each other at 90 degrees to avoid getting digital noise into the analog path. To maximize accuracy in video (high frequency) systems, however, avoid crossing analog and digital lines altogether. Furthermore, it is important to keep any clock lines isolated from ALL other lines, including other digital lines. Even the generally accepted 90 degree crossing should be avoided as even a little coupling can cause problems at high frequencies. 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. www.national.com Figure 3 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. 4.0 DYNAMIC PERFORMANCE The ADC08351 is ac tested and its dynamic performance is guaranteed. To meet the published specifications, the clock source driving the CLK input must be free of jitter. For best 10 5.0 TYPICAL APPLICATION CIRCUITS (Continued) Figure 5 shows a simple interface for a low impedance source located close to the converter. As discussed in Section 1.0, the series capacitor is optional. Notice the isolation of the ADC clock signal from the clock signals going elsewhere in the system. The reference input of this circuit is shown connected to the 3V supply. Video ADCs tend to have input current transients that can upset a driving source, causing distortion of the driving signal. The resistor at the ADC08351 input isolates the amplifier’s output from the current transients at the input to the converter. When the signal source is not located close to the converter, the signal should be buffered. Figure 6 shows an example of an appropriate buffer. The amplifier provides a gain of two to compensate for transmission losses. Operational amplifiers have better linearity when they operate with gain, so the input is attenuated with the 68Ω and 30Ω resistors at the non-inverting input. The 330Ω resistor in parallel with these two resistors provides for a 75Ω cable termination. Replacing this 330Ω resistor with one of 100Ω will provide a 50Ω termination. The circuit shown has a nominal gain of two. You can provide a gain adjustment by changing the 110Ω feedback resistor to a 100Ω resistor in series with a 20Ω potentiometer. The offset adjustment is used to bring the input signal within the common mode range of the converter. If a fixed offset is desired, the potentiometer and the 3.3k resistor may be replaced with a single resistor of 3k to 4k to the appropriate supply. The resistor value and the supply polarity used will depend upon the amount and polarity of offset needed. The CLC409 shown in Figure 6 was chosen for a low cost solution with good overall performance. Figure 7 shows an inverting DC coupled circuit. The above comments regarding Figure 6 generally apply to this circuit as well. ac performance, isolating the ADC clock from any digital circuitry should be done with adequate buffers, as with a clock tree. See Figure 4. 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. Even lines with 90˚ crossings have capacitive coupling, so try to avoid even these 90˚ crossings of the clock line. DS100895-26 FIGURE 4. Isolating the ADC Clock from Digital Circuitry Digital circuits create substantial supply and ground current transients. The logic noise thus generated could have significant impact upon system noise performance. The best 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 74HC(T) and 74AC(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. In general, slower logic families, such as 74LS and 74HC(T) will produce less high frequency noise than do high speed logic families, such as the 74F and 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 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 variation across the impedance of this relatively narrow ground connection. 11 www.national.com ADC08351 Applications Information ADC08351 Applications Information (Continued) DS100895-27 FIGURE 5. AC Coupled Circuit for a Low Impedance Source Located Near the Converter DS100895-28 FIGURE 6. Non-inverting Input Circuit for Remote Signal Source www.national.com 12 ADC08351 Applications Information (Continued) DS100895-29 FIGURE 7. Inverting Circuit with Bias Adjust To ensure that the signal you are presenting to the ADC being evaluated is spectrally pure, use a bandpass filter between the signal generator and the ADC input. One such possible filter is the elliptic filter shown in Figure 8. ACCURATELY EVALUATING THE ADC If a signal that is spectrally impure is presented to the ADC, the output from the ADC cannot be pure. Nearly all signal generators in use today produce signals that are not spectrally pure enough to adequately evaluate present-day ADCs. This is especially true at higher frequencies and at high resolutions. DS100895-31 FIGURE 8. This elliptic filter has a cutoff frequency of about 11MHz and is suitable for input frequencies of 5MHz to 10MHz. It should be driven by a generator of 75Ω source impedance and teminated with 75Ω. This termination may be provided by the ADC evaluation circuit. 7.0 COMMON APPLICATION PITFALLS Driving the inputs (analog or digital) beyond the power supply rails. For proper operation, all inputs should not go more than 300 mV beyond the supply rails. That is, more than 300 mV below the ground pins or 300 mV 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 or above the power supply. Since these conditions are of very short duration with very fast rise and fall times, they can In addition to being used to eliminate undesired frequencies from a desired signal, this filter can be used to filter a square wave, reducing 3rd and higher harmonics to negligible levels. When evaluating dynamic performance of an ADC, repeatability of measurements could be a problem unless coherent sampling is used. and ADC08351 evaluation system is available that can simplify evaluation of thsi product. 13 www.national.com ADC08351 Applications Information Using an inadequate amplifier to drive the analog input. As explained in Section 2.0, the capacitance seen at the input alternates between 4 pF and 11 pF with the clock. This dynamic capacitance is more difficult to drive than a fixed capacitance, so care should be taken in choosing a driving device. The CLC409, CLC440, LM6152, LM6154, LM6181 and LM6182 are good devices for driving the ADC08351. Also, an amplifier with insufficient gain-bandwidth may limit the overall frequency response of the overall circuit. Using an operational amplifier in an insufficient gain configuration to drive the analog input. Operational amplifiers, while some may be unity gain stable, generally exhibit more distortion at low in-circuit gains than at higher gains. 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. Simple gates with RC timing is generally inadequate. Not considering the timing relationships, especially tOD. Timing is always important and gets more critical with higher speeds. If the output data is latched or looked at when that data is in transition, you may see excessive noise and distortion of the output signal. (Continued) inject noise into the system and may be difficult to detect with an oscilloscope. A resistor of about 50Ω to 100Ω in series with the offending digital input will usually eliminate the problem. Care should be taken not to overdrive the inputs of the ADC08351 (or any device) with a device that is powered from supplies outside the range of the ADC08351 supply. Such practice may lead to conversion inaccuracies and even to device damage. Attempting to drive a high capacitance digital data bus. The more capacitance the output drivers have to charge for each conversion, the more instantaneous digital current is required from VD and DGND. These large charging current spikes can couple into the analog section, degrading dynamic performance. While adequate bypassing and maintaining separate analog and digital ground planes will reduce this problem on the board, this coupling can still occur on the ADC08351 die. Buffering the digital data outputs (with a 74ACQ541, for example) may be necessary if the data bus to be driven is heavily loaded. Dynamic performance can also be improved by adding series resistors at each digital output, reducing the energy coupled back into the converter output pins by limiting the output slew rate. A reasonable value for these resistors is about 47Ω. www.national.com 14 ADC08351 8-Bit, 42 MSPS, 40 mW A/D Converter Physical Dimensions inches (millimeters) unless otherwise noted 20-Lead TSSOP Order Number ADC08351CIMTC NS Package Number MTC20 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. 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