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MAX1419 Datasheet(PDF) 12 Page - Maxim Integrated Products |
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MAX1419 Datasheet(HTML) 12 Page - Maxim Integrated Products |
12 / 18 page 15-Bit, 65Msps ADC with -79.3dBFS Noise Floor for Baseband Applications 12 ______________________________________________________________________________________ Applications Information Differential, AC-Coupled Clock Input The clock inputs to the MAX1419 are designed to be driven with an AC-coupled differential signal, and best performance is achieved under these conditions. However, it is often the case that the available clock source is single ended. Figure 5 demonstrates one method for converting a single-ended clock signal into a differential signal through a transformer. In this exam- ple, the transformer turns ratio from the primary to sec- ondary side is 1:1.414. The impedance ratio from primary to secondary is the square of the turns ratio, or 1:2, so that terminating the secondary side with a 100 Ω differential resistance results in a 50 Ω load looking into the primary side of the transformer. The termination resistor in this example comprises the series combina- tion of two 50 Ω resistors with their common node AC- coupled to ground. Alternatively, a single 100 Ω resistor across the two inputs with no common-mode connec- tion could be employed. In the example of Figure 5, the secondary side of the transformer is coupled directly to the clock inputs. Since the clock inputs are self-biasing, the center tap of the transformer must be AC-coupled to ground or left floating. If the center tap of the secondary were DC- coupled to ground, then it would be necessary to add blocking capacitors in series with the clock inputs. Clock jitter is generally improved if the clock signal has a high slew rate at the time of its zero crossing. Therefore, if a sinusoidal source is used to drive the clock inputs, it is desirable that the clock amplitude be as large as possible to maximize the zero-crossing slew rate. The back-to-back Schottky diodes shown in Figure 5 are not required as long as the input signal is held to 3VP-P differential or less. If a larger amplitude signal is provided (to maximize the zero-crossing slew rate), then the diodes serve to limit the differential sig- nal swing at the clock inputs. Any differential mode noise coupled to the clock inputs translates to clock jitter and degrades the SNR perfor- mance of the MAX1419. Any differential mode coupling of the analog input signal into the clock inputs results in harmonic distortion. Consequently, it is important that the clock lines be well isolated from the analog signal input and from the digital outputs. See the PC Board Layout Considerations sections for more discussion on noise coupling. Differential, AC-Coupled Analog Input The analog inputs (INP and INN) are designed to be dri- ven with a differential AC-coupled signal. It is extremely important that these inputs be accurately balanced. Any common-mode signal applied to these inputs degrade even-order distortion terms. Therefore, any attempt at driving these inputs in a single-ended fashion results in significant even-order distortion terms. Figure 6 presents one method for converting a single- ended signal to a balanced differential signal using a transformer. The primary-to-secondary turns ratio in this example is 1:1.414. The impedance ratio is the square of the turns ratio, so in this example, the impedance ratio is 1:2. In order to achieve a 50 Ω input impedance at the primary side of the transformer, the secondary side is terminated with a 112 Ω differential load. This load, in shunt with the differential input resistance of the MAX1419, results in a 100 Ω differential load on the sec- ondary side. It is reasonable to use a larger transformer turns ratio in order to achieve a larger signal step-up, and this may be desirable in order to relax the drive requirements for the circuitry driving the MAX1419. MAX1419 50 Ω 50 Ω 0.1 µF 0.1 µF 0.01 µF 0.1µF 0.01 µF BACK-TO-BACK DIODE T2-1T–KK81 15 D0–D14 AVCC DVCC DRVCC GND CLKP CLKN INP INN Figure 5. Transformer-Coupled Clock Input Configuration |
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Similar Description - MAX1419 |
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