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SPT7866SCR Datasheet(PDF) 6 Page - Cadeka Microcircuits LLC. |
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SPT7866SCR Datasheet(HTML) 6 Page - Cadeka Microcircuits LLC. |
6 / 8 page 6 11/20/01 SPT7866 DIFFERENTIAL CLOCK INPUT The SPT7866 clock can be driven differentially or single- ended. When driven differentially, CLK and CLK accommo- date differential sinusodial signals centered around VDD/2. The peak-to-peak value should be 0.8 V. In order to pre- serve accuracy at high input frequency, it is important that the clock have low jitter. The differential clock input is made to allow a low-jitter clock design. To ensure low jitter, the differential input should be a pure sine wave with low white noise floor. SINGLE-ENDED CLOCK INPUT For single-ended operation, the CLK node is internally biased to 1.5 V, and should externally be decoupled to ground by a capacitor. A CMOS logic level clock (5 V or 3 V) is applied at the CLK node. (To get an inverted clock input, CLK should be decoupled and the clock signal ap- plied at the CLK node). The duty cycle of the clock should be close to 50%. Consecutive pipeline stages in the ADC are clocked in antiphase. With a 50% duty cycle, every stage has the same time for settling. If the duty cycle devi- ates from 50%, every second stage has a shorter time for settling; thus it operates less accurately, causing degrada- tion of SNR. In order to preserve accuracy at high input frequency, it is important that the clock have low jitter and steep edges. Rise/fall times should be kept shorter than 2 ns whenever possible. Overshoot should be minimized. Low jitter is es- pecially important when converting high-frequency input signals. Jitter causes the noise floor to rise proportionally to input signal frequency. Jitter may be caused by crosstalk on the PCB. It is therefore recommended that the clock trace on the PCB be made as short as possible. DIGITAL OUTPUTS The digital output data appears in offset binary code at CMOS logic levels. Full-scale negative input results in out- put code 000...0. Full-scale positive input results in output code 111...1. Output data is available 6 clock cycles after the data is sampled. The analog input is sampled one aperture delay (tAP) after the high-to-low clock transition. Output data should be sampled as shown in the timing dia- gram (figure 5). The OR pin is an out-of-range pin; if the outputs go either over or under range, OR is set high. PCB LAYOUT AND DECOUPLING A well designed PCB is necessary to get good spectral purity from any high-performance ADC. A multilayer PCB with a solid ground plane is recommended for optimum performance. If the system has a split analog and digital ground plane, it is recommended that all ground pins on the ADC be connected to the analog ground plane. It is our experience that this gives the best performance. The power supply pins should be bypassed using 100 nF surface mounted capacitors as close to the package pins as possible. Analog and digital supply pins should be separately filtered. Figure 3 – Driving Differential Inputs with a Differential Configuration Figure 4 – Driving Differential Inputs with a Single-Ended Configuration VIHD VICM VILD VID VIH VICM VIL N1 N N+1 N+2 N+3 Data N1 Data N Data N+1 Data N+2 Data Clock tAP tH tD AIN Clock Figure 5 – Timing Diagram |
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