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ASM5I961P Datasheet(PDF) 7 Page - Alliance Semiconductor Corporation |
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ASM5I961P Datasheet(HTML) 7 Page - Alliance Semiconductor Corporation |
7 / 14 page July 2005 ASM5I961P rev 0.2 Low Voltage Zero Delay Buffer 7 of 14 Notice: The information in this document is subject to change without notice. a step in the waveform, this step is caused by the impedance mismatch seen looking into the driver. The parallel combination of the 36 Ω series resistor plus the output impedance does not match the parallel combination of the line impedances. The voltage wave launched down the two lines will equal: VL = VS ( Zo / (Rs + Ro +Zo)) Zo = 50 Ω || 50Ω Rs = 36 Ω || 36Ω Ro = 14 Ω VL = 3.0 (25 / (18 + 14 + 25) = 3.0 (25 / 57) = 1.31V At the load end the voltage will double, due to the near unity reflection coefficient, to 2.62V. It will then increment towards the quiescent 3.0V in steps separated by one round trip delay (in this case 4.0nS). Figure 5. Single versus Dual Waveforms Since this step is well above the threshold region it will not cause any false clock triggering, however designers may be uncomfortable with unwanted reflections on the line. To better match the impedances when driving multiple lines the situation in Figure 6. should be used. In this case the series terminating resistors are reduced such that when the parallel combination is added to the output buffer impedance the line impedance is perfectly matched. 14 Ω + 22Ω || 22Ω = 50Ω || 50Ω 25 Ω = 25Ω Figure 6. Optimized Dual Line Termination Using the ASM5I961P in zero-delay applications Nested clock trees are typical applications for the ASM5I961P. Designs using the ASM5I961P, as LVCMOS PLL fanout buffer with zero insertion delay will show significantly lower clock skew than clock distributions developed from CMOS fanout buffers. The external feedback option of the ASM5I961P clock driver allows for its use as a zero delay buffer. By using the QFB output as a feedback to the PLL the propagation delay through the device is virtually eliminated. The PLL aligns the feedback clock output edge with the clock input reference edge resulting a near zero delay through the device. The maximum insertion delay of the device in zero-delay applications is measured between the reference clock input and any output. This effective delay consists of the static phase offset, I/O jitter (phase or long-term jitter), feedback path delay and the output-to-output skew error relative to the feedback output. Calculation of part-to-part skew The ASM5I961P zero delay buffer supports applications where critical clock signal timing can be maintained across several devices. If the reference clock inputs of two or more ASM5I961P are connected together, the maximum overall timing uncertainty from the common PCLK input to any output is: tSK(PP) = t(φ) + tSK(O) + tPD, LINE(FB) + tJIT(φ) ⋅ CF This maximum timing uncertainty consist of 4 components: static phase offset, output skew, feedback board trace delay and I/O (phase) jitter: Figure 7. ASM5I961P max. device-to-device skew Due to the statistical nature of I/O jitter a rms value (1 σ) is specified. I/O jitter numbers for other confidence factors (CF) can be derived from Table 8. ASM5I961P OUTPUT BUFFER 14Ω IN Z0=50Ω RS=22Ω Z0=50Ω RS=22Ω 3.0 2.5 2.0 1.5 1.0 0.5 0 2 4 6 8 10 12 14 TIME (nS) In OutA tD = 3.8956 OutB tD = 3.9386 QFBDevice 1 Any QDevice 1 QFBDevice 2 Any QDevice 2 Max. skew -t(Ø) tPD,LINE (FB) tJIT(Ø) +tSK(O) +t(Ø tJIT(Ø) +tSK(O) tSK(PP) PCLKCommon |
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