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AD10465BZ Arkusz danych(PDF) 10 Page - Analog Devices |
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AD10465BZ Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 20 page REV. 0 AD10465 –10– AD10465 on any input by using the other inputs as alternate locations for GND or an external resistor. The following chart summarizes the impedance options available at each input location: AIN1 = 100 Ω when A IN2 and AIN3 are open. AIN1 = 75 Ω when AIN3 is shorted to GND. AIN1 = 50 Ω when AIN2 is shorted to GND. AIN2 = 200 Ω when A IN3 is open. AIN2 = 100 Ω when AIN3 is shorted to GND. AIN2 = 75 Ω when AIN2 to AIN3 has an external resistor of 300 Ω, with AIN3 shorted to GND. AIN2 = 50 Ω when AIN2 to AIN3 has an external resistor of 100 Ω, with AIN3 shorted to GND. AIN3 = 400 Ω. AIN3 = 100 Ω when A IN3 has an external resistor of 133 Ω to GND. AIN3 = 75 Ω when A IN3 has an external resistor of 92 Ω to GND. AIN3 = 50 Ω when A IN3 has an external resistor of 57 Ω to GND. APPLYING THE AD10465 Encoding the AD10465 The AD10465 encode signal must be a high quality, extremely low phase noise source, to prevent degradation of performance. Maintaining 14-bit accuracy places a premium on encode clock phase noise. SNR performance can easily degrade by 3 dB to 4 dB with 32 MHz input signals when using a high-jitter clock source. See Analog Devices’ Application Note AN-501, “Aper- ture Uncertainty and ADC System Performance” for complete details. For optimum performance, the AD10465 must be clocked differentially. The encode signal is usually ac-coupled into the ENCODE and ENCODE pins via a transformer or capacitors. These pins are biased internally and require no additional bias. Shown below is one preferred method for clocking the AD10465. The clock source (low jitter) is converted from single-ended to differential using an RF transformer. The back-to-back Schottky diodes across the transformer secondary limit clock excursions into the AD10465 to approximately 0.8 V p-p differential. This helps prevent the large voltage swings of the clock from feeding through to the other portions of the AD10465, and limits the noise presented to the ENCODE inputs. A crystal clock oscillator can also be used to drive the RF transformer if an appropriate limiting resistor (typically 100 Ω) is placed in the series with the primary. T1-4T 100 0.1nF ENCODE ENCODE AD10465 HSMS2812 DIODES CLOCK SOURCE Figure 6. Crystal Clock Oscillator, Differential Encode If a low jitter ECL/PECL clock is available, another option is to ac-couple a differential ECL/PECL signal to the encode input pins as shown below. A device that offers excellent jitter perfor- mance is the MC100LVEL16 (or same family) from Motorola. ENCODE ENCODE AD10465 0.1 F ECL/ PECL VT VT 0.1 F Figure 7. Differential ECL for Encode Jitter Considerations The signal-to-noise ratio (SNR) for an ADC can be predicted. When normalized to ADC codes, Equation 1 accurately predicts the SNR based on three terms. These are jitter, average DNL error, and thermal noise. Each of these terms contributes to the noise within the converter. SNR f t rms V N ANALOG NOISE RMS N =− × + + ×× × () + 20 1 2 2 2 2 2 12 log / ε π J (1) fANALOG = analog input frequency. tJ RMS = rms jitter of the encode (rms sum of encode source and internal encode circuitry). ε = average DNL of the ADC (typically 0.50 LSB). N = Number of bits in the ADC. VNOISE RMS = V rms noise referred to the analog input of the ADC (typically 5 LSB). For a 14-bit analog-to-digital converter like the AD10465, aper- ture jitter can greatly affect the SNR performance as the analog frequency is increased. The chart below shows a family of curves that demonstrates the expected SNR performance of the AD10465 as jitter increases. The chart is derived from the above equation. For a complete discussion of aperture jitter, please consult Analog Devices’ Application Note AN-501, “Aperture Uncer- tainty and ADC System Performance.” RMS CLOCK JITTER – ps 0.1 60 AIN = 5MHz AIN = 10MHz AIN = 20MHz AIN = 32MHz 0.3 0.5 0.9 1.3 1.7 2.1 2.5 2.9 3.3 3.7 0.7 1.1 1.5 1.9 2.3 2.7 3.1 3.5 3.9 61 62 63 64 65 66 67 68 69 70 71 Figure 8. SNR vs. Jitter |
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