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ADN2865 Datasheet(PDF) 16 Page - Analog Devices |
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ADN2865 Datasheet(HTML) 16 Page - Analog Devices |
16 / 33 page ADN2865 Preliminary Technical Data Rev. PrA | Page 16 of 33 TERMINOLOGY Input Sensitivity and Input Overdrive Sensitivity and overdrive specifications for the quantizer involve offset voltage, gain, and noise. The relationship between the logic output of the quantizer and the analog voltage input is shown in Figure . For sufficiently large positive input voltage, the output is always Logic 1 and, similarly for negative inputs, the output is always Logic 0. However, the transitions between output Logic Levels 1 and 0 are not at precisely defined input voltage levels, but occur over a range of input voltages. Within this range of input voltages, the output might be either 1 or 0, or it might even fail to attain a valid logic state. The width of this zone is determined by the input voltage noise of the quantizer. The center of the zone is the quantizer input offset voltage. Input overdrive is the magnitude of signal required to guarantee the correct logic level with 1 × 10−10 confidence level. NOISE OUTPUT INPUT (V p-p) OFFSET OVERDRIVE SENSITIVITY (2 × OVERDRIVE) 1 0 Figure 14. Input Sensitivity and Input Overdrive Single-Ended vs. Differential AC coupling is typically used to drive the inputs to the quantizer. The inputs are internally dc biased to a common- mode potential of ~2.5 V. Driving the ADN2865 single-ended and observing the quantizer input with an oscilloscope probe at the point indicated in Figure shows a binary signal with an average value equal to the common-mode potential and instantaneous values both above and below the average value. It is convenient to measure the peak-to-peak amplitude of this signal and call the minimum required value the quantizer sensitivity. Referring to Figure , because both positive and negative offsets need to be accommodated, the sensitivity is twice the overdrive. The ADN2865 quantizer typically has 6 mV p-p sensitivity. PIN 50 50 VREF Quantizer + - VREF 10mVp-p scope probe 3k 2.5V Figure 15. Single-Ended Sensitivity Measurement Driving the ADN2865 differentially (see Figure ), sensitivity seems to improve from observing the quantizer input with an oscilloscope probe. This is an illusion caused by the use of a single-ended probe. A 5 mV p-p signal appears to drive the ADN2865 quantizer. However, the single-ended probe measures only half the signal. The true quantizer input signal is twice this value, because the other quantizer input is a comple- mentary signal to the signal being observed. SCOPE PROBE PIN 50 Ω 3k Ω 2.5V 50 Ω VREF QUANTIZER + – NIN 5mV p-p VREF 5mV p-p VREF Figure 16. Differential Sensitivity Measurement LOS Response Time LOS response time is the delay between removal of the input signal and indication of loss of signal (LOS) at the LOS output, Pin 22. When the inputs are dc-coupled, the LOS assert time of the AD2817 is 500 ns typically and the de-assert time is 400 ns typically,. In practice, the time constant produced by the ac coupling at the quantizer input and the 50 Ω on-chip input termination determines the LOS response time. |
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