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ADN2814 Datasheet(PDF) 12 Page - Analog Devices |
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ADN2814 Datasheet(HTML) 12 Page - Analog Devices |
12 / 28 page ADN2814 Data Sheet Rev. C | Page 12 of 28 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 12. 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 Level 1 and output Logic Level 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 may be either 1 or 0, or it may 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 12. 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 ADN2814 single-ended and observing the quantizer input with an oscilloscope probe at the point indicated in Figure 13 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 13, because both positive and negative offsets need to be accommodated, the sensitivity is twice the overdrive. The ADN2814 quantizer typically has 3.3 mV p-p sensitivity. SCOPE PROBE PIN 50 Ω 3k Ω 2.5V 50 Ω VREF ADN2814 QUANTIZER + – 10mV p-p VREF Figure 13. Single-Ended Sensitivity Measurement Driving the ADN2814 differentially (see Figure 14), 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 ADN2814 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 complementary 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 14. 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 AD2814 is 500 ns typical and the deassert time is 400 ns typical. 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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