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ADN2817 Datasheet(PDF) 15 Page - Analog Devices

No. de Pieza. ADN2817
Descripción  Continuous Rate 12.3Mb/s to 2.7Gb/s Clock and Data Recovery ICs
Descarga  35 Pages
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Fabricante  AD [Analog Devices]
Página de inicio  http://www.analog.com
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ADN2817 Datasheet(HTML) 15 Page - Analog Devices

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Preliminary Technical Data
ADN2817/ADN2818
Rev.Pr A | Page 15 of 35
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 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 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 ADN2817/ADN2818
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 ADN2817/ADN2818
quantizer typically has 6 mV p-p sensitivity.
PIN
50
50
VREF
Quantizer
+
-
VREF
10mVp-p
scope
probe
3k
2.5V
Figure 13. Single-Ended Sensitivity Measurement
Driving the ADN2817/ADN2818 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
ADN2817/ADN2818 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 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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