ADN2817 Analog Devices, Inc., ADN2817 Datasheet - Page 15

• Manufacturer Part Number: ADN2817
• Description: Continuous Rate 10 Mbps To 2.7gb/s Clock And Data Recovery Ics
• Manufacturer: Analog Devices, Inc.

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 23. 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
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
confidence level.
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 24 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 24, because both
positive and negative offsets need to be accommodated, the
sensitivity is twice the overdrive. The ADN2817 quantizer
typically has 5 mV p-p sensitivity. The ADN2818 does not have
a limiting amplifier at its input. The input sensitivity for the
ADN2818 is 200 mV p-p.
Figure 23. Input Sensitivity and Input Overdrive
OUTPUT
(2× OVERDRIVE)
SENSITIVITY
1
0
OVERDRIVE
OFFSET
NOISE
INPUT (V p-p)
−10
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Driving the ADN2817 differentially (see Figure 25), 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 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.
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 ADN2817 is 450 ns typically and the deassert time is 500 ns
typically. In practice, the time constant produced by the ac
coupling at the quantizer input and the 50 Ω on-chip input
termination determine the LOS response time.
VREF
VREF
5mV p-p
5mV p-p
Figure 24. Single-Ended Sensitivity Measurement
PIN
Figure 25. Differential Sensitivity Measurement
SCOPE
PROBE
VREF
NIN
PIN
10mV p-p
SCOPE
PROBE
VREF
50Ω
ADN2817/ADN2818
50Ω
+
QUANTIZER
–
3kΩ
50Ω
50Ω
QUANTIZER
+
–
VREF
2.5V
3kΩ
2.5V