ADN2865 Analog Devices, Inc., ADN2865 Datasheet - Page 16

• Manufacturer Part Number: ADN2865
• Description: Continuous Rate 12.3mb/s To 2.7gb/s Clock And Data Recovery Ic W/loop Timed Serdes
• Manufacturer: Analog Devices, Inc.

ADN2865
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
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.
Figure 14. Input Sensitivity and Input Overdrive
OUTPUT
(2
SENSITIVITY
×
OVERDRIVE)
1
0
OVERDRIVE
OFFSET
−10
confidence level.
NOISE
INPUT (V p-p)
Rev. PrA | Page 16 of 33
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.
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.
VREF
VREF
5mV p-p
5mV p-p
Figure 15. Single-Ended Sensitivity Measurement
PIN
Figure 16. Differential Sensitivity Measurement
scope
probe
VREF
Preliminary Technical Data
PIN
NIN
10mVp-p
SCOPE
PROBE
VREF
50
50
3k
Quantizer
+
-
50Ω
50Ω
2.5V
QUANTIZER
+
–
VREF
3kΩ
2.5V