ADN2804 Просмотр технического описания (PDF) - Analog Devices

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ADN2804

622 Mbps Clock and Data Recovery IC with Integrated Limiting Amplifier

Analog Devices 

ADN2804

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; 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.

OUTPUT

1

NOISE

0

OFFSET

OVERDRIVE

INPUT (V p-p)

SENSITIVITY

(2 × OVERDRIVE)

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 ADN2804 in a single-

ended fashion 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, the sensitivity is

twice the overdrive because both positive and negative offsets

need to be accommodated. The ADN2804 quantizer typically

has 3.3 mV p-p sensitivity.

Data Sheet

SCOPE

PROBE

PIN

10mV p-p

VREF

ADN2804

+

QUANTIZER

–

VREF

50Ω 50Ω

3kΩ

2.5V

Figure 13. Single-Ended Sensitivity Measurement

When the ADN2804 is driven differentially (see Figure 14),

sensitivity seems to improve if 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

ADN2804 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.

5mV p-p

VREF

SCOPE

PROBE

PIN

+

QUANTIZER

NIN

–

VREF

5mV p-p

VREF

50Ω 50Ω

3kΩ

2.5V

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 AD2804 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.

Rev. D | Page 12 of 24

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