ADN2817 datasheet(15/35 Pages) AD

15 / 35 page

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

× 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 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.

VREF

Quantizer

VREF

10mVp-p

scope

probe

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

2.5V

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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