ADN2811 datasheet(8/20 Pages) AD

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ADN2811

Rev. B | Page 8 of 20

DEFINITION OF TERMS

MAXIMUM, MINIMUM, AND TYPICAL

SPECIFICATIONS

Specifications for every parameter are derived from statistical

analyses of data taken on multiple devices from multiple wafer

lots. Typical specifications are the mean of the distribution of

the data for that parameter. If a parameter has a maximum (or a

minimum), that value is calculated by adding to (or subtracting

from) the mean six times the standard deviation of the

distribution. This procedure is intended to tolerate production

variations. If the mean shifts by 1.5 standard deviations, the

remaining 4.5 standard deviations still provide a failure rate of

only 3.4 parts per million. For all tested parameters, the test

limits are guardbanded to account for tester variation and

therefore guarantee that no device is shipped outside of data

sheet specifications.

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 6. For a 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 Levels 1 and 0 are not at precisely defined input

voltage levels but occur over a range of input voltages. Within

this zone of confusion, 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 of confusion 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.

INPUT (V p-p)

OUTPUT

NOISE

SENSITIVITY

× OVERDRIVE)

OFFSET

OVERDRIVE

Figure 6. 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 ~0.6 V. Driving the ADN2811 single-ended

and observing the quantizer input with an oscilloscope probe at

the point indicated in Figure 7 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 6, since both positive and

negative offsets need to be accommodated, the sensitivity is

twice the overdrive.

Ω

QUANTIZER

ADN2811

VREF

SCOPE

PROBE

VREF

10mV p-p

Figure 7. Single-Ended Sensitivity Measurement

Ω

QUANTIZER

ADN2811

VREF

NIN

SCOPE

PROBE

VREF

5mV p-p

Figure 8. Differential Sensitivity Measurement

Driving the ADN2811 differentially (see Figure 8), sensitivity

seems to improve by 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

ADN2811 quantizer. However, the single-ended probe measures

only half the signal. The true quantizer input signal is twice this

value since the other quantizer input is complementary to the

signal being observed.

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