REV. A

ADN2811

–7–

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 distribu-

tion. 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 to thus 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 4. 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 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

0

1

INPUT (V p-p)

OUTPUT

NOISE

SENSITIVITY

(2

OVERDRIVE)

OFFSET

OVERDRIVE

Figure 4. Input Sensitivity and Input Overdrive

SINGLE-ENDED VS. DIFFERENTIAL

AC-coupling is typically used to drive the inputs to the quan-

tizer. 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 5 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 4, since both positive and negative offsets need to be

accommodated, the sensitivity is twice the overdrive.

50

50

QUANTIZER

+

ADN2811

VREF

PIN

SCOPE

PROBE

VREF

10mV p-p

Figure 5. Single-Ended Sensitivity Measurement

50

50

QUANTIZER

+

ADN2811

VREF

NIN

PIN

SCOPE

PROBE

VREF

5mV p-p

Figure 6. Differential Sensitivity Measurement

Driving the ADN2811 differentially (see Figure 6), 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 mea-

sures only half the signal. The true quantizer input signal is

twice this value since the other quantizer input is a complemen-

tary signal to the signal being observed.

LOS Response Time

The LOS response time is the delay between the removal of

the input signal and the indication of loss of signal (LOS) at

SDOUT. The LOS response time of the ADN2811 is 300 ns

typ when the inputs are dc-coupled. In practice, the time con-

stant of the ac-coupling at the quantizer input determines the

LOS response time.