AD10465PCB datasheet(10/20 Pages) AD

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REV. 0

AD10465

–10–

AD10465 on any input by using the other inputs as alternate

locations for GND or an external resistor. The following chart

summarizes the impedance options available at each input

location:

AIN1 = 100

Ω when A

IN2 and AIN3 are open.

AIN1 = 75

Ω when AIN3 is shorted to GND.

AIN1 = 50

Ω when AIN2 is shorted to GND.

AIN2 = 200

Ω when A

IN3 is open.

AIN2 = 100

Ω when AIN3 is shorted to GND.

AIN2 = 75

Ω when AIN2 to AIN3 has an external resistor of

300

Ω, with AIN3 shorted to GND.

AIN2 = 50

Ω when AIN2 to AIN3 has an external resistor of

100

Ω, with AIN3 shorted to GND.

AIN3 = 400

Ω.

AIN3 = 100

Ω when A

IN3 has an external resistor of 133

Ω to

GND.

AIN3 = 75

Ω when A

IN3 has an external resistor of 92

Ω to

GND.

AIN3 = 50

Ω when A

IN3 has an external resistor of 57

Ω to

GND.

APPLYING THE AD10465

Encoding the AD10465

The AD10465 encode signal must be a high quality, extremely

low phase noise source, to prevent degradation of performance.

Maintaining 14-bit accuracy places a premium on encode clock

phase noise. SNR performance can easily degrade by 3 dB to

4 dB with 32 MHz input signals when using a high-jitter clock

source. See Analog Devices’ Application Note AN-501, “Aper-

ture Uncertainty and ADC System Performance” for complete

details. For optimum performance, the AD10465 must be clocked

differentially. The encode signal is usually ac-coupled into the

ENCODE and

ENCODE pins via a transformer or capacitors.

These pins are biased internally and require no additional bias.

Shown below is one preferred method for clocking the AD10465.

The clock source (low jitter) is converted from single-ended to

differential using an RF transformer. The back-to-back Schottky

diodes across the transformer secondary limit clock excursions

into the AD10465 to approximately 0.8 V p-p differential. This

helps prevent the large voltage swings of the clock from feeding

through to the other portions of the AD10465, and limits the

noise presented to the ENCODE inputs. A crystal clock oscillator

can also be used to drive the RF transformer if an appropriate

limiting resistor (typically 100

Ω) is placed in the series with

the primary.

T1-4T

100

0.1nF

ENCODE

AD10465

HSMS2812

DIODES

CLOCK

SOURCE

Figure 6. Crystal Clock Oscillator, Differential Encode

If a low jitter ECL/PECL clock is available, another option is to

ac-couple a differential ECL/PECL signal to the encode input

pins as shown below. A device that offers excellent jitter perfor-

mance is the MC100LVEL16 (or same family) from Motorola.

ENCODE

AD10465

0.1 F

ECL/

PECL

0.1 F

Figure 7. Differential ECL for Encode

Jitter Considerations

The signal-to-noise ratio (SNR) for an ADC can be predicted.

When normalized to ADC codes, Equation 1 accurately predicts

the SNR based on three terms. These are jitter, average DNL

error, and thermal noise. Each of these terms contributes to the

noise within the converter.

SNR

t rms

ANALOG

NOISE RMS

=− ×









××

() +





















log

(1)

fANALOG

= analog input frequency.

tJ RMS

= rms jitter of the encode (rms sum of encode

source and internal encode circuitry).

= average DNL of the ADC (typically 0.50 LSB).

= Number of bits in the ADC.

VNOISE RMS = V rms noise referred to the analog input of the

ADC (typically 5 LSB).

For a 14-bit analog-to-digital converter like the AD10465, aper-

ture jitter can greatly affect the SNR performance as the analog

frequency is increased. The chart below shows a family of curves

that demonstrates the expected SNR performance of the AD10465

as jitter increases. The chart is derived from the above equation.

For a complete discussion of aperture jitter, please consult

Analog Devices’ Application Note AN-501, “Aperture Uncer-

tainty and ADC System Performance.”

RMS CLOCK JITTER – ps

0.1

AIN = 5MHz

AIN = 10MHz

AIN = 20MHz

AIN = 32MHz

0.3

0.5

0.9

1.3

1.7

2.1

2.5

2.9

3.3

3.7

0.7

1.1

1.5

1.9

2.3

2.7

3.1

3.5

3.9

Figure 8. SNR vs. Jitter

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