REV. 0

AD13280

–10–

or AMP-IN-B-1 when an input of

±0.5 V full scale is desired. Use

AMP-IN-A-2 or AMP-IN-B-2 when

±1 V full scale is desired.

Each channel has an AMP-OUT which must be tied to either a

noninverting or inverting input of a differential amplifier with the

remaining input grounded. For example, Side A, AMP-OUT-A

(Pin 6) must be tied to A+IN (Pin 5) with A–IN (Pin 5) tied to

ground for noninverting operation or AMP-OUT-A (Pin 6) tied

to A–IN (Pin 4) with A+IN (Pin 5) tied to ground for inverting

operation.

USING THE DIFFERENTIAL INPUT

Each channel of the AD13280 was designed with two optional

differential inputs, A+IN, A–IN and B+IN, B–IN. The inputs

provide system designers with the ability to bypass the AD8037

amplifier and drive the AD8138 directly. The AD8138 differen-

tial ADC driver can be deployed in either a single-ended or

differential input configuration. The differential analog inputs

have a nominal input impedance of 620

Ω and nominal full-

scale input range of 1.2 V p-p. The AD8138 amplifier drives a

differential filter and the custom analog-to-digital converter. The

differential input configuration provides the lowest even-order

harmonics and signal-to-noise (SNR) performance improvement

of up to 3 dB (SNR = 73 dBFS). Exceptional care was taken in

the layout of the differential input signal paths. The differential

input transmission line characteristics are matched and balanced.

Equal attention to system level signal paths must be provided in

order to realize significant performance improvements.

APPLYING THE AD13280

Encoding the AD13280

The AD13280 encode signal must be a high quality, extremely

low phase noise source, to prevent degradation of performance.

Maintaining 12-bit accuracy at 80 MSPS places a premium on

encode clock phase noise. SNR performance can easily degrade

3 dB to 4 dB with 37 MHz input signals when using a high-jitter

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

“Aperture Uncertainty and ADC System Performance” for

complete details. For optimum performance, the AD13280 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 addi-

tional bias.

Shown below is one preferred method for clocking the AD13280.

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 AD13280 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 AD13280, 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

limited resistor (typically 100

Ω) is placed in the series with

the primary.

T1-4T

100

0.1 F

ENCODE

ENCODE

AD13280

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

ENCODE

AD13280

0.1 F

ECL/

PECL

VT

VT

0.1 F

Figure 7. Differential ECL for Encode

Jitter Consideration

The signal-to-noise ratio (SNR) for any 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

f

t

V

N

ANALOG

RMS

NOISE RMS

N

=×

+

()

















+× ×

×

+

























–

log

(

)

/

20

1

2

2

2

2

2

12

ε

π

J

(1)

= analog input frequency

= rms jitter of the encode (rms sum of encode

source and internal encode circuitry)

ε

= average DNL of the ADC (typically 0.50 LSB)

N

= Number of bits in the ADC

ADC (typically 5 LSB)

For a 12-bit analog-to-digital converter like the AD13280, 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 AD13280

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

For a complete discussion of aperture jitter, please consult Ana-

log Devices’ Application Note AN-501, “Aperture Uncertainty

and ADC System Performance.”

CLOCK JITTER – ps

60

61

62

63

64

65

66

67

68

69

70

71

59

58

Figure 8. SNR vs. Jitter