REV. 0

AD6645

–16–

Jitter Considerations

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

When normalized to ADC codes, the above equation 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.

internal encode circuitry)

= average DNL of the ADC (typically 0.41 LSB)

n = number of bits in the ADC

the ADC (typically 0.9 LSB rms)

For a 14-bit analog-to-digital converter, like the AD6645, aperture

jitter can greatly affect the SNR performance as the analog

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

that demonstrate the expected SNR performance of the AD6645 as

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

For a complete discussion of aperture jitter, please consult

AN-501, “Aperture Uncertainty and ADC System Performance.”

JITTER – ps

55

00.1

60

65

70

75

80

0.2

0.3

0.4

0.5

0.6

AIN = 110MHz

AIN = 150MHz

AIN = 190MHz

AIN = 30MHz

AIN = 70MHz

Figure 12. Jitter vs. SNR

SNR

F

t

V

j rms

n

N

n

=-

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176 20

2

1

2

22

2

2

2

2

1

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.

log

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e

ANALOG

OISE rms