AD6645(Rev0) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD6645 Datasheet PDF : 20 Pages
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AD6645
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.
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 0.41 LSB)
n = number of bits in the ADC
VNOISE rms = V rms thermal noise referred to the analog input of
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.”
80
75
70
AIN = 110MHz
65
AIN = 150MHz
AIN = 190MHz
60
AIN = 30MHz
AIN = 70MHz
55
0
0.1
0.2
0.3
0.4
0.5
0.6
JITTER – ps
Figure 12. Jitter vs. SNR
( ) SNR
È
= 1.76 - 20logÍ
ÎÍ
2p
¥
FANALOG
¥
t j rms
2
+
ÊËÁ
1+e
2n
ˆ¯˜
2
+
Ê
ËÁ
2
¥
1
2
¥ VNOISE
2n
rms
ˆ
¯˜
2
ù2
ú
ûú
–16–
REV. 0
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