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Understanding the Effect of Cl
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2009-09-23
百灵千岛酱
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Understanding the Effect of Clock Jitter on High Speed ADCs:Digitizing high speed signals to a high resolution requires
careful selection of a clock that will not compromise the
sampling performance of the Analog to Digital Converter
(ADC). In this article we hope to give the reader a better
understanding of clock jitter and how it affects the
performance of the high speed ADC.
As an example we will highlight the latest high performance
ADC from Linear Technology, the 16-bit, 160Msps
LTC2209. This ADC exhibits a signal to noise ratio (SNR)
of 77.4dB, with 100dB SFDR throughout much of the
baseband region. Like most high speed ADCs on the
market today, the LTC2209 uses a sample-and-hold (S&H)
circuit that essentially takes a snapshot of the ADC input
at an instant in time. When the S&H switch is closed,
the network at the input of the ADC is connected to the
sample capacitor. At the instant the switch is opened
one half clock cycle later, the voltage on the capacitor
is recorded and held. Variation in the time at which the
switch is opened is known as aperture uncertainty, or jitter,
and will result in an error voltage that is proportional
to the magnitude of the jitter and the input signal slew rate. In other words, the greater the input frequency and amplitude, the more susceptible you are to jitter on the clock source. Figure 1 demonstrates this relationship of slew rate proportional to jitter.
careful selection of a clock that will not compromise the
sampling performance of the Analog to Digital Converter
(ADC). In this article we hope to give the reader a better
understanding of clock jitter and how it affects the
performance of the high speed ADC.
As an example we will highlight the latest high performance
ADC from Linear Technology, the 16-bit, 160Msps
LTC2209. This ADC exhibits a signal to noise ratio (SNR)
of 77.4dB, with 100dB SFDR throughout much of the
baseband region. Like most high speed ADCs on the
market today, the LTC2209 uses a sample-and-hold (S&H)
circuit that essentially takes a snapshot of the ADC input
at an instant in time. When the S&H switch is closed,
the network at the input of the ADC is connected to the
sample capacitor. At the instant the switch is opened
one half clock cycle later, the voltage on the capacitor
is recorded and held. Variation in the time at which the
switch is opened is known as aperture uncertainty, or jitter,
and will result in an error voltage that is proportional
to the magnitude of the jitter and the input signal slew rate. In other words, the greater the input frequency and amplitude, the more susceptible you are to jitter on the clock source. Figure 1 demonstrates this relationship of slew rate proportional to jitter.
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