AD9148 Analog Devices, AD9148 Datasheet - Page 56

• Manufacturer Part Number: AD9148
• Manufacturer: Analog Devices

Specifications of AD9148

• Resolution (bits): 16bit
• Dac Update Rate: 1GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9148
CLOCK GENERATION
DAC INPUT CLOCK CONFIGURATIONS
The
directly or by clock multiplying. Clock multiplying employs
the on-chip, phased-locked loop (PLL) that accepts a reference
clock (REFCLK_x) operating at a submultiple of the desired
DACCLK rate, most commonly the data input frequency.
The PLL then multiplies the reference clock up to the desired
DACCLK frequency, which can then be used to generate all the
internal clocks required by the DAC. The clock multiplier provides
a high quality clock that meets the performance requirements of
most applications. Using the on-chip clock multiplier removes
the burden of generating and distributing the high speed DACCLK.
The second mode bypasses the clock multiplier circuitry and allows
DACCLK to be sourced directly through the CLK_x pins. This
mode enables the user to source a very high quality clock directly
to the DAC core. Sourcing the DACCLK directly through the
CLK_x pins may be necessary in demanding applications that
require the lowest possible DAC output noise, particularly at
higher output frequencies.
AD9148
LVPECL
DRIVER
DAC sample clock (DACCLK) can be sourced
200Ω
200Ω
Figure 73. Clock Receiver Circuitry and Recommended Drive Circuitry using LVPECL (Left) and LVDS (Right)
1000pF
1000pF
100Ω
CLK_P/
REFCLK_P
CLK_N/
REFCLK_N
DAC
5kΩ
5kΩ
1.25V
Rev. B | Page 56 of 72
DRIVER
DRIVING THE CLK_x AND REFCLK_x INPUTS
The REFCLK_x and CLK_x differential inputs share similar
clock receiver input circuitry. Figure 1 shows a simplified circuit
diagram of the input, along with a recommended drive circuit.
The on-chip clock receiver has a differential input impedance of
about 10 kΩ. It is self-biased to a common-mode voltage of about
1.25 V. The recommended circuit for driving the input is a pair
of ac coupling capacitors and a differential 100 Ω termination.
The minimum input drive level to either of the clock inputs is
100 mV ppd. The optimal performance is achieved when the clock
input signal is between 500 mV ppd and 1.6 V ppd. Whether using
the on-chip clock multiplier or sourcing the DACCLK directly,
it is necessary that the input clock signal to the device have low
jitter and fast edge rates to optimize the DAC noise performance.
DIRECT CLOCKING
When a high quality, sample rate clock is connected to the AD9148,
it provides the lowest noise spectral density at the DAC outputs.
To select the differential CLK inputs as the source for the DAC
sampling clock, set the PLL enable bit to 0 (Register 0x0A, Bit 7).
Setting this bit to 0 powers down the internal PLL clock
multiplier and selects the input from the CLK_x pins as the
source for the internal DACCLK.
The device also has duty-cycle correction circuitry and differential
input level correction circuitry. Enabling these circuits may provide
improved performance in some cases. The control bits for these
functions can be found in Register 0x08.
LVDS
1000pF
1000pF
100Ω
CLK_P/
REFCLK_P
CLK_N/
REFCLK_N
DAC
5kΩ
5kΩ
1.25V
Data Sheet