National Semiconductor ADS54J69, Руководство пользователя

Страница: 14 / 21

Alternate Hardware Configurations
www.ti.com
14 SLAU662B – January 2016 – Revised May 2016
Submit Documentation Feedback
Copyright © 2016, Texas Instruments Incorporated
ADS54J69EVM
To turn off the ADC clock provided by the LMK04828 to reduce switching noise, click on the LMK04828
tab, then click on Clock Outputs tab, then select Powerdown for DCLK Type under CLKout 2 and 3 , as
shown in Figure 9 .
Figure 9. LMK04828 Clock Outputs Tab
5.1.2 External LMK04828 Clock (Clock Distribution Mode)
The LMK04828 can be used as a clock distributor. In this case, the LMK04828 uses in input clock source
from LMK_CLKIN1 SMA connector (J6). SJP2 (XO_PWR) can be left open to turn off the onboard VCXO
to avoid crosstalk. To use this mode, load the configuration file named
LMK_Config_External_Clock.cfg .
This mode allows generation of frequencies that are not possible with the LMK when using the on-board
VCXO.
5.1.3 Clock Generator Using Onboard VCXO
The LMK04828 is used as a clock generator using the onboard 122.88 MHz VCXO. SJP2 must be shorted
to turn on the onboard VCXO. The internal PLLs of the LMK04828 can be used with the onboard VCXO to
generate the desired frequencies. To use this mode, load one of the configuration files named
LMK_Config_Onboard_xxxx_MSPS.cfg , where xxxx corresponds to the desired ADC sampling rate. A
10-MHz signal can be brought into the LMK_CLKIN1 input to synchronize to external instruments. This is
the board default mode of operation.
5.2 Analog Input Options
The ADS54J69EVM allows for a differential analog input configuration in addition to the default using the
single-ended transformer-coupled input. This option is described in the following section.
5.2.1 Differential Input
The analog input transformers can be bypassed in favor of a differential input source. This allows for a
wider range of input frequencies, including the possibility of DC coupling. To configure the EVM for a
differential analog input on Channel A, remove C6, C7, and R7 and install R3, R4, C1, and C3. For
channel B, remove R8, C14, and C15 and install R21, R22, C12, and C13. All resistor values are 0 Ohms
and capacitor values 0.1uF. For a DC-coupled application, swap the series capacitors with 0-
Ω resistors.
The input signal must be biased to the required ADC input common mode voltage.