AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 18

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

AD9775
NOTES ON SERIAL PORT OPERATION
The AD9775 serial port configuration bits reside in Bits 6 and 7
of Register Address 00h. It is important to note that the configura-
tion changes immediately upon writing to the last bit of the register.
For multibyte transfers, writing to this register may occur dur-
ing the middle of the communication cycle. Care must be taken
to compensate for this new configuration for the remaining
bytes of the current communication cycle.
The same considerations apply to setting the reset bit in Register
Address 00h. All other registers are set to their default values, but
the software reset doesn’t affect the bits in Register Address 00h.
It is recommended to use only single-byte transfers when chang-
ing serial port configurations or initiating a software reset.
A write to Bits 1, 2, and 3 of Address 00h with the same logic
levels as for Bits 7, 6, and 5 (bit pattern: XY1001YX binary)
allows the user to reprogram a lost serial port configuration and
to reset the registers to their default values. A second write to
Address 00h with reset bit low and serial port configuration as
specified above (XY) reprograms the OSC IN multiplier set-
ting. A changed f
200 f
MCLK
cycles (equals wake-up time).
SYSCLK
SCLK
SCLK
SDIO
SDIO
SDO
CS
CS
frequency is stable after a maximum of
Figure 5. Timing Diagram for Register Read from AD9775
Figure 4. Timing Diagram for Register Write to AD9775
t
DS
t
DS
INSTRUCTION BIT 7
DATA BIT N
t
PWH
t
DH
t
DV
t
SCLK
–18–
t
PWL
DAC OPERATION
The dual 14-bit DAC output of the AD9775, along with the
reference circuitry, gain, and offset registers, is shown in Figure 6.
Referring to the transfer functions in Equation 1, a reference
current is set by the internal 1.2 V reference, the external R
resistor, and the values in the coarse gain register. The fine gain
DAC subtracts a small amount from this and the result is input
to IDAC and QDAC, where it is scaled by an amount equal
to 1024/24. Figures 7a and 7b show the scaling effect of the
coarse and fine adjust DACs. IDAC and QDAC are PMOS
current source arrays, segmented in a 5-4-5 configuration. The
five most significant bits control an array of 31 current sources.
The next four bits consist of 15 current sources whose values
are all equal to 1/16 of an MSB current source. The five LSBs
are binary weighted fractions of the middle bit’s current sources.
All current sources are switched to either I
ing on the input code.
The fine adjustment of the gain of each channel allows for
improved balance of QAM modulated signals, resulting in
improved modulation accuracy and image rejection. In the
Applications section of this data sheet, performance data is
included that shows to what degree image rejection can be im-
proved when the AD9775 is used with an AD8345 quadrature
modulator from ADI.
INSTRUCTION BIT 6
DATA BIT N–1
OUTA
or I
OUTB
, depend-
REV. 0
SET