AD9726 Analog Devices, AD9726 Datasheet - Page 20

• Manufacturer Part Number: AD9726
• Manufacturer: Analog Devices

Specifications of AD9726

• Resolution (bits): 16bit
• Dac Update Rate: 400MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9726
MSB/LSB Transfers
The SPI can support both MSB- and LSB-justified serial data
byte formats. This functionality is determined by Bit 6 in SPI
Register 0x00. This bit defaults low, which is MSB justification.
In this mode, serial data bits are written to and/or read from
registers sequentially from Bit 7 to Bit 0.
If Bit 6 of SPI Register 0x00 is set high, the controller switches
to LSB justification. In this mode, data bits are written to or
read from registers sequentially from Bit 0 to Bit 7. Writing to
the instruction bytes is also affected by the active justification.
For multibyte transfers with MSB justification, the address in
the instruction byte is interpreted as a final address, and its value
is decremented automatically by the controller. For multibyte
transfers with LSB justification, the address in the instruction
byte is interpreted as an initial address, and its value is incremented
automatically by the controller.
Care must be exercised when switching from MSB to LSB
justification. The controller switches modes immediately once
all eight bits of SPI Register 0x00 are written (even if in the
process of a multibyte transfer). For this reason, a single byte
command is recommended when changing justification.
3-Wire and 4-Wire Operation
Bit 7 of SPI Register 0x00 defaults low, enabling 4-wire SPI
operation. In this mode, serial data is input from the SDIO pin,
and serial data is output on the SDO pin. Setting Bit 7 of SPI
Register 0x00 high enables 3-wire operation. In this mode,
SDIO becomes bidirectional and switches automatically from
input to output when necessary. The SDO pin in this mode is
unused and assumes a high impedance state.
As with MSB or LSB justification, care must be exercised when
switching operational modes. The change occurs immediately
once all eight bits of SPI Register 0x00 are written.
Writing and Reading Register Data
Bringing CSB low initiates a new communication cycle. The
next eight rising edges of SCLK latch data from SDIO into the
instruction byte. If Bit 7 of the instruction byte is low, a write
operation is enabled. If Bit 7 is high, a read operation is enabled.
For a write operation, a data byte is latched from the SDIO pin
into a register on the next eight rising edges of SCLK. If the
instruction byte Bit 6 and Bit 5 are not both 0, a multibyte
transfer latches data bytes into adjacent registers after each
successive set of eight rising SCLK edges. Depending upon
MSB or LSB justification, the controller decrements or
increments the address value in the instruction byte during
the cycle as necessary.
If a read operation is enabled, data bits from the register being
addressed appear on SDO (or SDIO) with each falling edge of
SCLK. Note that for a read operation, the eighth bit of the
instruction byte is latched on the eighth rising edge of SCLK,
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and the first output bit is enabled on the immediately following
falling SCLK edge.
For multibyte read sequences, the controller adjusts the register
address when necessary, and subsequent data bit values appear
at the output with each falling SCLK edge.
Disabling the SPI
Tie the SPI_DIS pin high to ADVDD to disable the serial port
inteface. In this state, the default DDR operational mode can be
changed to SDR by pulling the SDR_EN pin high to ADVDD.
In addition, with the SPI disabled, the sync logic no longer oper-
ates in a fully automatic mode. See the Sync Logic Operation
and Programming section for a full explanation of sync opera-
tional modes.
SPI PIN DESCRIPTION
The AD9726 SPI logic runs from the DBVDD supply rail, and
input/output thresholds are based upon a nominal 3.3 V level.
The maximum frequency of operation is 15 MHz.
Chip Select (CSB)
The CSB pin is an active low input. It begins and ends any
communication cycle and must remain low during the entire
cycle. An incomplete cycle is aborted if CSB is prematurely
returned high.
Serial Clock (SCLK)
The SCLK pin is used to synchronize data to and from the SPI
registers, and the controller state machine runs from this input.
It is, therefore, possible to read and write register data (but not
SMEM/FMEM) without a valid DAC clock. All input data is
registered on the rising edge of SCLK, and output data bits are
enabled on the falling edge of SCLK.
Serial Data Input/Output (SDIO)
Data is always written into the SPI on the SDIO pin. In 3-wire
mode, however, data is also driven out using this pin. The
switch from input to output occurs automatically between the
instruction and data transfer phases of a read operation. In the
default 4-wire mode, SDIO is unidirectional and input only.
Serial Data Output (SDO)
Serial data is driven out on the SDO pin when the SPI is in its
default 4-wire mode. In 3-wire mode (or whenever CSB is high),
SDO is set to a high impedance state.
CALIBRATION
To ensure linearity to the 16-bit level, the AD9726 incorporates
132 calibration DACs (CALDACs), which are used to linearize
the current output transfer function. Each CALDAC is a 6-bit
device and takes its input directly from static memory (SMEM).
There are 127 CALDACs associated with each major transition
of the 16-bit input data-word (that is, any transition involving
the upper 7 MSBs). A 128th CALDAC operates on the sum total
of the lower nine LSBs. The remaining four CALDACs (129 to
132) are used to adjust the DAC’s overall transfer function gain.