AD9726 Analog Devices, AD9726 Datasheet - Page 21

• 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

Linearity CALDACs operate inversely from their input; that is,
as their binary input value increases, the magnitude of their
current contribution seen at the AD9726 output decreases. Gain
CALDACs are an exception to this. Their contribution seen at
the AD9726 output is in direct proportion to their binary input.
Gain CALDACs are also half strength as compared to linearity
CALDACs, but they are intended to be used together as a unit
and thus, together, provide twice the current adjustment range.
Calibration Memory
During production testing, the linearity of the AD9726 is
measured and optimized. Values for all CALDACs are perma-
nently stored in nonvolatile factory memory (FMEM). At
reset, all factory memory contents are transferred to static
memory. CALMEM, Bits[5:4] in Register 0x0E, indicates a
factory calibrated state (CALMEM = 10b).
It is also possible at any time to transfer the contents of FMEM
to SMEM by asserting the MEMXFER bit in Register 0x0F. The
XFERSTAT indicator bit (Bit 5 in Register 0x0F) then reports
the successful completion of the transfer cycle, and MEMXFER
is cleared.
Note that the MEMXFER bit (and SELFCAL, Bit 6, Register
0x0F) cannot be asserted if any other memory access function is
currently enabled (that is, if any one of Bits[3:0] in Register
0x0F is high). Attempting to assert MEMXFER (or SELFCAL)
in this case clears any asserted bits in Register 0x0F, but the
requested cycle does not commence.
The factory-to-static memory data transfer cycle requires a
number of DAC clock cycles. The total depends on the value of
CALCLK. This value sets a divider used to create a slow version
of the DAC clock, which is intended to extend the settling time
available to the self-calibration cycle. However, this divided
clock is also used to sequence a memory transfer cycle.
The divider is set to its maximum value with CALCLK at its
default value. A memory transfer cycle requires about 15 ms at a
DAC clock frequency of 100 MHz. This time can be reduced by
50% for every increase in the value of CALCLK.
Accessing Calibration Memory
SMEM or FMEM locations can be read at any time by setting
the SMEMRD or FMEMRD bit in SPI Register 0x0F. Address
and data information can be input and/or output through SPI
Register 0x10 and SPI Register 0x11, respectively.
SMEM locations can also be written by setting the SMEMWR
bit in Register 0x0F. Register 0x10 and Register 0x11 are again
used for addresses and data. Any time after the SMEMWR bit
has been asserted, the device reports a user-calibrated state
(CALMEM = 11b) until another action changes the calibration
memory status.
To reset static memory at any time, assert the UNCAL bit in
Register 0x0F. All SMEM locations are then reset to their
default values (63). CALMEM reports an uncalibrated state
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(CALMEM = 00b). Note that UNCAL remains asserted (and
the contents of SMEM remains at default values) indefinitely.
UNCAL does not clear itself (like SWRESET) and must be
cleared by the user.
Note also that although SPI registers do not depend on the DAC
clock (they use SCLK to sequence the controller state machine),
SMEM and/or FMEM access does require a valid DAC clock.
SMEM/FMEM Read/Write Procedures
Static and factory memory is accessed through the SPI, but it is
not part of the SPI logic. For this reason, memory access requires
a valid DAC clock, while SPI register access does not.
Because the AD9726 SPI is so flexible, allowing single and
multiple byte reads and writes as well as MSB or LSB justified
data, there are a number of ways in which a user can access one
or more SMEM or FMEM locations.
To avoid potential errors, the following procedures for accessing
static or factory memory should be followed. These procedures
use only single-byte SPI commands to ensure the enabling of
addresses and the sequencing of memory access.
To read from SMEM or FMEM,
1.
2.
3.
4.
5.
To write to SMEM,
1.
2.
3.
4.
5.
Self-Calibration
The AD9726 features an internal self-calibration engine to
linearize the transfer function automatically. This can be very
useful at temperature extremes where factory calibration no
longer applies. The automated cycle can be initiated by asserting
the SELFCAL bit.
The self-calibration process calibrates all linearity and gain
CALDACs based upon a fixed internal reference current. Values
for all CALDACs are stored in volatile static memory. The
CALSTAT bit indicates the successful completion of the cycle,
Ensure that Bits [3:0] of Register 0x0F are clear.
Begin the sequence by writing the memory address value
to Register 0x10 with a single-byte SPI write command.
Assert the SMEMRD or FMEMRD bit in Register 0x0F
with another single-byte SPI write command.
Import the contents of Register 0x11 using a single-byte
SPI read command.
Clear the SMEMRD or FMEMRD bit with another single-
byte command.
Ensure that Bits [3:0] of Register 0x0F are clear.
Begin the sequence by writing the data value to
Register 0x11 using a single-byte SPI write command.
Assert the SMEMWR bit using a single-byte SPI write
command.
Place the memory address value in Register 0x10 using a
single-byte SPI write command.
Clear the SMEMWR bit with a fourth single-byte SPI write
command.
AD9726