AD9761_15 Datasheet, PDF (13/24 Pages) Analog Devices – Dual 10-Bit TxDAC

AD9761
The positive output compliance range is slightly dependent on
the full-scale output current, I_OUTFS. It degrades slightly from
its nominal 1.25 V for an I_OUTFS = 10 mA to 1.00 V for an
I_OUTFS = 2 mA. Applications requiring the AD9761’s output
(i.e., V_OUTA and/or V_OUTB) to extend to its output compliance
range should size R_LOAD accordingly. Operation beyond this
compliance range will adversely affect the AD9761’s linearity
performance and subsequently degrade its distortion perfor-
mance. Note that the optimum distortion performance of
the AD9761 is obtained by restricting its output(s) as seen at
IOUT(A/B) and QOUT(A/B) to within ±0.5 V.
DIGITAL INPUTS AND INTERLEAVED INTERFACE
CONSIDERATIONS
The AD9761 digital interface consists of 10 data input pins,
a clock input pin, and three control pins. It is designed to
support a clock rate up to 40 MSPS. The 10-bit parallel
data inputs follow standard positive binary coding, where
DB9 is the most significant bit (MSB) and DB0 is the
least significant bit (LSB). IOUTA (or QOUTA) produces
a full-scale output current when all data bits are at Logic 1.
IOUTB (or QOUTB) produces a complementary output,
with the full-scale current split between the two outputs as a
function of the input code.
I AND Q DATA
I
INPUT
REGISTER
I
FILTER
REGISTER
I DATA
Q
INPUT
REGISTER
Q
INPUT
REGISTER
Q DATA
the AD9761. If SELECT is low around the rising edge
of WRITE, the data is latched into the Q register of the
AD9761. If SELECT is kept in one state while data is
repeatedly writing to the AD9761, the data will be written
into the selected filter register at half the input data rate
since the data is always assumed to be interleaved.
The state machine controls the generation of the divided
clock and thus pairing of I and Q data inputs. After the
AD9761 is reset, the state machine keeps track of the paired
I and Q data. The state transition diagram is shown in Fig-
ure 11, in which all states are defined. A transition in state
occurs upon the rising edge of CLOCK and is a function
of the current state as well as status of SELECT, WRITE,
and SLEEP. The state machine is reset on the first rising
CLOCK edge while RESET remains high. Upon RESET
returning low, a state transition will occur on the first rising
edge of CLOCK. The most recent I and Q data samples
are transferred to the correct interpolation filter only upon
entering state FILTER DATA.
Note that it is possible to ensure proper pairing of I and Q data
inputs without issuing RESET high. This may be accomplished
by writing two or more successive Q data inputs followed by
a clock. In this case, the state machine will advance to either
the RESET or FILTER DATA state. The state machine
will advance to the ONE-I state upon writing I data followed
by a clock.
I or Q or N
Q
FILTER
ONE, I
I
DATA
N
I
Q or N
RESET
CLOCK
SELECT
RESET/SLEEP
WRITE
STATE
MACHINE
CLOCK
2
Figure 10. Block Diagram of Digital Interface
The AD9761 interfaces with a single 10-bit digital input
bus that supports interleaved I and Q input data. Figure 10
shows a simplified block diagram of the digital interface
circuitry consisting of two banks of edge triggered registers,
two multiplexers, and a state machine. Interleaved I and Q
input data is presented at the DATA input bus, where it is
then latched into the selected I or Q input register on the
rising edge of the WRITE input. The output of these input
registers is transferred in pairs to their respective interpola-
tor filters’ register after each Q write on the rising edge of
the CLOCK input (refer to Timing Diagram in Figure 1).
A state machine ensures the proper pairing of I and Q input
data to the interpolation filter’s inputs.
The SELECT signal at the time of the rising edge of the
WRITE signal determines which input register latches
the input data. If SELECT is high around the rising
edge of WRITE, the data is latched into the I register of
I = WRITE AND SELECT FOLLOWED BY A CLOCK
Q = WRITE AND SELECT FOLLOWED BY A CLOCK
N = CLOCK ONLY, NO WRITE
Figure 11. State Transition Diagram of AD9761
Digital Interface
An example helps illustrate the digital timing and control
requirements to ensure proper pairing of I and Q data. In
this example, the AD9761 is assumed to interface with a host
processor on a dedicated data bus and the state machine
is reset by asserting a Logic Level 1 to the RESET/SLEEP
input for a duration of one clock cycle. In the timing diagram
shown in Figure 12, WRITE and CLOCK are tied together
while SELECT is updated at the same instance as DATA.
Since SELECT is high upon RESET returning low, I data is
latched into the I input register on the first rising WRITE.
On the next rising WRITE edge, the Q data is latched into
its input register and the outputs of both input registers are
latched into their respective I and Q filter registers. The
sequence of events is repeated on the next rising WRITE edge
with the new I data being latched into the I input register.
The digital inputs are CMOS compatible with logic thresholds,
V_THRESHOLD, set to approximately half the digital positive
supply (DVDD) or V_THRESHOLD = DVDD/2 (±20%).
The internal digital circuitry of the AD9761 is capable of
operating over a digital supply range of 2.7 V to 5.5 V. As a
REV. C
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