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AD9761
–13–
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, 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.
STATE
MACHINE
"I" AND "Q" DATA
CLOCK
SELECT
RESET/SLEEP
WRITE
"Q" DATA
CLOCK
2
"I" DATA
"I"
INPUT
REGISTER
"I"
FILTER
REGISTER
"Q"
INPUT
REGISTER
"Q"
INPUT
REGISTER
Figure 28. 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 28 shows a
simplified block diagram of the digital interface circuitry consist-
ing 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 interpolator filters’ register after each Q write on the
rising edge of the CLOCK input (refer to Timing Diagram in
Figure 2). 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 the AD9761. If SELECT is low
around the rising edge of the 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 hence 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 Figure 29, in which all
the 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, 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.
ONE, I
RESET
FILTER
DATA
I or Q or N
N
I = WRITE & SELECT FOLLOWED BY A CLOCK
Q = WRITE &
SELECT
FOLLOWED BY A CLOCK
N = CLOCK ONLY, NO WRITE
I
I
Q
Q or N
Figure 29. 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 pro-
cessor 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 30, 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 respec-
tive 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 oper-
ating over a digital supply range of 2.7 V to 5.5 V. As a result,
the digital inputs can also accommodate TTL levels when DVDD
is set to accommodate the maximum high level voltage, V
OH(MAX)
,
of the TTL drivers. A DVDD of 3 V to 3.3 V will typically
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