AD9742-EB Analog Devices, AD9742-EB Datasheet - Page 11

• Manufacturer Part Number: AD9742-EB
• Description: 12-Bit/ 165 MSPS TxDAC D/A Converter
• Manufacturer: Analog Devices

IOUTA and IOUTB also have a negative and positive voltage
compliance range that must be adhered to in order to achieve
optimum performance. The negative output compliance range of
–1.0 V is set by the breakdown limits of the CMOS process.
Operation beyond this maximum limit may result in a breakdown
of the output stage and affect the reliability of the AD9742.
The positive output compliance range is slightly dependent on the
full-scale output current, I
nominal 1.2 V for an I
The optimum distortion performance for a single-ended or differ-
ential output is achieved when the maximum full-scale signal at
IOUTA and IOUTB does not exceed 0.5 V.
DIGITAL INPUTS
The AD9742’s digital section consists of 12 input bit channels
and a clock input. The 12-bit parallel data inputs follow stan-
dard positive binary coding where DB11 is the most significant
bit (MSB) and DB0 is the least significant bit (LSB). IOUTA
produces a full-scale output current when all data bits are at
Logic 1. IOUTB produces a complementary output with the
full-scale current split between the two outputs as a function of
the input code.
The digital interface is implemented using an edge-triggered master/
slave latch. The DAC output updates on the rising edge of the clock
and is designed to support a clock rate as high as 165 MSPS. The
clock can be operated at any duty cycle that meets the specified
latch pulsewidth. The setup and hold times can also be varied
within the clock cycle as long as the specified minimum times are
met, although the location of these transition edges may affect
digital feedthrough and distortion performance. Best performance
is typically achieved when the input data transitions on the falling
edge of a 50% duty cycle clock.
DAC TIMING
Input Clock and Data Timing Relationship
Dynamic performance in a DAC is dependent on the relation-
ship between the position of the clock edges and the point in
time at which the input data changes. The AD9742 is rising
edge triggered, and so exhibits dynamic performance sensitivity
when the data transition is close to this edge. In general, the
goal when applying the AD9742 is to make the data transition
close to the falling clock edge. This becomes more important as
the sample rate increases. Figure 7 shows the relationship of
SFDR to clock placement with different sample rates. Note that
at the lower sample rates, more tolerance is allowed in clock
placement, while at higher rates, more care must be taken.
REV. 0
Figure 6. Equivalent Digital Input
DIGITAL
INPUT
OUTFS
OUTFS
= 20 mA to 1.0 V for an I
. It degrades slightly from its
DVDD
OUTFS
= 2 mA.
–11–
Sleep Mode Operation
The AD9742 has a power-down function that turns off the
output current and reduces the supply current to less than 4 mA
over the specified supply range of 3.0 V to 3.6 V and tempera-
ture range. This mode can be activated by applying a logic level
1 to the SLEEP pin. The SLEEP pin logic threshold is equal to
0.5 ¥ AVDD. This digital input also contains an active pull-
down circuit that ensures the AD9742 remains enabled if this
input is left disconnected. The AD9742 takes less than 50 ns to
power down and approximately 5 ms to power back up.
POWER DISSIPATION
The power dissipation, P
several factors that include:
The power dissipation is directly proportional to the analog
supply current, I
I
and is insensitive to f
both the digital input waveform, f
DVDD. Figure 9 shows I
wave output ratios (f
DVDD = 3.3 V.
AVDD
•
•
•
•
Figure 7. SFDR vs. Clock Placement @ f
and 50 MHz
The power supply voltages (AVDD and DVDD)
The full-scale current output I
The update rate f
The reconstructed digital input waveform
is directly proportional to I
80
75
70
65
60
55
50
45
40
–3
AVDD
–2
TIME (ns) OF DATA CHANGE RELATIVE
OUT
CLOCK
CLOCK
, and the digital supply current, I
/f
TO RISING CLOCK EDGE
D
DVDD
–1
CLOCK
, of the AD9742 is dependent on
. Conversely, I
as a function of full-scale sine
) for various update rates with
OUTFS
0
CLOCK
OUTFS
as shown in Figure 8
, and digital supply
1
DVDD
f
f
OUT
OUT
is dependent on
= 50MHz
AD9742
OUT
= 20MHz
2
= 20 MHz
DVDD
3
.