AD9752 Analog Devices, AD9752 Datasheet - Page 12

• Manufacturer Part Number: AD9752
• Manufacturer: Analog Devices

Specifications of AD9752

• Resolution (bits): 12bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD9752
In summary, the AD9752 achieves the optimum distortion and
noise performance under the following conditions:
(1) Differential Operation.
(2) Positive voltage swing at IOUTA and IOUTB limited to
(3) I
(4) Analog Supply (AVDD) set at 5.0 V.
(5) Digital Supply (DVDD) set at 3.0 V to 3.3 V with appro-
Note that the ac performance of the AD9752 is characterized
under the above mentioned operating conditions.
DIGITAL INPUTS
The AD9752’s digital input consists of 12 data input pins and a
clock input pin. The 12-bit parallel data inputs follow standard
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 is updated following the
rising edge of the clock as shown in Figure 1 and is designed to
support a clock rate as high as 125 MSPS. The clock can be
operated at any duty cycle that meets the specified latch pulse-
width. 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.
The digital inputs are CMOS compatible with logic thresholds,
V
(DVDD) or
The internal digital circuitry of the AD9752 is capable of operating
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 of the TTL
drivers V
proper compatibility with most TTL logic families. Figure 23
shows the equivalent digital input circuit for the data and clock
inputs. The sleep mode input is similar with the exception that
it contains an active pull-down circuit, thus ensuring that the
AD9752 remains enabled if this input is left disconnected.
Since the AD9752 is capable of being updated up to 125 MSPS,
the quality of the clock and data input signals are important in
achieving the optimum performance. The drivers of the digital
THRESHOLD
+0.5 V.
priate logic levels.
OUTFS
OH(MAX)
set to 20 mA.
set to approximately half the digital positive supply
Figure 23. Equivalent Digital Input
V
. A DVDD of 3 V to 3.3 V will typically ensure
THRESHOLD
DIGITAL
INPUT
= DVDD/2 ( 20%)
DVDD
–12–
data interface circuitry should be specified to meet the mini-
mum setup and hold times of the AD9752 as well as its re-
quired min/max input logic level thresholds. Typically, the
selection of the slowest logic family that satisfies the above con-
ditions will result in the lowest data feedthrough and noise.
Digital signal paths should be kept short and run lengths
matched to avoid propagation delay mismatch. The insertion of
a low value resistor network (i.e., 20
AD9752 digital inputs and driver outputs may be helpful in reduc-
ing any overshooting and ringing at the digital inputs that con-
tribute to data feedthrough. For longer run lengths and high data
update rates, strip line techniques with proper termination resis-
tors should be considered to maintain “clean” digital inputs. Also,
operating the AD9752 with reduced logic swings and a corre-
sponding digital supply (DVDD) will also reduce data feedthrough.
The external clock driver circuitry should provide the AD9752
with a low jitter clock input meeting the min/max logic levels
while providing fast edges. Fast clock edges will help minimize
any jitter that will manifest itself as phase noise on a recon-
structed waveform. Thus, the clock input should be driven by
the fastest logic family suitable for the application.
Note, the clock input could also be driven via a sine wave, which is
centered around the digital threshold (i.e., DVDD/2), and meets
the min/max logic threshold. This will typically result in a slight
degradation in the phase noise, which becomes more noticeable
at higher sampling rates and output frequencies. Also, at higher
sampling rates, the 20% tolerance of the digital logic threshold
should be considered since it will affect the effective clock duty
cycle and subsequently cut into the required data setup and
hold times.
INPUT CLOCK/DATA TIMING RELATIONSHIP
SNR in a DAC is dependent on the relationship between the
position of the clock edges and the point in time at which the
input data changes. The AD9752 is positive edge triggered, and
so exhibits SNR sensitivity when the data transition is close to
this edge. In general, the goal when applying the AD9752 is to
make the data transitions shortly after the positive clock edge.
This becomes more important as the sample rate increases. Figure
24 shows the relationship of SNR to clock placement with dif-
ferent sample rates and different frequencies out. Note that at
the lower sample rates, much more tolerance is allowed in clock
placement, while at higher rates, much more care must be taken.
Figure 24. SNR vs. Clock Placement @ f
68
64
60
56
52
48
44
40
–8
–6
TIME OF DATA CHANGE RELATIVE TO
–4
RISING CLOCK EDGE – ns
–2
0
2
to 100 ) between the
F
4
S
= 125MSPS
F
S
6
= 65MSPS
OUT
8
= 10 MHz
10
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