AD9740-EB Analog Devices, AD9740-EB Datasheet - Page 13

• Manufacturer Part Number: AD9740-EB
• Description: 10-Bit/ 165 MSPS TxDAC D/A Converter
• Manufacturer: Analog Devices

AD9740 while meeting other system level objectives (i.e., cost,
power) should be selected. The op amp’s differential gain, its gain
setting resistor values, and full-scale output swing capabilities
should all be considered when optimizing this circuit.
The differential circuit shown in Figure 12 provides the necessary
level shifting required in a single-supply system. In this case,
AVDD, which is the positive analog supply for both the AD9740
and the op amp, is also used to level shift the differential output
of the AD9740 to midsupply (i.e., AVDD/2). The AD8041 is a
suitable op amp for this application.
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 13 shows the AD9740 configured to provide a unipolar
output range of approximately 0 V to 0.5 V for a doubly termi-
nated 50 Ω cable, since the nominal full-scale current, I
of 20 mA flows through the equivalent R
case, R
IOUTA or IOUTB. The unused output (IOUTA or IOUTB)
can be connected to ACOM directly or via a matching R
Different values of I
the positive compliance range is adhered to. One additional
consideration in this mode is the integral nonlinearity (INL) as
discussed in the Analog Output section of this data sheet. For
optimum INL performance, the single-ended, buffered voltage
output configuration is suggested.
SINGLE-ENDED, BUFFERED VOLTAGE OUTPUT
CONFIGURATION
Figure 14 shows a buffered single-ended output configuration in
which the op amp U1 performs an I-V conversion on the AD9740
output current. U1 maintains IOUTA (or IOUTB) at a virtual
ground, minimizing the nonlinear output impedance effect on the
DAC’s INL performance as discussed in the Analog Output
section. Although this single-ended configuration typically
provides the best dc linearity performance, its ac distortion
performance at higher DAC update rates may be limited by
U1’s slew rate capabilities. U1 provides a negative unipolar output
voltage and its full-scale output voltage is simply the product of R
and I
voltage output swing capabilities by scaling I
An improvement in ac distortion performance may result with a
reduced I
REV. 0
Figure 12. Single-Supply DC Differential Coupled Circuit
Figure 13. 0 V to 0.5 V Unbuffered Voltage Output
OUTFS
AD9740
LOAD
AD9740
OUTFS
IOUTA
IOUTB
. The full-scale output should be set within U1’s
IOUTA
IOUTB
represents the equivalent load resistance seen by
since U1 will be required to sink less signal current.
25
OUTFS
I
OUTFS
C
OPT
25
= 20mA
and R
25
225
225
LOAD
50
can be selected as long as
1k
LOAD
AD8041
V
500
OUTA
OUTFS
of 25 Ω. In this
50
1k
= 0V TO 0.5V
and/or R
LOAD
OUTFS
AVDD
FB
.
FB
,
.
–13–
POWER AND GROUNDING CONSIDERATIONS,
POWER SUPPLY REJECTION
Many applications seek high speed and high performance under
less than ideal operating conditions. In these application circuits,
the implementation and construction of the printed circuit board
is as important as the circuit design. Proper RF techniques must
be used for device selection, placement, and routing as well as power
supply bypassing and grounding to ensure optimum performance.
Figures 19 to 22 illustrate the recommended printed circuit board
ground, power, and signal plane layouts that are implemented on
the AD9740 evaluation board.
One factor that can measurably affect system performance is the
ability of the DAC output to reject dc variations or ac noise
superimposed on the analog or digital dc power distribution.
This is referred to as the power supply rejection ratio. For dc
variations of the power supply, the resulting performance of the
DAC directly corresponds to a gain error associated with the
DAC’s full-scale current, I
common in applications where the power distribution is gener-
ated by a switching power supply. Typically, switching power
supply noise will occur over the spectrum from tens of kHz to
several MHz. The PSRR vs frequency of the AD9740 AVDD
supply over this frequency range is shown in Figure 15.
Note that the units in Figure 15 are given in units of (amps out/
volts in). Noise on the analog power supply has the effect of
modulating the internal switches, and therefore the output current.
The voltage noise on AVDD, therefore, will be added in a
nonlinear manner to the desired IOUT. Due to the relative differ-
ent size of these switches, PSRR is very code dependent. This
can produce a mixing effect that can modulate low-frequency
power supply noise to higher frequencies. Worst-case PSRR for
either one of the differential DAC outputs will occur when the
AD9740
IOUTA
IOUTB
Figure 14. Unipolar Buffered Voltage Output
85
80
75
70
65
60
55
50
45
40
Figure 15. Power Supply Rejection Ratio
0
2
I
OUTFS
= 10mA
4
FREQUENCY – MHz
OUTFS
200
. AC noise on the dc supplies is
6
200
C
R
U1
OPT
FB
8
AD9740
10
V
OUT
= I
OUTFS
12
R
FB