参数资料
| 型号： | AD9774ASZRL |
| 厂商： | Analog Devices Inc |
| 文件页数： | 8/24页 |
| 文件大小： | 0K |
| 描述： | IC DAC 14BIT 32MSPS 44MQFP |
| 产品变化通告： | Product Discontinuance 27/Oct/2011 |
| 标准包装： | 800 |
| 系列： | TxDAC+® |
| 设置时间： | 35ns |
| 位数： | 14 |
| 数据接口： | 并联 |
| 转换器数目： | 1 |
| 电压电源： | 模拟和数字 |
| 功率耗散（最大）： | 1.13W |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 44-QFP |
| 供应商设备封装： | 44-MQFP（10x10） |
| 包装： | 带卷 (TR) |
| 输出数目和类型： | 2 电流，单极；2 电流，双极 |
| 采样率（每秒）： | 32M |
AD9774
–16–
REV. B
APPLYING THE AD9774
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9774. Unless otherwise noted, it is assumed
that IOUTFS is set to a nominal 20 mA. For applications requir-
ing the optimum dynamic performance, a differential output
configuration is suggested. A differential output configuration
may consist of either an RF transformer or a differential op amp
configuration. The transformer configuration provides the opti-
mum high frequency performance and is recommended for any
application allowing for ac coupling. The differential op amp
configuration is suitable for applications requiring dc coupling, a
bipolar output, signal gain and/or level shifting.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if IOUTA and/or IOUTB is connected to an approximately
sized load resistor, RLOAD, referred to ACOM. This configura-
tion may be more suitable for a single-supply system requiring a
dc-coupled, ground referred output voltage. Alternatively, an
amplifier could be configured as an I-V converter, thus convert-
ing IOUTA or IOUTB into a negative unipolar voltage. This
configuration provides the best dc linearity since IOUTA or
IOUTB is maintained at a virtual ground.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 36. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. An RF transformer such
as the Mini-Circuits T1-1T provides excellent rejection of
common-mode distortion (i.e., even-order harmonics) and noise
over a wide frequency range. It also provides electrical isolation
and the ability to deliver twice the power to the load. Trans-
formers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only.
RLOAD
AD9774
22
21
MINI-CIRCUITS
T1-1T
OPTIONAL RDIFF
IOUTA
IOUTB
Figure 36. Differential Output Using a Transformer
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path
for both IOUTA and IOUTB. The complementary voltages
appearing at IOUTA and IOUTB (i.e., VOUTA and VOUTB)
swing symmetrically around ACOM and should be maintained
with the specified output compliance range of the AD9774. A
differential resistor, RDIFF, may be inserted in applications in
which the output of the transformer is connected to the load,
RLOAD, via a passive reconstruction filter or cable. RDIFF is deter-
mined by the transformer’s impedance ratio and provides the
proper source termination that results in a low VSWR. Note that
approximately half the signal power will be dissipated across RDIFF.
DIFFERENTIAL USING AN OP AMP
An op amp can also be used to perform a differential-to-single-
ended conversion as shown in Figure 37. The AD9774 is
configured with two equal load resistors, RLOAD, of 25 . The
differential voltage developed across IOUTA and IOUTB is
converted to a single-ended signal via the differential op amp
configuration. An optional capacitor can be installed across
IOUTA and IOUTB, forming a real pole in a low-pass filter.
The addition of this capacitor also enhances the op amp’s distor-
tion performance by preventing the DAC’s high slewing output
from overloading the op amp’s input.
AD9774
22
IOUTA
IOUTB 21
COPT
500
225
500
25
AD8055
Figure 37. DC Differential Coupling Using an Op Amp
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differ-
ential op amp circuit using the AD8055 is configured to provide
some additional signal gain. The op amp must operate from a
dual supply since its output is approximately
±1.0 V. A high
speed amplifier capable of preserving the differential perform-
ance of the AD9774 while meeting other system level objectives
(i.e., cost, power) should be selected. The op amps 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 38 provides the neces-
sary level-shifting required in a single supply system. In this case,
AVDD, which is the positive analog supply for both the AD9774
and the op amp, is also used to level-shift the differential output
of the AD9774 to midsupply (i.e., AVDD/2). The AD8041 is a
suitable op amp for this application.
AD9774
22
IOUTA
IOUTB 21
COPT
500
225
1k
25
AD8041
1k
AVDD
Figure 38. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 39 shows the AD9774 configured to provide a unipolar
output range of approximately 0 V to +0.5 V for a doubly termi-
nated 50
cable since the nominal full-scale current, I
OUTFS, of
20 mA flows through the equivalent RLOAD of 25 . In this case,
RLOAD represents the equivalent load resistance seen by IOUTA.
The unused output (IOUTB) can be connected to ACOM di-
rectly. Different values of IOUTFS and RLOAD can be selected as
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