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DAC908E/2K5G4 参数 Datasheet PDF下载

DAC908E/2K5G4图片预览
型号: DAC908E/2K5G4
PDF下载: 下载PDF文件 查看货源
内容描述: 8位, 165MSPS数位类比转换器 [8-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER]
分类和应用: 转换器数模转换器光电二极管
文件页数/大小: 23 页 / 789 K
品牌: TI [ TEXAS INSTRUMENTS ]
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As shown in Figure 3, the transformer’s center tap must be  
connected to ground to enable the necessary DC-current  
flow for both outputs. Some applications may require a solid  
termination, in which case a differential resistor, RDIFF, may  
be inserted as shown. Note that this will reduce the available  
signal power by approximately one half.  
of the resistors typically sets the limit for the achievable  
common-mode rejection. An improvement can be obtained  
by fine tuning resistor R4.  
This configuration typically delivers a lower level of ac  
performance than the previously discussed transformer solu-  
tion because the amplifier introduces another source of  
distortion. Suitable amplifiers should be selected based on  
their slew-rate, harmonic distortion, and output swing capa-  
bilities. High-speed amplifiers like the OPA680 or OPA687  
may be considered. The ac performance of this circuit may  
be improved by adding a small capacitor, CDIFF, between the  
ADT1-1WT  
(Mini-Circuits)  
1:1  
IOUT  
outputs IOUT and IOUT, as shown in Figure 4. This will intro-  
50Ω  
Optional  
RDIFF  
DAC908  
RL  
duce a real pole to create a low-pass filter in order to slew-  
limit the DACs fast output signal steps, which otherwise  
could drive the amplifier into slew-limitations or into an  
overload condition; both would cause excessive distortion.  
The difference amplifier can easily be modified to add a  
level shift for applications requiring the single-ended output  
voltage to be unipolar, i.e., swing between 0V and +2V.  
IOUT  
50Ω  
FIGURE 3. Differential Output Configuration Using an RF  
Transformer.  
DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION  
The circuit example of Figure 5 shows the signal output  
currents connected into the summing junction of the  
OPA2680, which is set up as a transimpedance stage, or  
I-to-V converter. With this circuit, the DAC’s output will be  
kept at a virtual ground, minimizing the effects of output  
impedance variations, and resulting in the best DC linearity  
(INL). However, as mentioned previously, the amplifier  
may be driven into slew-rate limitations, and produce un-  
wanted distortion. This may occur especially at high DAC  
update rates.  
DIFFERENTIAL CONFIGURATION USING AN OP AMP  
If the application requires a DC-coupled output, a difference  
amplifier may be considered, as shown in Figure 4. Four  
external resistors are needed to configure the voltage-feed-  
back op amp OPA680 as a difference amplifier performing  
the differential to single-ended conversion. Under the shown  
configuration, the DAC908 generates a differential output  
signal of 0.5Vp-p at the load resistors, RL. The resistor  
values shown were selected to result in a symmetric 25Ω  
loading for each of the current outputs since the input  
impedance of the difference amplifier is in parallel to resis-  
tors RL, and should be considered.  
+5V  
50  
1/2  
OPA2680  
VOUT = IOUT RF  
R2  
402Ω  
RF1  
CF1  
DAC908  
R1  
200Ω  
IOUT  
IOUT  
CD1  
VOUT  
DAC908  
IOUT  
OPA680  
R3  
200Ω  
CDIFF  
RF2  
CF2  
5V +5V  
RL  
28.7Ω  
R4  
402Ω  
RL  
26.1Ω  
CD2  
IOUT  
1/2  
OPA2680  
VOUT = IOUT RF  
FIGURE 4. Difference Amplifier Provides Differential to  
Single-Ended Conversion and DC-Coupling.  
50Ω  
5V  
The OPA680 is configured for a gain of two. Therefore,  
operating the DAC908 with a 20mA full-scale output will  
produce a voltage output of ±1V. This requires the amplifier  
to operate off of a dual power supply (±5V). The tolerance  
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680  
Forms Differential Transimpedance Amplifier.  
DAC908  
12  
SBAS109B