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參數(shù)資料
| 型號： | OPA685U |
| 元件分類： | 運(yùn)動控制電子 |
| 英文描述： | Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM |
| 中文描述： | 超寬帶，電流反饋運(yùn)算放大器商標(biāo)使用禁用 |
| 文件頁數(shù)： | 21/28頁 |
| 文件大小： | 283K |
| 代理商： | OPA685U |
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21
OPA685
For a 20mA peak output current DAC, the mid-scale current
of 10mA will give a 2V DC output common-mode operating
voltage due to the 200
resistor to ground at their outputs. The
total AC impedance at each output is 50
, giving a
±
0.5V
swing around this 2V common-mode voltage for the DAC.
These resistors also act as a current divider sending 75% of the
DAC output current through the feedback resistor (464
). The
blocking capacitor references the OPA685 output voltage to
ground, and turns the unipolar DAC output current into a
bipolar swing of 0.75 20mA 464
= 7Vp-p at each
amplifier output. Each output is exactly 180
°
out-of-phase
from the other, producing double 7Vp-p into the matching
resistors. To limit the peak output current and improve distor-
tion, the circuit of Figure 10 is set up with a 1.4:1 step-down
transformer. This reflects the 50
load to be 100
at the
primary side of the transformer. For the maximum 14Vp-p
swing across the outputs of the two amplifiers, the matching
resistors will drop this to 7Vp-p at the input of the transformer,
then down to 5Vp-p maximum at the 50
load at the output
of the transformer.
HIGH-SPEED ADC INPUT DRIVERS
The OPA685 is ideally suited to the demanding input drive
requirements of emerging ultra high-speed and high perfor-
mance ADCs. As a single amplifier stage, 10-bit converters
through 100MSPS may be driven, while 8-bit converters
may be driven with input frequencies in excess of 100MHz.
Emerging differential input ADCs can also benefit from a
purely differential input interface using two OPA685s to get
a significant improvement in even-order harmonics along
with a somewhat improved 3rd-order harmonic suppression.
SINGLE-ENDED ADC INPUT INTERFACE
Figure 11 shows an example single +5V supply ADC driver
where the AC gain is set to –8V/V. The converter is shown
with both an inverting and non-inverting input. The inverting
input is used here to make the overall channel non-inverting.
The Typical Performance Curves for the non-inverting gain
of +8 show very flat frequency response for +5V only
operation when driving into the 20pF load capacitor shown in
Figure 11. The inverting configuration of Figure 11 was
selected for higher slew rate and lower distortion perfor-
mance. It will give > 300MHz bandwidth at this gain of –8.
Harmonic distortion for a 2Vp-p output signal will be < –
50dBc through 50MHz.
DIFFERENTIAL ADC DRIVER
For applications requiring the lowest harmonic distortion
through very high frequencies, a balanced differential cir-
cuit using the OPA685 offers the best performance. Figure
12 shows an example of this approach where an input step-
up transformer is used to convert to a differential signal and
improve the Noise Figure.
FIGURE 11. Single Supply, Wideband ADC Driver.
FIGURE 12. Very Wideband Differential ADC Driver.
OPA685
42.2
V
O
= 2.5V
DC
– 8V
I
400
50
+5V
Power supply decoupling
not shown.
+5V
ADC
2k
2k
IN
20
0.1
μ
F
20pF
IN
CM
V
I
DIS
+5V
–5V
OPA685
22pF
22pF
+5V
–5V
600
OPA685
1:2
50
Source
V
I
Noise
Figure
11.8dB
V
CM
V
CM
600
100
100
43.2
Power supply decoupling
not shown.
43.2
V
O
ADC Input
V
O
V
I
= 12V/V (21.6dB)
DIS
DIS
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA685U/2K5 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
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| OPA686N | 制造商:BB 制造商全稱:BB 功能描述:Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER |
| OPA686N/250 | 制造商:Rochester Electronics LLC 功能描述:- Bulk 制造商:Texas Instruments 功能描述: |
| OPA686N/3K | 制造商:Rochester Electronics LLC 功能描述:- Bulk 制造商:Texas Instruments 功能描述: |
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