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| 型号： | AD8021ARZ-REEL7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 16/29页 |
| 文件大小： | 0K |
| 描述： | IC OPAMP VF LN LP LDIST 8SOIC |
| 设计资源： | Driving the AD7366/7 Bipolar SAR ADC in Low-Distortion DC-Coupled Appls (CN0042) |
| 标准包装： | 1 |
| 放大器类型： | 电压反馈 |
| 电路数： | 1 |
| 转换速率： | 460 V/µs |
| -3db带宽： | 560MHz |
| 电流 - 输入偏压： | 7.5µA |
| 电压 - 输入偏移： | 400µV |
| 电流 - 电源： | 7.8mA |
| 电流 - 输出 / 通道： | 70mA |
| 电压 - 电源，单路/双路(±)： | 4.5 V ~ 24 V，±2.25 V ~ 12 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SO |
| 包装： | 标准包装 |
| 其它名称： | AD8021ARZ-REEL7DKR |
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AD8021
Rev. F | Page 22 of 28
DRIVING 16-BIT ADCs
Low noise and adjustable compensation make the AD8021
especially suitable as a buffer/driver for high resolution ADCs.
As seen in Figure 19, the harmonic distortion is better than 90 dBc
at frequencies between 100 kHz and 1 MHz. This is an
advantage for complex waveforms that contain high frequency
information, because the phase and gain integrity of the sampled
waveform can be preserved throughout the conversion process.
The increase in loop gain results in improved output regulation
and lower noise when the converter input changes state during
a sample. This advantage is particularly apparent when using
16-bit high resolution ADCs with high sampling rates.
Figure 63 shows a typical ADC driver configuration. The
AD8021 is in an inverting gain of 7.5, fC is 65 kHz, and its
output voltage is 10 V p-p. The results are listed in Table 7.
+
–
IN
HI
IN
HI
50
Ω
RG
200
Ω
56pF
RF
1.5k
Ω
CC
10pF
–12V
AD7665
570kSPS
16
B
ITS
+5V
6
5
3
2
590
Ω
+12V
AD8021
01888-063
Figure 63. Inverting ADC Driver, Gain = 7.5, fC = 65 kHz
Table 7. Summary of ADC Driver Performance (fC = 65 kHz,
VOUT = 10 V p-p)
Parameter
Measurement
Unit
Second Harmonic Distortion
101.3
dBc
Third Harmonic Distortion
109.5
dBc
THD
100.0
dBc
SFDR
+100.3
dBc
Figure 64 shows another ADC driver connection. The circuit
was tested with a noninverting gain of 10.1 and an output
voltage of approximately 20 V p-p for optimum resolution and
noise performance. No filtering was used. An FFT was
performed using Analog Devices evaluation software for the
50
Ω
+5V
AD8021
+
–
–12V
+12V
AD7665
570kSPS
50
Ω
3
2
RF
750
Ω
OPTIONAL CF
IN
LO
IN
6
50
Ω
HI
ADC
CC
5
RG
82.5
Ω
16
BI
T
S
01888-
064
Figure 64. Noninverting ADC Driver, Gain = 10, fC = 100 kHz
Table 8. Summary of ADC Driver Performance
(fC = 100 kHz, VOUT = 20 V p-p)
Parameter
Measurement
Unit
Second Harmonic Distortion
92.6
dBc
Third Harmonic Distortion
86.4
dBc
THD
84.4
dBc
SFDR
+5.4
dBc
DIFFERENTIAL DRIVER
The AD8021 is uniquely suited as a low noise differential driver
for many ADCs, balanced lines, and other applications requiring
differential drive. If pairs of internally compensated op amps are
configured as inverter and follower, the noise gain of the inverter
is higher than that of the follower section, resulting in an
imbalance in the frequency response (see Figure 66).
A better solution takes advantage of the external compensation
feature of the AD8021. By reducing the CCOMP value of the
inverter, its bandwidth can be increased to match that of the
follower, avoiding compromises in gain bandwidth and phase
delay. The inverting and noninverting bandwidths can be
closely matched using the compensation feature, thus
minimizing distortion.
Figure 65 illustrates an inverter-follower driver circuit operating
at a gain of 2, using individually compensated AD8021s. The
values of feedback and load resistors were selected to provide a
total load of less than 1 kΩ, and the equivalent resistances seen
at each op amp’s inputs were matched to minimize offset voltage
and drift. Figure 67 is a plot of the resulting ac responses of
driver halves.
AD8021
+
–
3
2
6
7pF
249
Ω
499
Ω
G = +2
499
Ω
49.9
Ω
1k
Ω
VOUT1
5
–VS
AD8021
+
–
3
2
6
5pF
232
Ω
G = –2
664
Ω
1k
Ω
VOUT2
5
–VS
332
Ω
VIN
01888-065
Figure 65. Differential Amplifier
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