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| 型号： | ADA4938-2ACPZ-RL |
| 厂商： | Analog Devices Inc |
| 文件页数： | 17/28页 |
| 文件大小： | 0K |
| 描述： | IC ADC DRIVER DUAL DIFF 24LFCSP |
| 标准包装： | 5,000 |
| 类型： | ADC 驱动器 |
| 应用： | 数据采集 |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-VFQFN 裸露焊盘，CSP |
| 供应商设备封装： | 24-LFCSP-VQ（4x4） |
| 包装： | 带卷 (TR) |
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ADA4938-1/ADA4938-2
Rev. A | Page 24 of 28
HIGH PERFORMANCE ADC DRIVING
The ADA4938-x is ideally suited for dc-coupled baseband
applications. The circuit in Figure 69 shows a front-end connection
for an ADA4938-x driving an AD9446, 16-bit, 80 MSPS ADC.
The AD9446 achieves its optimum performance when it is
driven differentially. The ADA4938-x eliminates the need for a
transformer to drive the ADC, performs a single-ended-to-
differential conversion, buffers the driving signal, and provides
appropriate level shifting for dc coupling.
The ADA4938-x is configured with a single 10 V supply and
unity gain for a single-ended input to differential output. The
61.9 Ω termination resistor, in parallel with the single-ended
input impedance of 267 Ω, provides a 50 Ω termination for the
source. The additional 26 Ω (226 Ω total) at the inverting input
balances the parallel impedance of the 50 Ω source and the
termination resistor driving the noninverting input.
The signal generator has a symmetric, ground-referenced bipolar
output. The VOCM pin of the ADA4938-x is biased with an external
resistor divider to obtain the desired 3.5 V output common-mode.
One-half of the common-mode voltage is fed back to the summing
nodes, biasing IN and +IN at 1.75 V. For a common-mode vol-
tage of 3.5 V, each ADA4938-x output swings between 2.7 V
and 4.3 V, providing a 3.2 V p-p differential output.
The output of the amplifier is dc-coupled to the ADC through a
second-order, low-pass filter with a 3 dB frequency of 50 MHz.
The filter reduces the noise bandwidth of the amplifier and
isolates the driver outputs from the ADC inputs.
The AD9446 is configured for a 4.0 V p-p full-scale input by
setting R1 = R2 = 1 kΩ between the VREF pin and SENSE pin
in Figure 69.
The circuit in Figure 70 shows a simplified front-end connection
for an ADA4938-x driving an AD9246, 14-bit, 125 MSPS ADC.
The AD9246 achieves its optimum performance when it is
driven differentially. The ADA4938-x eliminates the need for a
transformer to drive the ADC, performs a single-ended-to-
differential conversion, buffers the driving signal, and provides
appropriate level shifting for dc coupling.
The ADA4938-x is configured with dual ±5 V supplies and a
gain of ~2 V/V for a single-ended input to differential output.
The 76.8 Ω termination resistor, in parallel with the single-
ended input impedance of 137 Ω, provides a 50 Ω dc termination
for the source. The additional 30.1 Ω (120 Ω total) at the inverting
input balances the parallel dc impedance of the 50 Ω source and
the termination resistor driving the noninverting input.
The signal generator has a symmetric, ground-referenced
bipolar output. The VOCM pin of the ADA4938-x is connected to
the CML pin of the AD9246 to set the output common-mode
level at the appropriate point. A portion of this is fed back to the
summing nodes, biasing IN and +IN at 0.55 V. For a common-
mode voltage of 0.9 V, each ADA4938 output swings between
0.4 V and 1.4 V, providing a 2 V p-p differential output.
The output is dc-coupled to a single-pole, low-pass filter. The filter
reduces the noise bandwidth of the amplifier and provides some
level of isolation from the switched capacitor inputs of the ADC.
The AD9246 is set for a 2 V p-p full-scale input by connecting the
SENSE pin to AGND. The inputs of the AD9246 are biased at
1 V by connecting the CML output, as shown in Figure 70.
VIN+
VIN–
47pF
30nH
24.3
24.3
50
SIGNAL
GENERATOR
226
200
VOCM
10V
ADA4938
+
61.9
200
200
16
BUFFER T/H
ADC
CLOCK/
TIMING
REF
SENSE
AGND
VREF
AD9446
3.3V (A)
AVDD1
5V (A)
AVDD2
3.3V (D)
DRVDD
06
59
2-
05
4
10V
R1
R2
Figure 69. ADA4938 Driving an AD9446, 16-Bit, 80 MSPS ADC
1.8V
DRVDD
AVDD
VIN–
VIN+
AD9246
AGND
CML
SENSE
D13 TO
D0
10pF
33
VOCM
33
50
VIN
90
90
76.8
+5V
–5V
ADA4938
+
200
200
30.1
0.1F
06
59
2-
0
5
6
Figure 70. ADA4938 Driving an AD9246, a 14-Bit, 125 MSPS ADC
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