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參數(shù)資料
| 型號： | AD7450BRZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 5/22頁 |
| 文件大小： | 0K |
| 描述： | IC ADC 12BIT DIFF IN 1MSPS 8SOIC |
| 標(biāo)準(zhǔn)包裝： | 98 |
| 位數(shù)： | 12 |
| 采樣率（每秒）： | 1M |
| 數(shù)據(jù)接口： | DSP，MICROWIRE?，QSPI?，串行，SPI? |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 功率耗散（最大）： | 9.25mW |
| 電壓電源： | 單電源 |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-SOIC（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 8-SOIC |
| 包裝： | 管件 |
| 輸入數(shù)目和類型： | 1 個差分，單極 |
| 配用： | EVAL-AD7450CBZ-ND - BOARD EVALUATION FOR AD7450 |
AD7450
–13–
Differential Amplifier
An ideal method of applying differential drive to the AD7450 is to
use a differential amplifier, such as the AD8138. This part can be
used as a single-ended-to-differential amplifier or as a differential-
to-differential amplifier. In both cases, the analog input needs to
be bipolar. It also provides common-mode level shifting and buffer-
ing of the bipolar input signal. Figure 14 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The positive
and negative outputs of the AD8138 are connected to the respective
inputs on the ADC via a pair of series resistors to minimize the
effects of switched capacitance on the front end of the ADC.
The RC low-pass filter on each analog input is recommended in
ac applications to remove the high-frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components.
If the analog input source being used has zero impedance then all
four resistors (Rg1, Rg2, Rf1, and Rf2) should be the same. If the
source has a 50
impedance and a 50 termination, for example,
the value of Rg2 should be increased by 25
to balance this paral-
lel impedance on the input and thus ensure that both the positive
and negative analog inputs have the same gain (see Figure 14).
The outputs of the amplifier are perfectly matched, balanced
differential outputs of identical amplitude and exactly 180
o out
of phase.
The AD8138 is specified with 3 V, 5 V, and
±5 V power supplies,
but the best results are obtained when it is supplied by
±5 V.
A lower cost device that could also be used in this configuration
with slight differences in characteristics to the AD8138, but with
similar performance and operation, is the AD8132.
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7450. The circuit configurations shown in
Figures 15a and 15b show how a dual op amp can be used to
convert a single-ended signal into a differential signal for both a
bipolar and a unipolar input signal, respectively.
The voltage applied to Point A sets up the common-mode voltage.
In both diagrams, it is connected in some way to the reference,
but any value in the common-mode range can be input here to
set up the common mode. Examples of suitable dual op amps
that could be used in this configuration to provide differential
drive to the AD7450 are the AD8042, AD8056, and AD8022.
Care must be taken when choosing the op amp, since the selec-
tion will depend on the required power supply and the system
performance objectives. The driver circuits in Figure 15a and
Figure 15b are optimized for dc coupling applications requiring
optimum distortion performance.
The differential op amp driver circuit in Figure 15a is configured
to convert and level shift a single-ended, ground referenced
(bipolar) signal to a differential signal centered at the VREF level
of the ADC.
V+
V–
V+
V–
27
390
220
10k
EXTERNAL
VREF
220
VDD
VIN+
VIN–
AD7450
220
20k
0.1 F
A
VREF
2
VREF p-p
GND
Figure 15a. Dual Op Amp Circuit to Convert a
Single-Ended Bipolar Input into a Differential Input
2.5V
3.75V
1.25V
Rs*
Rf2
2.5V
3.75V
1.25V
EXTERNAL
VREF (2.5V)
VREF
VIN+
AD7450
VIN–
AD8138
C*
*MOUNT AS CLOSE TO THE AD7450
AS POSSIBLE AND ENSURE HIGH
PRECISION Rs AND Cs ARE USED
Rs – 50R; C – 1nF;
Rg1 = Rf1 = Rf2 = 499R; Rg2 = 523R
Rf1
Rg1
VOCM
51R
Rg2
GND
+2.5V
–2.5V
Figure 14. Using the AD8138 as a Single-Ended-to-Differential Amplifier
Rev. A
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