Differential operation requires VIN+ " />
參數資料
型號: AD7450ABRT-R2
廠商: Analog Devices Inc
文件頁數: 11/29頁
文件大?。?/td> 0K
描述: IC ADC 12BIT W/DIFF INP SOT-23-8
標準包裝: 1
位數: 12
采樣率(每秒): 1M
數據接口: DSP,MICROWIRE?,QSPI?,串行,SPI?
轉換器數目: 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: SOT-23-8
供應商設備封裝: SOT-23-8
包裝: 剪切帶 (CT)
輸入數目和類型: 1 個差分,單極
配用: EVAL-AD7450CBZ-ND - BOARD EVALUATION FOR AD7450
其它名稱: AD7450ABRT-R2CT
AD7440/AD7450A
Rev. C | Page 18 of 28
DRIVING DIFFERENTIAL INPUTS
Differential operation requires VIN+ and VIN– to be driven
simultaneously with two equal signals that are 180° out of
phase. The common mode must be set up externally and has a
range determined by VREF, the power supply, and the particular
amplifier used to drive the analog inputs (see Figure 28 and
Figure 29). Differential modes of operation with either an ac or
dc input provide the best THD performance over a wide
frequency range. Because not all applications have a signal
preconditioned for differential operation, there is often a need
to perform single-ended-to-differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the
AD7440/AD7450A 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 buffering of the bipolar input
signal. Figure 34 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 ADCs. The RC
low-pass filter on each analog input is recommended in ac
applications to remove high frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are very highly balanced over a wide frequency range
without requiring tightly matched external components.
If the analog input source being used has zero impedance, all
four resistors (RG1, RG2, RF1, and RF2) should be the same. If
the source has a 50 Ω impedance and a 50 Ω termination, for
example, the value of RG2 should be increased by 25 Ω to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the same
gain (see Figure 34). The outputs of the amplifier are perfectly
matched, balanced differential outputs of identical amplitude
and are exactly 180° out of phase.
The AD8138 is specified with +3 V, +5 V, and ±5 V power
supplies, but the best results are obtained with a ±5 V supply.
The AD8132 is 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.
03051-A
-034
+2.5V
GND
–2.5V
AD8138
51
Ω
RG1
RS*
C*
RS*
RG2
RF2
VOCM
RF1
3.75V
VIN+
VIN–
VREF
2.5V
1.25V
3.75V
2.5V
1.25V
AD7440/
AD7450A
*MOUNT AS CLOSE TO THE AD7440/AD7450A AS POSSIBLE
AND ENSURE HIGH PRECISION RS AND CS ARE USED.
RS–50Ω; C–1nF
RG1 = RF1 = RF2 = 499Ω; RG2 = 523Ω
EXTERNAL
VREF (2.5V)
Figure 34. Using the AD8138 as a Single-Ended-to-Differential Amplifier
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