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| 型號： | AD8372ACPZ-R7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 2/16頁 |
| 文件大小： | 0K |
| 描述： | IC AMP VGA DIFF DUAL LN 32LFCSP |
| 產(chǎn)品培訓(xùn)模塊： | Differential Circuit Design Techniques for Communication Applications |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 放大器類型： | 可變增益 |
| 電路數(shù)： | 2 |
| 輸出類型： | 差分 |
| -3db帶寬： | 130MHz |
| 電流 - 輸入偏壓： | 400nA |
| 電流 - 電源： | 106mA |
| 電壓 - 電源，單路/雙路(±)： | 4.5 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 32-VFQFN 裸露焊盤，CSP |
| 供應(yīng)商設(shè)備封裝： | 32-LFCSP-VQ（5x5） |
| 包裝： | 標(biāo)準(zhǔn)包裝 |
| 產(chǎn)品目錄頁面： | 775 (CN2011-ZH PDF) |
| 其它名稱： | AD8372ACPZ-R7DKR |
AD8372
Rev. B | Page 10 of 16
THEORY OF OPERATION
The AD8372 is a dual differential variable gain amplifier. Each
amplifier consists of a 150 Ω digitally controlled 6 dB attenuator
followed by a 1 dB vernier and a fixed gain transconductance
amplifier.
The differential output on each amplifier consists of a pair of
open-collector transistors. It is recommended that each open-
collector output be biased to +5 V with a high value inductor.
A 33 μH inductor, such as the Coilcraft 1812LS-333XJL, is an
excellent choice for this component. A 250 Ω resistor should be
placed across the differential outputs to provide a current-to-
voltage conversion and as a source impedance for passive
filtering, post AD8372.
The gain for each side is based on a 250 Ω differential load and
varies as the RLOAD changes per the following equations:
Gain = 20log(RLOAD/250), for voltage gain
Gain = 10log(RLOAD/250), for power gain
The dependency of the gain on the load is due to the open-
collector output stage that is biased using external chokes. The
inductance of the chokes and the resistance of the load deter-
mine the low frequency pole of the amplifier. The high frequency
pole is set by the parasitic capacitance of the chokes and outputs
in parallel with the output resistance.
The total supply current of 106 mA per side consists of 70 mA
for the combined outputs and about 36 mA through the power
supply pins. Each side has an external resistor (REXT) to ground
to set the transconductance of the output stage. For optimum
distortion, 106 mA total current per side is recommended,
making the REXT value about 2.0 kΩ. Each side has a 2.4 V
reference pin and that same common-mode voltage appears on
the inputs. This reference should be decoupled using a 0.1 μF
capacitor. The part can be powered down to less than 2.6 mA by
setting the ENB pin low for the appropriate side.
The noise figure of the AD8372 is 7.8 dB at maximum gain and
increases as the gain is reduced. The increase in noise figure is
equal to the reduction in gain.
The linearity of the part measured at the output is first-order
independent of the gain setting.
Layout considerations should include minimizing capacitance
on the outputs by avoiding ground planes under the chokes, and
equalizing the output line lengths for phase balance.
SINGLE-ENDED AND DIFFERENTIAL SIGNALS
The AD8372 is designed to be used by applying differential
signals to the inputs and using the differential output drive of
the device to drive the next device in the signal chain. The
excellent distortion performance of the AD8372 is due
primarily to the use of differential signaling techniques to
cancel various distortion components in the device. In addition,
all ac characterization is done using differential signal paths.
Using this device with either the input or the output in a single-
ended circuit significantly degrades the overall performance of
the AD8372.
PASSIVE FILTER TECHNIQUES
The AD8372 has a 100 Ω differential input impedance. For
optimal performance, the differential output load should be
250 Ω. When designing passive filters around the AD8372,
these impedances must be taken into account.
DIGITAL GAIN CONTROL
The digital gain control interface consists of the following pins:
SDI, SDO, CLK, and LATCH. The interface is active when the
LATCH pin is shifted low. Gain words are written into the
AD8372 via the SDI pin, and read back from the SDO pin. The
first bit clocked into the data input pin determines whether the
interface is in write or read mode. The second bit is a don’t care
bit, while the remaining six bits program the gain. In read
mode, the SDO pin clocks out the 6-bit gain word, LSB to MSB.
The gain can be programmed between 9 dB and 32 dB in 1 dB
gain code is given in Table 2.
DRIVING ANALOG-TO-DIGITAL CONVERTERS
The AD8372 is designed with the intention of driving high
speed, high dynamic range ADCs. The circuit in Figure 14
represents a simplified front end of one-half of the AD8372 dual
VGA driving an AD9445 14-bit, 125 MHz analog-to-digital
converter (ADC). The input of the AD8372 is driven
differentially using a 1:3 impedance ratio transformer, which
also matches the 150 Ω input resistance to a 50 Ω source. The
open-collector outputs are biased through the 33 μH inductors
and are ac-coupled from the 142 Ω load resistors that, in
parallel with the 2 kΩ input resistance of the ADC, provide a
250 Ω load for gain accuracy.
The ADC is ac-coupled from the 142 Ω resistors to negate a dc
effect on the input common-mode voltage of the AD9445.
Including the series 33 Ω resistors improves the isolation of the
AD8372 from the switching currents caused by the ADC input
sample and hold. The AD9445 represents a 2 kΩ differential
load and requires a 2 V p-p signal when VREF = 1 V for a full-
scale output. This circuit provides variable gain, isolation, and
source matching for the AD9445. Using this circuit with the
AD8372 in a gain of 32 dB (maximum gain), an SFDR
performance of 74.5 dBc is achieved at 85 MHz (see Figure 15).
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| AD8372ACPZ-R71 | 制造商:AD 制造商全稱:Analog Devices 功能描述:41 dB Range, 1 dB Step Size, Programmable Dual VGA |
| AD8372ACPZ-WP | 制造商:Analog Devices 功能描述: 制造商:Rochester Electronics LLC 功能描述: |
| AD8372ACPZ-WP1 | 制造商:AD 制造商全稱:Analog Devices 功能描述:41 dB Range, 1 dB Step Size, Programmable Dual VGA |
| AD8372-EVALZ | 功能描述:AD8372 2 - Dual Channels per IC Variable Gain Amplifier Evaluation Board 制造商:analog devices inc. 系列:- 零件狀態(tài):有效 每 IC 通道數(shù):2 - 雙 放大器類型:可變增益 輸出類型:差分 壓擺率:- -3db 帶寬:130MHz 電流 - 輸出/通道:- 工作溫度:-40°C ~ 85°C 電流 - 電源（主 IC）:106mA 電壓 - 電源，單/雙（±）:4.5 V ~ 5.5 V 板類型:完全填充 所含物品:板 使用的 IC/零件:AD8372 標(biāo)準(zhǔn)包裝:1 |
| AD8372-EVALZ1 | 制造商:AD 制造商全稱:Analog Devices 功能描述:41 dB Range, 1 dB Step Size, Programmable Dual VGA |
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