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參數(shù)資料
| 型號： | AD8305ACPZ-RL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 9/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC LOGARITH CONV 100DB 16-LFCSP |
| 設計資源： | Interfacing ADL5315 to Translinear Logarithmic Amplifier (CN0056) Interfacing ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector |
| 標準包裝： | 1 |
| 類型： | 對數(shù)轉換器 |
| 應用： | 光纖 |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-VFQFN 裸露焊盤，CSP |
| 供應商設備封裝： | 16-LFCSP-VQ |
| 包裝： | 標準包裝 |
| 產(chǎn)品目錄頁面： | 550 (CN2011-ZH PDF) |
| 其它名稱： | AD8305ACPZ-RL7DKR |
AD8305
Rev. B | Page 17 of 24
LOG-RATIO APPLICATIONS
It is often desirable to determine the ratio of two currents, for
example, in absorbance measurements. These are commonly
used to assess the attenuation of a passive optical component,
such as an optical filter or variable optical attenuator. In these
situations, a reference detector is used to measure the incident
power entering the component. The exiting power is then
measured using a second detector and the ratio is calculated to
determine the attenuation factor. Because the AD8305 is
fundamentally a ratiometric device, having nearly identical
logging systems for both numerator and denominator (IPD and
IREF, respectively), it can greatly simplify such measurements.
Figure 38 illustrates the AD8305 log-ratio capabilities in optical
absorbance measurements. Here a reference detector diode is
used to provide the reference current, IREF, proportional to the
optical reference power level. A second detector measures the
transmitted signal power, proportional to IPD. The AD8305
calculates the logarithm of the ratio of these two currents, as
shown in Equation 11, and which is reformulated in power
terms in Equation 12. Both of these equations include the
internal factor of 10,000 introduced by the output offset applied
to VLOG via pin VRDZ. If the true (nonoffset) log ratio shown in
Equation 4 is preferred, VRDZ should be grounded to remove
the offset. As already noted, the use of a negative supply at Pin
VNEG allows both VLOG and the buffer output to swing below
ground, and also allow the input pins INPT and IREF to be set
to ground potential. Therefore, the AD8305 may also be used to
determine the log ratio of two voltages.
Figure 38 also illustrates how a second order Sallen-Key low-
pass filter can be realized using two external capacitors and one
resistor. Here, the corner frequency is set to 1 kHz and the filter
Q is chosen to provide an optimally flat (overshoot-free) pulse
response. To scale this frequency either up or down, simply
scale the capacitors by the appropriate factor. Note that one of
the resistors needed to realize this filter is the output resistance
of 4.55 kΩ present at Pin VLOG. While this does not ratio
exactly to the external resistor, which may slightly alter the Q of
the filter, the effect on pulse response is be negligible for most
purposes. Note that the gain of the buffer (×2.5) is an integral
part of this illustrative filter design; in general, the filter may be
redesigned for other closed-loop gains.
The transfer characteristics can be expressed in terms of optical
power. If we assume that the two detectors have equal
responsivities, the relationship is
VOUT = 0.5 V log10(104 × PSIG/PREF)
(11)
Using the identity log10(AB) = log10A + log10B and defining the
attenuation as 10 × log10(PSIG/PREF), the overall transfer
characteristic can be written as
VOUT = 2 50 mV/dB × α
(12)
where α = 10 × log10(PSIG/PREF)
Figure 39 illustrates the linear-in-dB relationship between the
absorbance and the output of the circuit in Figure 38.
ATTENUATION (dB)
2.5
50
0
5
V
LO
G
(V
)
10
15
20
25
30
35
40
45
2.0
1.5
1.0
0.5
0
03
05
3-
0
38
Figure 39. Example of an Absorbance Transfer Function
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| AD8305BRU | 制造商:Analog Devices 功能描述:- Bulk |
| AD8305-EVAL | 制造商:Analog Devices 功能描述:Evaluation Board For IC Log Amp Single 12V 制造商:Analog Devices 功能描述:AD8305 EVALUATION BOARD - Bulk |
| AD8305-EVALZ | 制造商:Analog Devices 功能描述:EVALUATION BOARDS - Bulk |
| AD8306 | 制造商:AD 制造商全稱:Analog Devices 功能描述:5 MHz-400 MHz 100 dB High Precision Limiting-Logarithmic Amplifier |
| AD8306ACHIPS | 制造商:AD 制造商全稱:Analog Devices 功能描述:5 MHz-400 MHz 100 dB High Precision Limiting-Logarithmic Amplifier |
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