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| 型號： | AD8305ACP-R2 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 6/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC LOGARITHM CONV 100DB 16-LFCSP |
| 產(chǎn)品變化通告： | Product Discontinuance 27/July/2010 |
| 設(shè)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(biāo)準(zhǔn)包裝： | 1 |
| 類型： | 對數(shù)轉(zhuǎn)換器 |
| 應(yīng)用： | 光纖 |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-VFQFN 裸露焊盤，CSP |
| 供應(yīng)商設(shè)備封裝： | 16-LFCSP-VQ |
| 包裝： | 標(biāo)準(zhǔn)包裝 |
| 其它名稱： | AD8305ACP-R2DKR |
AD8305
Rev. B | Page 14 of 24
APPLICATIONS
The AD8305 is easy to use in optical supervisory systems and in
similar situations where a wide ranging current is to be
converted to its logarithmic equivalent, which is represented in
decibel terms. Basic connections for measuring a single-current
input are shown in Figure 35, which also includes various
nonessential components.
BIAS
GENERATOR
VLOG
COMM
VNEG
VSUM
IREF
0.5V
80k
0.5V
TEMPERATURE
COMPENSATION
VBE2
6.69k
Q1
COMM
20k
451
VREF
VRDZ
14.2k
Q2
INPT
COMM
2.5V
VPOS
BFIN
SCAL
VOUT
ILOG
IPD
200k
0.5 log10
IPD
1nA
1nF
1k
VBIAS
1nF
1k
12k
+5V
CFLT
10nF
8k
–
+
VBE1
03
053
-03
4
Figure 35. Basic Connections for Fixed Intercept Use
The 2 V difference in voltage between the VREF and INPT pins
in conjunction with the external 200 kΩ resistor RREF provide a
reference current, IREF, of 10 μA into Pin IREF. Connecting pin
VRDZ to VREF raises the voltage at VLOG by 0.8 V, effectively
lowering the intercept current, IINTC, by a factor of 104 to
position it at 1 nA. A wide range of other values for IREF, from
under 100 nA to over 1 mA, may be used. The effect of such
changes is shown in Figure 5.
Any temperature variation in RREF must be taken into account
when estimating the stability of the intercept. Also, the overall
noise increases when using very low values of IREF. In fixed
intercept applications, there is little benefit in using a large
reference current, since this only compresses the low current
end of the dynamic range when operated from a single supply,
here shown as 5 V. The capacitor between VSUM and ground is
recommended to minimize the noise on this node and to help
provide a clean reference current.
Because the basic scaling at VLOG is 0.2 V/decade, and a swing
of 4 V at the buffer output would correspond to 20 decades, it is
often useful to raise the slope to make better use of the rail-to- rail
voltage range. For illustrative purposes, the circuit in Figure 35
provides an overall slope of 0.5 V/ decade (25 mV/dB). Thus,
using IREF = 10 μA, VLOG runs from 0.2 V at IPD = 10 nA to 1.4 V
at IPD = 1 mA while the buffer output runs from 0.5 V to 3.5 V,
corresponding to a dynamic range of 120 dB (electrical, that is,
60 dB optical power).
The optional capacitor from VLOG to ground forms a single-
pole low-pass filter in combination with the 4.55 kΩ resistance
at this pin. For example, using a CFLT of 10 nF, the 3 dB corner
frequency is 3.5 kHz. Such filtering is useful in minimizing the
output noise, particularly when IPD is small. Multipole filters are
more effective in reducing the total noise; examples are provided in
the AD8304 data sheet.
The dynamic response of this overall input system is influenced
by the external RC networks connected from the two inputs
(INPT, IREF) to ground. These are required to stabilize the
input systems over the full current range. The bandwidth
changes with the input current due to the widely varying pole
frequency. The RC network adds a zero to the input system to
ensure stability over the full range of input current levels. The
network values shown in Figure 35 usually suffice, but some
experimentation may be necessary when the photodiode
capacitance is high.
Although the two current inputs are similar, some care is
needed to operate the reference input at extremes of current
(<100 nA) and temperature (<0°C). Modifying the RC network
to 4.7 nF and 2 kΩ is recommended for measuring 10 nA at
40°C. By inspecting the transient response to perturbations in
IREF at representative current levels, the capacitor value can be
adjusted to provide fast rise and fall times with acceptable
settling. To fine tune the network zero, the resistor value should be
adjusted.
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