AD8305(2003) 데이터 시트보기 (PDF) - Analog Devices
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AD8305 Datasheet PDF : 20 Pages
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AD8305
Managing Intercept and Slope
When using a single supply, VRDZ should be directly connected
to VREF to allow operation over the entire five-decade input
current range. As noted previously, this introduces an accurate
offset voltage of 0.8 V at the VLOG pin, equivalent to four decades,
resulting in a logarithmic transfer function that can be written as:
( ) VLOG = VY log10 104 ¥ IPD/IREF
( ) = VY log10 IPD/IINTC
(6)
where IINTC = IREF/104
Thus, the effective intercept current IINTC is only one ten-
thousandth of IREF, corresponding to 1 nA when using the
recommended value of IREF = 10 mA.
The slope can be reduced by attaching a resistor to the VLOG
pin. This is strongly discouraged, in view of the fact that the
on-chip resistors will not ratio correctly to the added resistance.
Also, it is rare that one would want to lower the basic slope of
10 mV/dB; if this is needed, it should be effected at the low
impedance output of the buffer, which is provided to avoid such
miscalibration and also allow higher slopes to be used.
The AD8305 buffer is essentially an uncommitted op amp with
rail-to-rail output swing, good load-driving capabilities and a
unity-gain bandwidth of >12 MHz. In addition to allowing the
introduction of gain, using standard feedback networks and
thereby increasing the slope voltage VY, the buffer can be used
to implement multipole low-pass filters, threshold detectors,
and a variety of other functions. Further details of these can be
found in the AD8304 data sheet.
Response Time and Noise Considerations
The response time and output noise of the AD8305 are funda-
mentally a function of the signal current IPD. For small currents,
the bandwidth is proportional to IPD, as shown in TPC 13. The
output low frequency voltage-noise spectral-density is a function
of IPD (TPC 15) and also increases for small values of IREF.
Details of the noise and bandwidth performance of translinear
log amps can be found in the AD8304 Data Sheet.
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 2, which also includes various nonessential com-
ponents, as will be explained.
VRDZ
+5V
VPOS
( ) 0.5 log10
IPD
1nA
VOUT
VREF
200k⍀
80k⍀ 2.5V
20k⍀
BIAS
GENERATOR
0.5V
IREF
VBIAS
1k⍀
1nF
IPD INPT
1k⍀
VSUM
1nF
0.5V
COMM
Q2
Q1
SCAL
VBE2
14.2k⍀
BFIN
– TEMPERATURE ILOG 451⍀
+ COMPENSATION
VBE1
6.69k⍀
COMM
1nF
VNEG
COMM
12k⍀
8k⍀
VLOG
CFLT
10nF
Figure 2. Basic Connections for Fixed Intercept Use
The 2 V difference in voltage between the VREF and INPT pins
in conjunction with the external 200 kW resistor RREF provide a
reference current IREF of 10 mA 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 TPC 3.
Any temperature variation in RREF must be taken into account
when estimating the stability of the intercept. Also, the overall
noise will increase 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.
Since the basic scaling at VLOG is 0.2 V/decade, and thus a swing
of 4 V at the buffer output would correspond to 20 decades, it will
often be useful to raise the slope to make better use of the rail-
to-rail voltage range. For illustrative purposes, the circuit in
Figure 2 provides an overall slope of 0.5 V/decade (25 mV/dB).
Thus, using IREF = 10 mA, 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 kW 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.
–10–
REV. A
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