AD8305 Analog Devices, AD8305 Datasheet - Page 12

• Manufacturer Part Number: AD8305
• Description: 100 Db-range (10nA-1mA) Logarithmic Converter
• Manufacturer: Analog Devices

AD8305
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 enter-
ing the component. The exiting power is then measured using a
second detector and the ratio is calculated to determine the
attenuation factor. Since the AD8305 is fundamentally a ratiometric
device, having nearly identical logging systems for both numerator
and denominator (I
simplify such measurements.
Figure 5 illustrates the AD8305’s log-ratio capabilities in optical
absorbance measurements. Here a reference detector diode is used
to provide the reference current, I
reference power level. A second detector measures the transmitted
signal power, proportional to I
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 V
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 will
allow both V
and also allow the input pins INPT and IREF to be set to
ground potential. Thus, the AD8305 may also be used to deter-
mine the log ratio of two voltages.
Figure 5 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 kW present at Pin VLOG. While this will not ratio
LOG
REFERENCE
DETECTOR
SIGNAL
DETECTOR
and the buffer output to swing below ground,
+5V
PD
and I
P
P
SIG
REF
1k
REF
1nF
1k
1nF
VRDZ
VREF
, respectively), it can greatly
VSUM
IREF
I
INPT
PD
REF
I
PD
REF
. The AD8305 calculates the
, proportional to the optical
0.5V
0.5V
Figure 5. Optical Absorbance Measurement
20k
COMM
VNEG
Q2
Q1
80k
2.5 V
–
+
V
LOG
V
BE2
VPOS
BE1
COMPENSATION
GENERATOR
TEMPERATURE
via pin
+5V
BIAS
–12–
COMM
6.69k
14.2k
I
LOG
exactly to the external resistor, which may slightly alter the Q of
the filter, the effect on pulse response will 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
Using the identity log
attenuation as –10
acteristic can be written as
where
Figure 6 illustrates the linear-in-dB relationship between the
absorbance and the output of the circuit in Figure 5.
Figure 6. Example of an Absorbance Transfer Function
451
COMM
V
V
VOUT
VLOG
SCAL
OUT
OUT
BFIN
a =
2.5
2.0
1.5
1.0
0.5
0
=
0
–
=
10
2
0 5
.
–
5
¥
12.1k
44.2k
28.0k
50
V
33nF
log (
log
mV dB
10
10
10
log
10
(AB) = log
P
(
15
10
10
SIG
(P
4
¥
ATTENUATION – dB
18nF
SIG
20
¥
a
P
REF
P
/ P
SIG
25
0.5 log
)
REF
10
/
A + log
P
), the overall transfer char-
REF
30
10
( )
)
I
REF
I
PD
35
10
B and defining the
+ 2
40
45
REV. A
50
(11)
(12)