Datasheet

MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
14 ______________________________________________________________________________________
Scale Factor
The scale factor, K, is the slope of the logarithmic output.
For the LOGV1 amplifier, K = 0.25V/decade. When oper-
ating from dual supplies, adjust the overall scale factor
for the MAX4206 using the uncommitted LOGV2 amplifi-
er and the following equation, which refers to Figure 6:
Select R
2
between 1kΩ and 100kΩ.
Design Example
Desired:
Dual-Supply Operation
Logarithmic intercept: 1µA
Overall scale factor = 1V/decade
Select R
1
= 10kΩ:
Measuring Optical Absorbance
A photodiode provides a convenient means of measur-
ing optical power, as diode current is proportional to
the incident optical power. Measure absolute optical
power using a single photodiode connected at LOGIIN,
with the MAX4206’s internal current reference driving
REFIIN. Alternatively, connect a photodiode to each of
the MAX4206’s logging inputs, LOGIIN and REFIIN, to
measure relative optical power (Figure 7).
In absorbance measurement instrumentation, a refer-
ence light source is split into two paths. The unfiltered
path is incident upon the photodiode of the reference
channel, REFIIN. The other path passes through a sam-
ple of interest, with the resulting filtered light incident on
the photodiode of the second channel, LOGIIN. The
MAX4206 outputs provide voltages proportional to the
log ratio of the two optical powers—an indicator of the
optical absorbance of the sample.
In wavelength-locking applications, often found in
fiberoptic communication modules, two photodiode cur-
rents provide a means of determining whether a given
optical channel is tuned to the desired optical frequency.
In this application, two bandpass optical filters with over-
lapping “skirts” precede each photodiode. With proper fil-
ter selection, the MAX4206 output can vary monotonically
(ideally linearly) with optical frequency.
Rk
V decade
k210
1
025
40=×






=ΩΩ
/
.
R
V
A
k
SET
=
×
=
05
10 1
50
.
µ
Ω
RR
K
21
025
=






.
MAX4206
V
EE
GND
REFIIN
LOGIIN
I
IN
CMVIN
REFIOUT
CMVOUT
REFISET
SCALE
LOGV2
OSADJ
LOGV1
REFVOUT
V
CC
V
CC
R
COMP
100
Ω
C
COMP
100pF
R
SET
500k
Ω
R
OS
0
Ω
R1
10k
Ω
R2
30k
Ω
0.1
µ
F
0.1
µ
F
V
OUT
0.1
µ
F
R
COMP
100
Ω
C
COMP
100pF
Figure 5. Single-Supply Typical Operating Circuit
MAX4206
V
EE
V
EE
GND
REFIIN
REFIOUT
LOGIIN
I
IN
REFISET
SCALE
LOGV2
LOGV1
CMVIN
CMVOUT
V
CC
V
CC
R
COMP
100
Ω
C
COMP
100pF
R
SET
50k
Ω
R1
10k
Ω
R2
40k
Ω
0.1
µ
F
0.1
µ
F
0.1
µ
F
V
OUT
R
COMP
100
Ω
C
COMP
100pF
OSADJ
REFVOUT
R4
R3
Figure 6. Dual-Supply Typical Operating Circuit