MAX4207 Maxim, MAX4207 Datasheet - Page 12

• Manufacturer Part Number: MAX4207
• Description: The MAX4207 logarithmic amplifier computes the log ratio of an input current relative to a reference current (externally or internally generated) and provides a corresponding voltage output with a default -0
• Manufacturer: Maxim

Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
Table 1. MAX4207 Example Configurations
The inverting configuration utilized by the MAX4207
facilitates large output-offset voltage adjustments. The
magnitude of the offset voltage is given by the following
equation:
A resistive divider between REFVOUT, OSADJ, and
GND can be used to adjust V
The scale factor, K, is the slope of the logarithmic output.
For the LOGV1 amplifier, K = -0.25V/decade. Adjust the
overall scale factor for the MAX4207 using the uncom-
mitted LOGV2 amplifier and the following equation,
which refers to Figure 5:
Select R
Desired:
Logarithmic intercept: 1µA
Overall scale factor = +1V/decade
Select R
12
LOGV2 AMPLIFIER CONFIGURATION
Inverting
Noninverting
______________________________________________________________________________________
2
1
between 1kΩ and 100kΩ.
= 10kΩ:
R
2 10
V
=
OSADJ
R
V
R
SET
OS
2
k
Ω
=
=
=
R
=
×
V
10 1
1
V
OSADJ
1
0 5
REFOUT
V decade
−
×
.
0 25
/
−
K
.
V
0 25
µ
OSADJ
.
A



1
=



+
50
R
R
R
3
k
2
(see Figure 5).
1
R
=
Ω
+



4
40
Design Example
R
4
k
Output Offset
I
I
I
I
Ω
INPUT CONDITIONS



LOG
LOG
LOG
LOG
Scale Factor
> I
< I
> I
< I
REF
REF
REF
REF
(constant)
(constant)
(constant)
(constant)
Figure 6 shows the MAX4207 in an optical-power
measurement circuit, common in fiberoptic applications.
The MAX4007 current monitor converts the sensed APD
current to an output current that drives the MAX4207
LOGIIN input (APD current is scaled by 0.1). The
MAX4007 also buffers the high-voltage APD voltages
from the lower MAX4207 voltages. The MAX4207’s inter-
nal current reference sources 10nA (R
REFIIN input. This configuration sets the logarithmic inter-
cept to 10nA, corresponding to an APD current of 100nA.
The unity-gain configuration of the output buffer maintains
the -0.25V/decade gain present at the LOGV1 output.
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 MAX4207’s internal current reference driving
REFIIN. Alternatively, connect a photodiode to each of
the MAX4207’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
MAX4207 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 frequen-
cy. In this application, two bandpass optical filters with
overlapping “skirts” precede each photodiode. With
proper filter selection, the MAX4207 output can vary
monotonically (ideally linearly) with optical frequency.
Negative
Negative
V
Positive
Positive
LOGV1
Photodiode Current Monitoring
Measuring Optical Absorbance
SET
Negative
Negative
V
Positive
Positive
LOGV2
= 5MΩ) to the