Datasheet

MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
______________________________________________________________________________________ 15
Photodiode Current Monitoring
Figure 8 shows the MAX4206 in a single-supply, optical-
power measurement circuit, common in fiberoptic
applications. The MAX4007 current monitor converts
the sensed APD current to an output current that drives
the MAX4206 LOGIIN input (APD current is scaled by
0.1). The MAX4007 also buffers the high-voltage APD
voltages from the lower MAX4206 voltages. The
MAX4206’s internal current reference sources 10nA
(R
SET
= 5MΩ) to the REFIIN input. This configuration
sets the logarithmic intercept 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.
Capacitive Loads
The MAX4206 drives capacitive loads of up to 50pF.
Reactive loads decrease phase margin and can pro-
duce excessive ringing and oscillation. Use an isolation
resistor in series with LOGV1 or LOGV2 to reduce the
effect of large capacitive loads. Recall that the combi-
nation of the capacitive load and the small isolation
resistor limits AC performance.
Power Dissipation
The LOGV1 and LOGV2 amplifiers are capable of
sourcing or sinking in excess of 30mA. Ensure that the
continuous power dissipation rating for the MAX4206 is
not exceeded.
TQFN Package
The 16-lead thin QFN package has an exposed paddle
that provides a heat-removal path, as well as excellent
electrical grounding to the PC board. The MAX4206’s
exposed pad is internally connected to V
EE
, and can
either be connected to the PC board V
EE
plane or left
unconnected. Ensure that only V
EE
traces are routed
under the exposed paddle.
Layout and Bypassing
Bypass V
CC
and V
EE
to GND with ceramic 0.1µF
capacitors. Place the capacitors as close to the device
as possible. Bypass REFVOUT and/or CMVOUT to
GND with a 0.1µF ceramic capacitor for increased
noise immunity and a clean reference current. For low-
current operation, it is recommended to use metal
guard rings around LOGIIN, REFIIN, and REFISET.
Connect this guard ring to CMVOUT.
Evaluation Kit
An evaluation kit is available for the MAX4206. The kit is
flexible and can be configured for either single-supply
or dual-supply operation. The scale factor and refer-
ence current are selectable. Refer to the MAX4206
Evaluation Kit data sheet for more information.
Chip Information
TRANSISTOR COUNT: 754
PROCESS: BiCMOS
MAX4206
V
EE
GND
0.1
µ
F
0.1
µ
F
0.1
µ
F
REFIIN
LOGIIN
V
CC
CMVIN
REFIOUT
CMVOUT
REFISET
V
CC
R
1
R
3
R
2
R
4
V
CC
SCALE
LOGV1
LOGV2
OSADJ
REFVOUT
100pF
100
Ω
100
Ω
100pF
Figure 7. Measuring Optical Absorbance