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MAX4206ETE+T

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P17
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
______________________________________________________________________________________ 13
Frequency Response and Noise Considerations
The MAX4206 bandwidth is proportional to the magni-
tude of the I
REF
and I
LOG
currents, whereas the noise is
inversely proportional to I
REF
and I
LOG
currents.
Common Mode
A common-mode input voltage, V
CMVOUT
, of 0.5V is
available at CMVOUT and can be used to bias the log-
ging and reference amplifier inputs by connecting
CMVOUT to CMVIN. An external voltage between 0.5V
and 1V can be applied to CMVIN to bias the logging
and reference transistor collectors and to optimize the
performance required for both single- and dual-supply
operation.
Adjusting the Logarithmic Intercept
Adjust the logarithmic intercept by changing the refer-
ence current, I
REF
. A resistor from REFISET to GND
(see Figures 5 and 6) adjusts the reference current,
according to the following equation:
where V
REFISET
is 0.5V. Select R
SET
between 5kΩ and
5MΩ. REFIOUT current range is 10nA to 10µA only.
Single-Supply Operation
When operating from a single +2.7V to +11V supply,
I
LOG
must be greater than I
REF
, resulting in a positive
slope of the log output voltages, LOGV1 and LOGV2.
Bias the log and reference amplifiers by connecting
CMVOUT to CMVIN or connecting an external voltage
reference between 0.5V and 1V to CMVIN. For single-
supply operation, connect V
EE
to GND.
Output Offset
Select R
OS
and I
OS
to adjust the output offset voltage
(see Figure 5). The magnitude of the offset voltage is
given by:
V
OS
= R
OS
✕ I
OSADJ
Scale Factor
The scale factor, K, is the slope of the logarithmic out-
put. For the LOGV1 amplifier, K = 0.25V/decade. When
operating in a single-supply configuration, adjust the
overall scale factor for the MAX4206 using the uncom-
mitted LOGV2 amplifier and the following equation,
which refers to Figure 5:
Select R1 between 1kΩ and 100kΩ, with an ideal value
of 10kΩ. The noninverting amplifier ensures that the
overall scale factor is greater than or equal to
0.25V/decade for single-supply operation.
Design Example
Desired:
Single-Supply Operation
Logarithmic intercept: 100nA
Overall scale factor = 1V/decade
Because there is no offset current applied to the circuit
(R
OS
= 0Ω), the reference current, I
REF
, equals the log
intercept of 100µA. Therefore,
Select R
1
= 10kΩ:
Dual-Supply Operation
When operating from dual ±2.7 to ±5.5V supplies, it is
not required that I
LOG
be greater than I
REF
. A positive
output voltage results at LOGV1 when I
LOG
exceeds
I
REF
. A negative output voltage results at LOGV1 when
I
LOG
is less than I
REF
. Bias the log and reference
amplifiers by connecting CMVOUT to CMVIN or con-
nect an external 0.5V to 1V reference to CMVIN. For
dual-supply operation with CMVIN < 0.5V, refer to the
MAX4207 data sheet.
Output Offset
The uncommitted amplifier in the inverting configuration
utilized by the MAX4206 facilitates large output-offset
voltage adjustments when operated with dual supplies.
The magnitude of the offset voltage is given by the fol-
lowing equation:
A resistive divider between REFVOUT, OSADJ, and
GND can be used to adjust V
OSADJ
(see Figure 6).
VV
R
RR
OSADJ REFOUT
=
+
4
34
VV
R
R
OS OSADJ
=+
1
2
1
Rk
VV
k210
1
025
130= −
=ΩΩ
/
.
R
V
nA
k
SET
=
×
=
05
10 100
500
.
Ω
RR
K
21
025
1= −
.
R
V
I
SET
REFISET
REF
=
×10
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
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
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P17

MAX4206ETE+T

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Manufacturer:
Maxim Integrated
Description:
Logarithmic Amplifiers Transimpedance w/ 100Db Dynamic Range
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