General Description
The MAX4206 logarithmic amplifier computes the log
ratio of an input current relative to a reference current
(externally or internally generated) and provides a cor-
responding voltage output with a default 0.25V/decade
scale factor. The device operates from a single +2.7V
to +11V supply or from dual ±2.7V to ±5.5V supplies
and is capable of measuring five decades of input cur-
rent across a 10nA to 1mA range.
The MAX4206’s uncommitted op amp can be used for
a variety of functions, including filtering noise, adding
offset, and adding additional gain. A 0.5V reference is
also included to generate an optional precision current
reference using an external resistor, which adjusts the
log intercept of the MAX4206. The output-offset voltage
and the adjustable scale factor are also set using exter-
nal resistors.
The MAX4206 is available in a space-saving 16-pin thin
QFN package (4mm x 4mm x 0.8mm), and is specified
for operation over the -40°C to +85°C extended temper-
ature range.
Applications
Photodiode Current Monitoring
Portable Instrumentation
Medical Instrumentation
Analog Signal Processing
Features
♦+2.7V to +11V Single-Supply Operation
♦±2.7V to ±5.5V Dual-Supply Operation
♦5 Decades of Dynamic Range (10nA to 1mA)
♦Monotonic Over a 1nA to 1mA Range
♦0.25V/Decade Internally Trimmed Output Scale
Factor
♦Adjustable Output Scale Factor
♦Adjustable Output Offset Voltage
♦Internal 10nA to 10µA Reference Current Source
♦0.5V Input Common-Mode Voltage
♦Small 16-Pin Thin QFN Package (4mm x 4mm x
0.8mm)
♦-40°C to +85°C Operating Temperature Range
♦Evaluation Kit Available
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
MAX4206
VEE
GND
REFIIN
CMVIN
REFIOUT
LOGIIN
IIN
CMVOUT
REFISET
SCALE
LOGV2
OSADJ
LOGV1
REFVOUT
VCC
VCC
RCOMP
CCOMP
RSET
ROS
R1
R2
0.1µF
0.1µF
VOUT
0.1µF
RCOMP
CCOMP
Typical Operating Circuit
19-3071; Rev 1; 6/04
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART TEMP RANGE PIN-PACKAGE
MAX4206ETE -40°C to +85°C 16 Thin QFN-EP*
*EP = Exposed paddle.
16 15 14 13
CMVIN
LOGIIN
REFIIN
REFIOUT
9
10
11
12
N.C.
VCC
REFISET
CMVOUT
4
3
2
1
VEE
GND
REFVOUT
N.C.
5678
LOGV1
OSADJ
SCALE
LOGV2
MAX4206
TOP VIEW
(LEADS ON BOTTOM)
THIN QFN
Pin Configuration
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(All voltages referenced to GND, unless otherwise noted.)
VCC.........................................................................-0.3V to +12V
VEE............................................................................-6V to +0.3V
Supply Voltage (VCC to VEE) .............................................. +12V
REFVOUT....................................................(VEE - 0.3V) to +3.0V
OSADJ, SCALE, REFISET...........................(VEE - 0.3V) to +5.5V
REFIIN, LOGIIN ........................................(VEE - 0.3V) to VCMVIN
LOGV1, LOGV2, CMVOUT,
REFIOUT ......................................(VEE - 0.3V) to (VCC + 0.3V)
CMVIN............................................................(VEE - 0.3V) to +1V
Continuous Current (REFIIN, LOGIIN) ................................10mA
Continuous Power Dissipation (TA= +70°C)
16-Pin Thin QFN (derate 16.9mW/°C above +70°C) ....1349mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature .....................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
DC ELECTRICAL CHARACTERISTICS—Single-Supply Operation
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
Supply Voltage VCC (Note 2) 2.7
11.0
V
TA = +25°C 3.9 5
Supply Current ICC TA = -40°C to +85°C 7 mA
Minimum 10 nA
LO GIIN C ur r ent Rang e ( N otes 3, 4)
ILOG Maximum 1 mA
Minimum 10 nA
RE FIIN C ur r ent Rang e ( N otes 3, 4)
IREF Maximum 1 mA
Common-Mode Voltage
VCMVOUT 480 500 520
mV
Common-Mode Voltage Input
Range
VCMVIN
0.5 1.0 V
TA = +25°C ±2 ±5
Log Conformity Error VLC
IREF = 10nA,
ILOG= 10nA to 1mA,
K = 0.25V/decade
(Note 4)
TA = -40°C to +85°C ±10
mV
TA = +25°C
237.5 250 262.5
Logarithmic Slope (Scale Factor)
KTA = -40°C to +85°C (Note 4)
231.25 268.75
mV/
Logarithmic Slope (Scale Factor)
Temperature Drift TA = -40°C to +85°C 80
µV/
°C
Input Offset Voltage VIO TA = +25°C, |VCMVIN - VREFIIN|,
|VCMVIN - VLOGIIN|15mV
Input Offset Voltage Temperature
Drift VIOS |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN|6
TA = +25°C
1.218 1.238 1.258
Voltage Reference Output
VREFVOUT
TA = -40°C to +85°C (Note 4)
1.195 1.275
V
V ol tag e Refer ence O utp ut C ur r ent IREFVOUT
1mA
TA = +25°C
490 500 510
C ur r ent Refer ence O utp ut V ol tag eV
REFISET
TA = -40°C to +85°C (Note 4)
482 518
mV
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
_______________________________________________________________________________________ 3
AC ELECTRICAL CHARACTERISTICS—Single-Supply Operation
(VCC = +5V, VEE = GND = 0, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
DC ELECTRICAL CHARACTERISTICS—Single-Supply Operation (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
LOGV2 BUFFER
TA = +25°C 0.4 2
Input Offset Voltage VIO TA = -40°C to +85°C (Note 4) 6 mV
Input Bias Current IB(Note 4)
0.01
1nA
VOH RL to GND = 2kΩVCC -
0.2
VCC -
0.3
Output Voltage Range
VOL RL to GND = 2kΩ0.2
0.08
V
IOUT+ Sourcing 34
Output Short-Circuit Current IOUT- Sinking 58 mA
Slew Rate SR 12 V/µs
Unity-Gain Bandwidth GBW 5
MHz
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
LOGV2 Total Noise 0.1Hz to 10Hz, total output-referred noise,
IREF = 10nA, ILOG = 100nA 17 µVRMS
LOGV2 Spot Noise Density f = 5kHz, IREF = 10nA, ILOG = 100nA 0.8 µV/√Hz
REFVOUT Total Noise 1Hz to 10Hz, total output-referred noise 3.3 µVRMS
REFVOUT Spot Noise Density f = 5kHz 266 nV/√Hz
REFISET Total Noise 1Hz to 10Hz, total output-referred noise 0.67 µVRMS
REFISET Spot Noise Density f = 5kHz 23 nV/√Hz
Small-Signal Unity-Gain
Bandwidth
IREF = 1µA, ILOG = 10µA, RCOMP = 300Ω,
CCOMP = 32pF 1MHz
DC ELECTRICAL CHARACTERISTICS—Dual-Supply Operation
(VCC = +5V, VEE = -5V, GND = 0, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
VCC 2.7 5.5
Supply Voltage (Note 2) VEE
-2.7 -5.5
V
TA = +25°C 5 6
Supply Current ICC TA = -40°C to +85°C 7.5 mA
Minimum 10 nA
LO GIIN C ur r ent Rang e ( N otes 3, 4)
ILOG Maximum 1 mA
Minimum 10 nA
RE FIIN C ur r ent Rang e ( N otes 3, 4)
IREF Maximum 1 mA
Common-Mode Voltage
VCMVOUT 480 500 520
mV
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
4_______________________________________________________________________________________
DC ELECTRICAL CHARACTERISTICS—Dual-Supply Operation (continued)
(VCC = +5V, VEE = -5V, GND = 0, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Common-Mode Voltage Input
Range
VCMVIN
0.5 1.0 V
TA = +25°C ±2 ±5
Log Conformity Error VLC
IREF = 10nA,
ILOG= 10nA to 1mA,
K = 0.25V/decade
(Note 4)
TA = -40°C to +85°C ±10
mV
TA = +25°C
237.5 250 262.5
Logarithmic Slope (Scale Factor)
KTA = -40°C to +85°C
231.25 268.75
mV/
decade
Logarithmic Slope (Scale Factor)
Temperature Drift TA = -40°C to +85°C 80
µV/
decade/
°C
Input Offset Voltage VIO TA = +25°C, |VCMVIN - VREFIIN|,
|VCMVIN - VLOGIIN|15mV
Input Offset Voltage
Temperature Drift VIOS |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN|6
µV/°C
TA = +25°C
1.218 1.238 1.258
Voltage Reference Output
VREFVOUT
TA = -40°C to +85°C (Note 4)
1.195 1.275
V
Voltage Reference Output
Current
IREFVOUT
1mA
TA = +25°C
490 500 510
Current Reference Output
Voltage
VREFISET
TA = -40°C to +85°C (Note 4)
482 518
mV
LOGV2 BUFFER
TA = +25°C 0.4 2
Input Offset Voltage VIO TA = -40°C to +85°C (Note 4) 6 mV
Input Bias Current IB(Note 4)
0.01
1nA
VOH RL to GND = 2kΩVCC -
0.2
VCC -
0.3
Output Voltage Range
VOL RL to GND = 2kΩVEE +
0.2
VEE +
0.08
V
IOUT+ Sourcing 34
Output Short-Circuit Current IOUT- Sinking 58 mA
Slew Rate SR 12 V/µs
Unity-Gain Bandwidth GBW 5 MHz
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
_______________________________________________________________________________________ 5
AC ELECTRICAL CHARACTERISTICS—Dual-Supply Operation
(VCC = +5V, VEE = -5V, GND = 0, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
LOGV2 Total Noise 0.1Hz to 10Hz, total output-referred noise,
IREF = 10nA, ILOG = 100nA 17
µVRMS
LOGV2 Spot Noise Density f = 5kHz, IREF = 10nA, ILOG = 100nA 0.8
µV/√Hz
REFVOUT Total Noise 1Hz to 10Hz, total output-referred noise 3.3
µVRMS
REFVOUT Spot Noise Density f = 5kHz
266
nV/√Hz
REFISET Total Noise 1Hz to 10Hz, total output-referred noise
0.67
µVRMS
REFISET Spot Noise Density f = 5kHz 23
nV/√Hz
Small-Signal Unity-Gain
Bandwidth
IREF = 1µA, ILOG = 10µA, RCOMP = 300Ω,
CCOMP = 32pF 1
MHz
Note 1: All devices are 100% production tested at TA= +25°C. All temperature limits are guaranteed by design.
Note 2: Guaranteed and functionally verified.
Note 3: Log conformity error less than ±5mV with scale factor = 0.25V/decade.
Note 4: Guaranteed by design.
Typical Operating Characteristics
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
VLOGV1 vs. ILOG
MAX4206 toc01
ILOG (A)
VLOGV1 (V)
1m100µ
100n 1µ10µ
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
-0.25
10n 10m
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = GND
1m100µ
10n 100n 1µ10µ
1n 10m
VLOGV1 vs. ILOG
(IREF = 10nA)
MAX4206 toc02
ILOG (A)
VLOGV1 (V)
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
-0.25
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = -5V
1m100µ
10n 100n 1µ10µ10m
VLOGV1 vs. ILOG
MAX4206 toc03
ILOG (A)
VLOGV1 (V)
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
-0.25
IREF = 10nA
TA = -40°C TO +85°C
VCC = +2.7V
VEE = GND
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
6_______________________________________________________________________________________
-0.25
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
-0.50
VLOGV1 vs. ILOG
(IREF = 10nA TO 1mA)
MAX4206 toc04
ILOG (A)
VLOGV1 (V)
1m100µ
100n 1µ10µ
10n 10m
10nA
100nA
1µA
10µA
100µA
1mA
VLOGV1 vs. IREF
(ILOG = 10nA TO 1mA)
MAX4206 toc05
IREF (A)
VLOGV1 (V)
100µ
10µ
1µ
100n10n
-0.25
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
-0.50
1n 1m
1µA10µA100µA
1mA
100nA
10nA
-15
-10
-5
0
5
10
15
20
-20
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
MAX4206 toc06
ILOG (A)
ERROR (mV)
1m100µ
100n 1µ10µ
10n 10m
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = GND
TA = -40°C
1m100µ
10n 100n 1µ10µ
1n 10m
-15
-10
-5
0
5
10
15
20
-20
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
MAX4206 toc07
ILOG (A)
ERROR (mV)
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = -5V
TA = -40°C
-15
-10
-5
0
5
10
15
20
-20
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
MAX4206 toc08
ILOG (A)
ERROR (mV)
1m100µ
100n 1µ10µ
10n 10m
IREF = 10nA
TA = -40°C TO +85°C
VCC = +2.7V
VEE = GND
TA = -40°C
-15
-10
-5
0
5
10
15
20
-20
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
MAX4206 toc09
ILOG (A)
ERROR (mV)
1m100µ
100n 1µ10µ
10n 10m
IREF = 10nA
SINGLE SUPPLY: VCC = +2.7V, +5V, 11V,
VEE = GND
DUAL SUPPLY: VCC = +5V
VEE = -5V
VLOGIIN - VCMVIN vs. ILOG
MAX4206 toc10
ILOG (A)
VLOGIIN - VCMVIN (mV)
1m100µ100n 1µ10µ
5
4
3
2
1
0
-1
-2
-3
-4
-5
10n1n 10m
IREF = 10nA
VLOGV2 VOLTAGE-NOISE DENSITY
vs. FREQUENCY
MAX4206 toc11
FREQUENCY (Hz)
1M100k10k1k10010
0.1
1
10
0.01
110M
NOISE DENSITY (µV/√Hz)
IREF = ILOG
100nA
10nA
1µA
10µA
TOTAL WIDEBAND VOLTAGE NOISE
AT VLOGV2 vs. ILOG
MAX4206 toc12
ILOG (A)
VOLTAGE NOISE (mVRMS)
100µ10µ1µ100n
1
2
3
4
5
0
10n 1m
IREF = ILOG
f = 1Hz TO 1MHz
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
_______________________________________________________________________________________ 7
ILOG PULSE RESPONSE
(IREF = 100nA,
VCC = 5V, VEE = GND)
MAX4206 toc13
VLOGV1 (V)
20µs/div
0.50V
1.0V 100µA TO 1mA
10µA TO 100µA
1µA TO 10µA
100nA TO 1µA
0.25V
0.75V
0.75V
0.50V
0.25V
0
ILOG PULSE RESPONSE
(IREF = 100nA,
VCC = 5V, VEE = -5V)
MAX4206 toc14
VLOGV1 (V)
20µs/div
0.50V
1.0V 100µA TO 1mA
10µA TO 100µA
1µA TO 10µA
100nA TO 1µA
0.25V
0.75V
0.75V
0.50V
0.25V
0
IREF PULSE RESPONSE
(ILOG = 1mA)
MAX4206 toc15
VLOGV1 (V)
20µs/div
0.50V
1.0V 1µA TO 100nA
10µA TO 1µA
100µA TO 10µA
1mA TO 100µA
0.25V
0.75V
0.75V
0.50V
0.25V
0
0
10
5
20
15
25
30
240 250245 255
LOGARITHMIC SLOPE DISTRIBUTION
MAX4206 toc16
SLOPE (mV/DECADE)
COUNT (%)
260
0
5
15
10
20
25
1.232 1.238 1.2401.234 1.236 1.242
VREFVOUT DISTRIBUTION
MAX4206 toc17
VREFVOUT (V)
COUNT (%)
RL = 100kΩ
1.244
0
6
4
2
12
8
10
14
16
-1.0 0.5 1.0-0.5 0 1.5 2.0 2.5
INPUT OFFSET VOLTAGE DISTRIBUTION
MAX4206 toc18
INPUT OFFSET VOLTAGE (mV)
COUNT (%)
INPUT OFFSET VOLTAGE = VLOGIIN - VCMVIN
3.0
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
8_______________________________________________________________________________________
0
-100 10 100 1k 10k 100k 1M
REFERENCE POWER-SUPPLY
REJECTION RATIO vs. FREQUENCY
-80
MAX4206 toc23
FREQUENCY (Hz)
REFERENCE PSRR (dB)
-60
-40
-20
-90
-70
-50
-30
-10 CREFVOUT = 0.1µF
IREFVOUT = 1mA
REFERENCE LINE-TRANSIENT RESPONSE
MAX4206 toc24
10µs/div
1.238V
0V
VCC
2V/div
VREFVOUT
200mV/div
CREFVOUT = 0F
REFERENCE LOAD-TRANSIENT RESPONSE
MAX4206 toc25
100µs/div
1.238V
0mA
IREFVOUT
1mA/div
VREFVOUT
100mV/div
CREFVOUT = 0F
REFERENCE TURN-ON TRANSIENT RESPONSE
MAX4206 toc26
10µs/div
0V
0V
VCC
2.5V/div
VREFVOUT
500mV/div
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
-10
-4
-6
-8
-2
0
2
4
6
8
10
-50 0-25 25 50 75 100
OUTPUT OFFSET VOLTAGE
vs. TEMPERATURE
MAX4206 toc19
TEMPERATURE (°C)
VLOGV1 (mV)
IREF = 1µA
ILOG 1µA
1.20
1.23
1.22
1.21
1.24
1.25
1.26
1.27
1.28
1.29
1.30
-50 0-25 25 50 75 100
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. TEMPERATURE
MAX4206 toc20
TEMPERATURE (°C)
REFERENCE OUTPUT VOLTAGE (V)
1.20
1.23
1.22
1.21
1.24
1.25
1.26
1.27
1.28
1.29
1.30
-1.0 -0.5 0.50 1.0
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. LOAD CURRENT
MAX4206 toc21
LOAD CURRENT (mA)
REFERENCE OUTPUT VOLTAGE (V)
1.200
1.215
1.210
1.205
1.220
1.225
1.230
1.235
1.240
1.245
1.250
-1.0 -0.5 0.501.0
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. SUPPLY VOLTAGE
MAX4206 toc22
LOAD CURRENT (mA)
REFERENCE OUTPUT VOLTAGE (V)
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
_______________________________________________________________________________________ 9
SMALL-SIGNAL AC RESPONSE
ILOG TO VLOGV1
MAX4206 toc27
FREQUENCY (Hz)
NORMALIZED GAIN (dB)
1M100k10k1k
-50
-40
-30
-20
-10
0
10
-60
100 10M
ILOG = 100µA
ILOG = 1mA
ILOG = 10µA
ILOG = 1µA
ILOG = 100nA
CCOMP = 33pF
RCOMP = 330Ω
SMALL-SIGNAL AC RESPONSE
ILOG TO VLOGV1
MAX4206 toc28
FREQUENCY (Hz)
NORMALIZED GAIN (dB)
1M100k10k1k
-50
-40
-30
-20
-10
0
10
-60
100 10M
ILOG = 100µA
ILOG = 1mA
ILOG = 10µA
ILOG = 1µA
ILOG = 100nA
CCOMP = 100pF
RCOMP = 1kΩ
SMALL-SIGNAL AC RESPONSE
OF BUFFER
MAX4206 toc29
FREQUENCY (Hz)
NORMALIZED GAIN (dB)
10M1M100k
-9
-6
-3
0
3
-12
10k 100M
AV = 2V/V
AV = 4V/V
AV = 1V/V
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ,
TA= +25°C, unless otherwise noted.)
Pin Description
PIN NAME FUNCTION
1, 9 N.C. No Connection. Not internally connected.
2
REFVOUT
1.238V Reference Voltage Output. Bypass REFVOUT to GND with a 0 to 1µF capacitor (optional).
3GND Ground
4V
EE Negative Power Supply. Bypass VEE to GND with a 0.1µF capacitor.
5LOGV1 Logarithmic Amplifier Voltage Output 1. The output scale factor of LOGV1 is 0.25V/decade.
6OSADJ
Offset Adjust Input. When operating from a single power supply, current applied to OSADJ adjusts
the output offset voltage (see the Output Offset section).
7SCALE Scale Factor Input. Adjust the output scale factor for LOGV2 using a resistive divider (see the Scale
Factor section).
8LOGV2 Logarithmic Amplifier Voltage Output 2. Adjust the output scale factor for LOGV2 using a resistive
divider (see the Scale Factor section).
10 VCC Positive Power Supply. Bypass VCC to GND with a 0.1µF capacitor.
11 REFISET Current Reference Adjust Input. A resistor (RSET), from REFISET to GND, adjusts the current at
REFIOUT (see the Adjusting the Logarithmic Intercept section).
12
CMVOUT
0.5V Common-Mode Voltage Reference Output. Bypass CMVOUT to GND with a 0.1µF capacitor.
13
REFIOUT
Current Reference Output. The internal current reference output is available at REFIOUT.
14 REFIIN Current Reference Input. Apply an external reference current at REFIIN. IREFIIN is the reference
current used by the logarithmic amplifier when generating LOGV1.
15 LOGIIN Current Input to Logarithmic Amplifier. LOGIIN is typically connected to a photodiode anode or other
external current source.
16 CMVIN Common-Mode Voltage Input. VCMVIN is the common-mode voltage for the input and reference
amplifiers (see the Common Mode section).
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
10 ______________________________________________________________________________________
Detailed Description
Theory
Figure 2 shows a simplified model of a logarithmic
amplifier. Two transistors convert the currents applied
at LOGIIN and REFIIN to logarithmic voltages accord-
ing to the following equation:
where:
VBE = base-emitter voltage of a bipolar transistor
k = 1.381 x 10-23 J/K
T = absolute temperature (K)
q = 1.602 x 10 –19 C
IC= collector current
IS= reverse saturation current
The logarithmic amplifier compares VBE1 to the refer-
ence voltage VBE2, which is a logarithmic voltage for a
known reference current, IREF. The temperature depen-
dencies of a logarithmic amplifier relate to the thermal
voltage, (kT/q), and IS. Matched transistors eliminate
the IStemperature dependence of the amplifier in the
following manner:
VVV
kT
q
I
I
kT
q
I
I
kT
q
I
I
I
I
kT
q
I
I
OUT BE BE
LOG
S
REF
S
LOG
S
REF
S
LOG
REF
=−
=
−
=
−
=
12
ln ln
ln ln
ln
=
()
=×
kT
q
I
I
KI
Isee Figure
LOG
REF
LOG
REF
ln( ) log
log ( )
10
3
10
10
VkT
q
I
I
BE C
S
=
ln
MAX4206
LOGIIN
CMVIN
VCC
REFIIN
VCC
CURRENT
CORRECTION
VCC
LOGV1
SCALE
OSADJ
LOGV2
VCC
VCC
REFISET
CURRENT MIRROR
CMVOUTREFVOUT
REFIOUT
GND
VEE
0.5V
1.238V
VEE
VEE
SUMMING
AMPLIFIER
AND
TEMPERATURE
COMPENSATION
Figure 1. Functional Diagram
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
______________________________________________________________________________________ 11
where:
K = scale factor (V/decade)
ILOG = the input current at LOGIIN
IREF = the reference current at REFIIN
The MAX4206 uses internal temperature compensation
to virtually eliminate the effects of the thermal voltage,
(kT/q), on the amplifier’s scale factor, maintaining a
constant slope over temperature.
Definitions
Transfer Function
The ideal logarithmic amplifier transfer function is:
Adjust K (see the Scale Factor section) to increase the
transfer-function slope as illustrated in Figure 3. Adjust
IREF using REFISET (see the Adjusting the Logarithmic
Intercept section) to shift the logarithmic intercept to the
left or right as illustrated in Figure 4.
Log Conformity
Log conformity is the maximum deviation of the
MAX4206’s output from the best-fit straight line of the
VLOGV1 versus log (ILOG/IREF) curve. It is expressed as
a percent of the full-scale output or an output voltage.
Referred-to-Input and Referred-to-Output Errors
The log nature of the MAX4206 insures that any addi-
tive error at LOGV1 corresponds to multiplicative error
at the input, regardless of input level.
Total Error
Total error (TE) is defined as the deviation of the output
voltage, VLOGV1, from the ideal transfer function (see
the Transfer Function section):
Total error is a combination of the associated gain,
input offset current, input bias current, output offset
voltage, and transfer characteristic nonlinearity (log
conformity) errors:
where VLC and VOSOUT are the log conformity and out-
put offset voltages, respectively. Output offset is
defined as the offset occurring at the output of the
MAX4206 when equal currents are presented to ILOG
and IREF. Because the MAX4206 is configured with
a gain of K = 0.25V/decade, a 4 should multiply the
(±VLC ±VOSOUT) term, if VLC and VOSOUT were derived
from this default configuration.
VKKII
II VV
LOGV LOG BIAS
REF BIAS LC OSOUT210
1
2
14=± −
−
±± ±
()
()log∆
VVTE
LOGV IDEAL1 =±
VK I
I
IDEAL LOG
REF
=×
log10
LOGIIN
CMVIN
VCC
REFIIN
VCC
VBE1
VBE2
VEE
VEE
ILOG
IREF
Figure 2. Simplified Model of a Logarithmic Amplifier
IDEAL TRANSFER FUNCTION
WITH VARYING K
MAX4206 fig03
CURRENT RATIO (ILOG/IREF)
NORMALIZED OUTPUT VOLTAGE (V)
100.1
-2
-3
-1
0
1
2
3
4
-4
0.001 1000
VOUT = K LOG (ILOG/IREF)K = 1
K = 0.5
K = 0.25
Figure 3. Ideal Transfer Function with Varying K
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
12 ______________________________________________________________________________________
IBIAS1 and IBIAS2 are currents in the order of 20pA, sig-
nificantly smaller than ILOG and IREF, and can therefore
be eliminated:
Expanding this expression:
The first term of this expression is the ideal component
of VLOGV1. The remainder of the expression is the TE:
In the second term, one can generally remove the
products relating to ∆K, because ∆K is generally much
less than 1. Hence, a good approximation for TE is
given by:
As an example, consider the following situation:
Full-scale input = 5V
ILOG = 100µA
IREF = 100nA
K = 1 ±5% V/decade (note that the uncommitted ampli-
fier is configured for a gain of 4)
VLC = ±5mV (obtained from the Electrical Character-
istics table)
VOSOUT = ±2mV (typ)
TA= +25°C
Substituting into the total error approximation,
TE ≅±(1V/decade)(0.05log10 (100µA/100nA)
±4 (±5mV ±2mV) = ±[0.15V ±4(±7mV)]
As a worst case, one finds TE ≅±178mV or ±3.6% of
full scale.
When expressed as a voltage, TE increases in proportion
with an increase in gain as the contributing errors are
defined at a specific gain. Calibration using a look-up
table eliminates the effects of gain and output offset
errors, leaving conformity error as the only factor con-
tributing to total error. For further accuracy, consider tem-
perature monitoring as part of the calibration process.
Applications Information
Input Current Range
Five decades of input current across a 10nA to 1mA
range are acceptable for ILOG and IREF. The effects of
leakage currents increase as ILOG and IREF fall below
10nA. Bandwidth decreases at low ILOG values (see
the Frequency Response and Noise Considerations
section). As ILOG and IREF increase to 1mA or higher,
transistors become less logarithmic in nature. The
MAX4206 incorporates leakage current compensation
and high-current correction circuits to compensate for
these errors.
Frequency Compensation
The MAX4206’s frequency response is a function of the
input current magnitude and the selected compensation
network at LOGIIN and REFIIN. The compensation net-
work comprised of CCOMP and RCOMP ensures stability
over the specified range of input currents by introducing
an additional pole/zero to the system. For the typical
application, select CCOMP = 100pF and RCOMP = 100Ω.
Where high bandwidth at low current is required, CCOMP
= 32pF and RCOMP = 330Ωare suitable compen-
sation values.
TE K K I
IVV
LOG
REF LC OSOUT
≅±
±± ±
()
∆log10 4
TE K K I
IKKVV
LOG
REF LC OSOUT
≅±
±±±±
()
∆∆log ( )
10 41
VK
I
IKK I
I
KKVV
LOGV LOG
REF
LOG
REF
LC OSOUT
210 10
41
≅
±
±±±±
()
log log
( )
∆
∆
VKKI
IVV
LOGV LOG
REF LC OSOUT210
14≅±
±± ±
()
()log∆
IDEAL TRANSFER FUNCTION
WITH VARYING IREF
MAX4206 fig04
ILOG (A)
OUTPUT VOLTAGE (V)
100µ
1µ10µ100n10n
-1.0
-0.5
0
0.5
1.0
1.5
-1.5
1n 1m
IREF = 100µA
IREF = 1µA
IREF = 10nA
Figure 4. Ideal Transfer Function with Varying IREF
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 IREF and ILOG currents, whereas the noise is
inversely proportional to IREF and ILOG currents.
Common Mode
A common-mode input voltage, VCMVOUT, 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, IREF. A resistor from REFISET to GND
(see Figures 5 and 6) adjusts the reference current,
according to the following equation:
where VREFISET is 0.5V. Select RSET between 5kΩand
5MΩ. REFIOUT current range is 10nA to 10µA only.
Single-Supply Operation
When operating from a single +2.7V to +11V supply,
ILOG must be greater than IREF, 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 VEE to GND.
Output Offset
Select ROS and IOS to adjust the output offset voltage
(see Figure 5). The magnitude of the offset voltage is
given by:
VOS = ROS ✕IOSADJ
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 1kΩand 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
(ROS = 0Ω), the reference current, IREF, equals the log
intercept of 100µA. Therefore,
Select R1= 10kΩ:
Dual-Supply Operation
When operating from dual ±2.7 to ±5.5V supplies, it is
not required that ILOG be greater than IREF. A positive
output voltage results at LOGV1 when ILOG exceeds
IREF. A negative output voltage results at LOGV1 when
ILOG is less than IREF. 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 VOSADJ (see Figure 6).
VV R
RR
OSADJ REFOUT
=+
4
34
VV R
R
OS OSADJ
=+
12
1
Rk
VV k210 1
025 130=−
=ΩΩ
/
.
RV
nA k
SET =×=
05
10 100 500
.Ω
RR K
21
025 1=−
.
RV
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 R2between 1kΩand 100kΩ.
Design Example
Desired:
Dual-Supply Operation
Logarithmic intercept: 1µA
Overall scale factor = 1V/decade
Select R1= 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
/
.
RV
Ak
SET =×=
05
10 1 50
.
µΩ
RRK
21
025
=
.
MAX4206
VEE
GND
REFIIN
LOGIIN
IIN
CMVIN
REFIOUT
CMVOUT
REFISET
SCALE
LOGV2
OSADJ
LOGV1
REFVOUT
VCC
VCC
RCOMP
100Ω
CCOMP
100pF
RSET
500kΩ
ROS
0Ω
R1
10kΩ
R2
30kΩ
0.1µF
0.1µF
VOUT
0.1µF
RCOMP
100Ω
CCOMP
100pF
Figure 5. Single-Supply Typical Operating Circuit
MAX4206
VEE
VEE
GND
REFIIN
REFIOUT
LOGIIN
IIN
REFISET
SCALE
LOGV2
LOGV1
CMVIN
CMVOUT
VCC
VCC
RCOMP
100Ω
CCOMP
100pF
RSET
50kΩ
R1
10kΩ
R2
40kΩ
0.1µF
0.1µF
0.1µF
VOUT
RCOMP
100Ω
CCOMP
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
(RSET = 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 VEE, and can
either be connected to the PC board VEE plane or left
unconnected. Ensure that only VEE traces are routed
under the exposed paddle.
Layout and Bypassing
Bypass VCC and VEE 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
VEE
GND
0.1µF
0.1µF
0.1µF
REFIIN
LOGIIN
VCC
CMVIN
REFIOUT
CMVOUT
REFISET
VCC
R1
R3
R2
R4
VCC
SCALE
LOGV1
LOGV2
OSADJ
REFVOUT
100pF
100Ω
100Ω
100pF
Figure 7. Measuring Optical Absorbance
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
16 ______________________________________________________________________________________
VEE
GND
REFVOUT
CMVIN
REFIOUT
CMVOUT
REFISET
VCC
SCALE
LOGV2
OSADJ
LOGV1
REFIIN
FIBER CABLE APD
+2.7V TO +76V 2.2µH
2.2µF0.22
µF
BIAS
REF
CLAMP
OUT
TIA
GND
TO LIMITING
AMPLIFIER
HIGH-SPEED DATA PATH
0.1µF
100pF
VCC
5MΩ
100pF
100Ω
100Ω
OUTPUT
PHOTODIODE BIAS
0.1µF
IAPD/10
IAPD
0.1µF
MAX4206
MAX4007
Figure 8. Logarithmic Current-Sensing Amplifier with Sourcing Input
MAX4206
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17
©2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
C
1
2
21-0139
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
C
2
2
21-0139
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm