EVALUATION KIT AVAILABLE
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
General Description
Benefits and Features
The MAX4206 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 0.25V/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 current 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 external resistors.
The MAX4206 is available in a space-saving 16-pin
TQFN package (4mm x 4mm x 0.8mm), and is specified for operation over the -40°C to +85°C extended
temperature range.
• Single Supply Capability Reduces System Power
Applications
Supply Requirements
• +2.7V to +11V Single-Supply Operation
• Also Supports ±2.7V to ±5.5V Dual-Supply
Operation if Required
• Wide Dynamic Range Supports Most Photodiode
Current Measurement Applications
• 5 Decades of Dynamic Range (10nA to 1mA)
• Monotonic Over a 1nA to 1mA Range
• Adjustable Output Scale Factor
• 0.25V/Decade Internally Trimmed Output Scale
Factor
• Adjustable Output Offset Voltage
• Internal 10nA to 10µA Reference Current Source
• 0.5V Input Common-Mode Voltage
• Small Package and Few External Components
Saves Space and Simplifies Design-In
• 16-Pin TQFN Package (4mm x 4mm x0.8mm)
Ordering Information
PART
Photodiode Current Monitoring
MAX4206ETE
Portable Instrumentation
TEMP RANGE
PIN-PACKAGE
-40°C to +85°C
16 TQFN-EP*
*EP = Exposed pad.
Medical Instrumentation
Typical Operating Circuit
Analog Signal Processing
VCC
Pin Configuration
IIN
REFIIN
LOGIIN
CMVIN
REFIOUT
0.1µF
TOP VIEW
(LEADS ON BOTTOM)
VCC
LOGV2
LOGIIN
R2
CCOMP
16
15
14
REFIOUT
13
RCOMP
N.C.
1
REFVOUT
2
GND
3
VEE
4
MAX4206
VOUT
12
CMVOUT
11
REFISET
10
VCC
9
N.C.
SCALE
REFIIN
CCOMP
R1
MAX4206
RCOMP
CMVIN
CMVOUT
LOGV1
REFVOUT
0.1µF
6
7
8
LOGV1
OSADJ
SCALE
LOGV2
0.1µF
5
THIN QFN
19-3071; Rev 2; 5/15
ROS
OSADJ
REFISET
RSET
GND
VEE
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
Absolute Maximum Ratings
(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
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.
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
Supply Voltage
VCC
Supply Current
ICC
LOGIIN Current Range (Notes 3, 4)
ILOG
REFIIN Current Range (Notes 3, 4)
Common-Mode Voltage
Common-Mode Voltage Input
Range
Log Conformity Error
Logarithmic Slope (Scale Factor)
IREF
CONDITIONS
(Note 2)
MIN
TA = +25°C
3.9
TA = -40°C to +85°C
Minimum
Minimum
0.5
TA = +25°C
500
±2
TA = +25°C
TA = -40°C to +85°C (Note 4)
237.5
250
231.25
TA = -40°C to +85°C
80
Input Offset Voltage Temperature
Drift
VIOS
|VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN|
6
Current Reference Output Voltage
www.maximintegrated.com
VREFISET
1
mA
520
mV
1.0
V
±5
TA = +25°C
1.218
TA = -40°C to +85°C (Note 4)
1.195
262.5
268.75
1
IREFVOUT
mA
±10
TA = +25°C, |VCMVIN - VREFIIN|,
|VCMVIN - VLOGIIN|
Voltage Reference Output Current
1
mV
TA = -40°C to +85°C
VIO
VREFVOUT
mA
nA
Input Offset Voltage
Voltage Reference Output
5
nA
Maximum
VCMVIN
Logarithmic Slope (Scale Factor)
Temperature Drift
V
10
480
K
UNITS
11.0
10
Maximum
IREF = 10nA,
ILOG= 10nA to 1mA,
K = 0.25V/decade
(Note 4)
MAX
7
VCMVOUT
VLC
TYP
2.7
1.238
µV/
decade/
°C
5
490
1.258
1.275
TA = -40°C to +85°C (Note 4)
482
500
mV
µV/°C
1
TA = +25°C
mV/
decade
V
mA
510
518
mV
Maxim Integrated | 2
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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
0.4
2
UNITS
LOGV2 BUFFER
Input Offset Voltage
Input Bias Current
Output Voltage Range
Output Short-Circuit Current
Slew Rate
Unity-Gain Bandwidth
VIO
IB
TA = +25°C
TA = -40°C to +85°C (Note 4)
6
mV
(Note 4)
0.01
1
nA
VOH
RL to GND = 2kΩ
VCC 0.2
VCC 0.3
V
VOL
RL to GND = 2kΩ
0.2
0.08
IOUT+
Sourcing
34
IOUT-
Sinking
58
mA
SR
12
V/µs
GBW
5
MHz
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.)
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
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
Supply Voltage (Note 2)
SYMBOL
MAX
VEE
-2.7
-5.5
LOGIIN Current Range (Notes 3, 4)
ILOG
www.maximintegrated.com
TYP
5.5
ICC
Common-Mode Voltage
MIN
2.7
Supply Current
REFIIN Current Range (Notes 3, 4)
CONDITIONS
VCC
IREF
VCMVOUT
TA = +25°C
5
TA = -40°C to +85°C
Minimum
10
mA
1
mA
1
mA
520
mV
10
nA
Maximum
480
V
nA
Maximum
Minimum
6
7.5
UNITS
500
Maxim Integrated | 3
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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
Common-Mode Voltage Input
Range
Log Conformity Error
Logarithmic Slope (Scale Factor)
SYMBOL
CONDITIONS
K
Logarithmic Slope (Scale Factor)
Temperature Drift
TYP
0.5
VCMVIN
VLC
MIN
IREF = 10nA,
ILOG= 10nA to 1mA,
K = 0.25V/decade
(Note 4)
TA = +25°C
±2
TA = +25°C
TA = -40°C to +85°C
237.5
250
231.25
TA = -40°C to +85°C
80
1
Input Offset Voltage
Temperature Drift
VIOS
|VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN|
6
IREFVOUT
Current Reference Output
Voltage
VREFISET
V
±5
TA = +25°C
1.218
TA = -40°C to +85°C (Note 4)
1.195
262.5
268.75
TA = +25°C, |VCMVIN - VREFIIN|,
|VCMVIN - VLOGIIN|
Voltage Reference Output
Current
1.0
±10
VIO
VREFVOUT
UNITS
mV
TA = -40°C to +85°C
Input Offset Voltage
Voltage Reference Output
MAX
1.238
µV/
decade/
°C
5
490
TA = -40°C to +85°C (Note 4)
482
1.258
1.275
500
mV
µV/°C
1
TA = +25°C
mV/
decade
V
mA
510
518
mV
LOGV2 BUFFER
Input Offset Voltage
Input Bias Current
VIO
IB
TA = +25°C
0.4
TA = -40°C to +85°C (Note 4)
(Note 4)
0.01
1
VOH
RL to GND = 2kΩ
VCC 0.2
VCC 0.3
VOL
RL to GND = 2kΩ
Slew Rate
Unity-Gain Bandwidth
www.maximintegrated.com
mV
nA
V
Output Voltage Range
Output Short-Circuit Current
2
6
VEE +
0.2
VEE +
0.08
IOUT+
Sourcing
34
IOUT-
Sinking
58
mA
SR
12
V/µs
GBW
5
MHz
Maxim Integrated | 4
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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:
Note 2:
Note 3:
Note 4:
All devices are 100% production tested at TA = +25°C. All temperature limits are guaranteed by design.
Guaranteed and functionally verified.
Log conformity error less than ±5mV with scale factor = 0.25V/decade.
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
(IREF = 10nA)
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = -5V
1.50
1.25
0.75
0.50
1.50
1.25
1.00
VLOGV1 (V)
1.00
VLOGV1 (V)
VLOGV1 (V)
1.25
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = GND
1.75
MAX4206 toc02
1.50
VLOGV1 vs. ILOG
1.75
MAX4206 toc01
1.75
0.75
0.50
0.75
0.50
0.25
0.25
0
0
0
-0.25
-0.25
10n
100n
1μ
10μ
ILOG (A)
www.maximintegrated.com
100μ
1m
10m
IREF = 10nA
TA = -40°C TO +85°C
VCC = +2.7V
VEE = GND
1.00
0.25
-0.25
MAX4206 toc03
VLOGV1 vs. ILOG
1n
10n
100n
1μ
10μ 100μ
ILOG (A)
1m
10m
10n
100n
1μ
10μ
100μ
1m
10m
ILOG (A)
Maxim Integrated | 5
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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.)
100μA
1.50
10nA
0.50
1.00
0.75
0.25
0.25
0
0
-0.25
-0.25
-0.50
100n
1μ
10μ
100μ
1m
-15
10μA 100μA
10nA 100nA 1μA
-20
1n
10n
100n
1μ
10μ
100μ
10n
1m
100n
1μ
10μ
100μ
1m
ILOG (A)
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
IREF = 10nA
TA = -40°C TO +85°C
VCC = +2.7V
VEE = GND
15
-5
TA = -40°C
-10
-15
10n
100n
1μ
10μ 100μ
1m
5
0
-5
TA = -40°C
5
0
-5
-10
-10
-15
-15
IREF = 10nA
SINGLE SUPPLY: VCC = +2.7V, +5V, 11V,
VEE = GND
DUAL SUPPLY: VCC = +5V
VEE = -5V
-20
10n
10m
10m
10
-20
-20
15
ERROR (mV)
ERROR (mV)
0
20
MAX4206 toc08
MAX4206 toc07
20
10
5
1n
TA = -40°C
IREF (A)
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = -5V
10
0
-5
ILOG (A)
20
15
10m
5
-10
-0.50
10n
ERROR (mV)
1mA
0.50
IREF = 10nA
TA = -40°C TO +85°C
VCC = +5V
VEE = GND
MAX4206 toc09
0.75
10
ERROR (mV)
1μA
100nA
1.00
15
1.25
VLOGV1 (V)
VLOGV1 (V)
1.75
10μA
1.25
20
MAX4206 toc05
1mA
1.50
2.00
MAX4206 toc04
2.00
1.75
NORMALIZED LOG CONFORMANCE
ERROR vs. ILOG
VLOGV1 vs. IREF
(ILOG = 10nA TO 1mA)
MAX4206 toc06
VLOGV1 vs. ILOG
(IREF = 10nA TO 1mA)
100n
1μ
10μ
100μ
1m
10n
10m
100n
1μ
10μ
100μ
1m
10m
ILOG (A)
ILOG (A)
ILOG (A)
LOGV2
NOISE DENSITY (μV/√Hz)
VLOGIIN - VCMVIN (mV)
2
1
0
-1
-2
100nA
1μA
IREF = ILOG
f = 1Hz TO 1MHz
1
10μA
0.1
4
MAX4206 toc12
3
10nA
VOLTAGE NOISE (mVRMS)
IREF = 10nA
5
MAX4206 toc11
4
10
MAX4206 toc10
5
AT VLOGV2 vs. ILOG
vs. FREQUENCY
VLOGIIN - VCMVIN vs. ILOG
3
2
1
-3
-4
IREF = ILOG
0
0.01
-5
1n
10n
100n
1μ
10μ 100μ
ILOG (A)
www.maximintegrated.com
1m
10m
1
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
10n
100n
1μ
10μ
100μ
1m
ILOG (A)
Maxim Integrated | 6
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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.)
ILOG PULSE RESPONSE
(IREF = 100nA,
VCC = 5V, VEE = GND)
ILOG PULSE RESPONSE
(IREF = 100nA,
VCC = 5V, VEE = -5V)
MAX4206 toc13
1.0V
MAX4206 toc14
1.0V
100μA TO 1mA
100μA TO 1mA
0.75V
0.75V
0.75V
10μA TO 100μA
VLOGV1 (V)
VLOGV1 (V)
0.75V
0.50V
0.50V
1μA TO 10μA
0.25V
10μA TO 100μA
0.50V
0.50V
1μA TO 10μA
0.25V
0.25V
100nA TO 1μA
0.25V
0
100nA TO 1μA
0
20μs/div
20μs/div
IREF PULSE RESPONSE
(ILOG = 1mA)
LOGARITHMIC SLOPE DISTRIBUTION
MAX4206 toc15
1.0V
1μA TO 100nA
MAX4206 toc16
30
25
0.75V
10μA TO 1μA
0.50V
0.50V
100μA TO 10μA
20
COUNT (%)
VLOGV1 (V)
0.75V
15
10
0.25V
5
0.25V
1mA TO 100μA
0
0
240
245
20μs/div
VREFVOUT DISTRIBUTION
260
255
RL = 100kΩ
20
INPUT OFFSET VOLTAGE = VLOGIIN - VCMVIN
14
MAX4206 toc18
INPUT OFFSET VOLTAGE DISTRIBUTION
16
MAX4206 toc17
25
250
SLOPE (mV/DECADE)
15
COUNT (%)
COUNT (%)
12
10
10
8
6
4
5
2
0
1.232 1.234
0
1.236 1.238
1.240 1.242
VREFVOUT (V)
www.maximintegrated.com
1.244
-1.0 -0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
INPUT OFFSET VOLTAGE (mV)
Maxim Integrated | 7
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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.)TA = +25°C, unless otherwise noted.)
VLOGV1 (mV)
4
2
0
-2
-4
-6
1.29
1.28
1.27
1.26
1.25
1.24
1.23
1.22
-8
1.21
-10
1.20
-50
-25
0
25
50
TEMPERATURE (°C)
75
100
1.240
1.230
1.225
1.220
1.215
1.210
-25
0
25
50
TEMPERATURE (°C)
75
-20
1.23
1.22
-1.0
-0.5
0
0.5
1.0
REFERENCE LINE-TRANSIENT RESPONSE
-30
VCC
2V/div
0V
-40
-50
-60
-70
-90
-100
0.5
1.24
MAX4206 toc24
CREFVOUT = 0.1μF
IREFVOUT = 1mA
-10
1.200
0
1.25
LOAD CURRENT (mA)
0
1.205
-0.5
1.26
100
VREFVOUT
200mV/div
-80
-1.0
1.27
REFERENCE POWER-SUPPLY
REJECTION RATIO vs. FREQUENCY
REFERENCE PSRR (dB)
1.235
1.28
1.20
-50
MAX4206 toc22
REFERENCE OUTPUT VOLTAGE (V)
1.245
1.29
1.21
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. SUPPLY VOLTAGE
1.250
1.30
MAX4206 toc21
6
MAX4206 toc20
8
1.30
MAX4206 toc23
IREF = 1μA
ILOG 1μA
REFERENCE OUTPUT VOLTAGE (V)
MAX4206 toc19
10
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. LOAD CURRENT
REFERENCE OUTPUT VOLTAGE (VREFVOUT)
vs. TEMPERATURE
REFERENCE OUTPUT VOLTAGE (V)
OUTPUT OFFSET VOLTAGE
vs. TEMPERATURE
1.0
1.238V
CREFVOUT = 0F
10
LOAD CURRENT (mA)
100
1k
10k
100k
10μs/div
1M
FREQUENCY (Hz)
REFERENCE LOAD-TRANSIENT RESPONSE
REFERENCE TURN-ON TRANSIENT RESPONSE
MAX4206 toc25
MAX4206 toc26
CREFVOUT = 0F
IREFVOUT
1mA/div
0mA
VCC
2.5V/div
0V
VREFVOUT
100mV/div
1.238V
VREFVOUT
500mV/div
0V
100μs/div
www.maximintegrated.com
10μs/div
Maxim Integrated | 8
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
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.)TA = +25°C, unless otherwise noted.)
-20
ILOG = 10μA
-30
ILOG = 1μA
-40
ILOG = 100nA
-50
ILOG = 100μA
-10
ILOG = 10μA
-20
ILOG = 1μA
-30
ILOG = 100nA
-40
-60
10k
100k
1M
FREQUENCY (Hz)
AV = 2V/V
-3
AV = 4V/V
-6
CCOMP = 100pF
RCOMP = 1kΩ
-12
-60
1k
AV = 1V/V
0
-9
-50
CCOMP = 33pF
RCOMP = 330Ω
100
3
NORMALIZED GAIN (dB)
ILOG = 1mA
-10
ILOG = 1mA
0
NORMALIZED GAIN (dB)
NORMALIZED GAIN (dB)
0
10
MAX4206 toc28
ILOG = 100μA
MAX4206 toc27
10
SMALL-SIGNAL AC RESPONSE
OF BUFFER
SMALL-SIGNAL AC RESPONSE
ILOG TO VLOGV1
MAX4206 toc29
SMALL-SIGNAL AC RESPONSE
ILOG TO VLOGV1
10M
100
1k
10k
100k
1M
10M
10k
100k
1M
10M
100M
FREQUENCY (Hz)
FREQUENCY (Hz)
Pin Description
PIN
NAME
1, 9
N.C.
2
REFVOUT
3
GND
Ground
4
VEE
Negative Power Supply. Bypass VEE to GND with a 0.1µF capacitor.
5
LOGV1
Logarithmic Amplifier Voltage Output 1. The output scale factor of LOGV1 is 0.25V/decade.
6
OSADJ
Offset Adjust Input. When operating from a single power supply, current applied to OSADJ adjusts
the output offset voltage (see the Output Offset section).
7
SCALE
Scale Factor Input. Adjust the output scale factor for LOGV2 using a resistive divider (see the Scale
Factor section).
8
LOGV2
Logarithmic Amplifier Voltage Output 2. Adjust the output scale factor for LOGV2 using a resistive
divider (see the Scale Factor section).
10
VCC
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).
www.maximintegrated.com
FUNCTION
No Connection. Not internally connected.
1.238V Reference Voltage Output. Bypass REFVOUT to GND with a 0 to 1µF capacitor (optional).
Positive Power Supply. Bypass VCC to GND with a 0.1µF capacitor.
Maxim Integrated | 9
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
VCC
REFVOUT
CMVOUT
CURRENT MIRROR
VCC
CURRENT
CORRECTION
LOGIIN
REFIOUT
1.238V
VCC
0.5V
CMVIN
VEE
REFISET
VCC
LOGV2
REFIIN
SUMMING
AMPLIFIER
AND
TEMPERATURE
COMPENSATION
VCC
SCALE
OSADJ
VEE
MAX4206
GND
VEE
LOGV1
Figure 1. Functional Diagram
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 according to the following equation:
⎛I ⎞
⎛ kT ⎞
VBE = ⎜ ⎟ ln ⎜ C ⎟
⎝ q⎠
⎝ IS ⎠
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 reference voltage VBE2, which is a logarithmic voltage for a
known reference current, IREF. The temperature depenwww.maximintegrated.com
dencies of a logarithmic amplifier relate to the thermal
voltage, (kT/q), and IS. Matched transistors eliminate
the IS temperature dependence of the amplifier in the
following manner:
VOUT = VBE1 − VBE2
⎞ ⎛ kT ⎞ ⎛ I
⎞
⎛ kT ⎞ ⎛ I
= ⎜ ⎟ ln⎜ LOG ⎟ − ⎜ ⎟ ln⎜ REF ⎟
⎝ q ⎠ ⎝ IS ⎠ ⎝ q ⎠ ⎝ IS ⎠
⎡ ⎛ ILOG ⎞
⎛I
⎞⎤
− ln⎜ REF ⎟ ⎥
⎢ln⎜
⎟
⎝ IS ⎠ ⎥⎦
⎢⎣ ⎝ IS ⎠
⎞⎤
⎛ kT ⎞ ⎡ ⎛ I
= ⎜ ⎟ ⎢ln⎜ LOG ⎟ ⎥
⎝ q ⎠ ⎢⎣ ⎝ IREF ⎠ ⎥⎦
⎡
⎛I
⎞⎤
⎛ kT ⎞
= ⎜ ⎟ (ln(10)) ⎢log10 ⎜ LOG ⎟ ⎥
⎝ q⎠
⎝ IREF ⎠ ⎥⎦
⎢⎣
⎛I
⎞
= K × log10 ⎜ LOG ⎟
⎝ IREF ⎠
⎛ kT ⎞
=⎜ ⎟
⎝ q⎠
(see Figure 3)
Maxim Integrated | 10
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
IDEAL TRANSFER FUNCTION
WITH VARYING K
VBE1
4
CMVIN
VEE
IREF
VCC
VBE2
REFIIN
NORMALIZED OUTPUT VOLTAGE (V)
LOGIIN
3
VOUT = K LOG (ILOG/IREF)
MAX4206 fig03
ILOG
VCC
K=1
K = 0.5
K = 0.25
2
1
0
-1
-2
-3
-4
0.001
VEE
0.1
10
1000
CURRENT RATIO (ILOG/IREF)
Figure 2. Simplified Model of a Logarithmic Amplifier
Figure 3. Ideal Transfer Function with Varying K
where:
K = scale factor (V/decade)
ILOG = the input current at LOGIIN
IREF = the reference current at REFIIN
Referred-to-Input and Referred-to-Output Errors
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.
The log nature of the MAX4206 insures that any additive 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):
VLOGV1 = VIDEAL ± TE
Definitions
Transfer Function
The ideal logarithmic amplifier transfer function is:
⎛I
⎞
VIDEAL = K × log10 ⎜ LOG ⎟
⎝ IREF ⎠
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.
www.maximintegrated.com
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:
⎡
⎤
⎛I
⎞
−I
VLOGV 2 = K(1 ± ΔK) ⎢log10 ⎜ LOG BIAS1 ⎟ ± 4( ± VLC ± VOSOUT )⎥
⎝ IREF − IBIAS2 ⎠
⎢⎣
⎥⎦
where VLC and VOSOUT are the log conformity and output 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.
Maxim Integrated | 11
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
IBIAS1 and IBIAS2 are currents in the order of 20pA, significantly smaller than ILOG and IREF, and can therefore
be eliminated:
IDEAL TRANSFER FUNCTION
WITH VARYING IREF
⎡
⎤
⎛I
⎞
VLOGV 2 ≅ K(1 ± ΔK) ⎢log10 ⎜ LOG ⎟ ± 4( ± VLC ± VOSOUT )⎥
⎝ IREF ⎠
⎢⎣
⎥⎦
⎛I
⎞
⎛I
⎞
VLOGV 2 ≅ K log10 ⎜ LOG ⎟ ± KΔK log10 ⎜ LOG ⎟
⎝ IREF ⎠
⎝ IREF ⎠
± 4K(1 ± ΔK)( ± VLC ± VOSOUT )
The first term of this expression is the ideal component
of VLOGV1. The remainder of the expression is the TE:
⎛I
⎞
TE ≅ ±KΔK log10 ⎜ LOG ⎟ ± 4K(1 ± ΔK)( ± VLC ± VOSOUT )
⎝ IREF ⎠
In the second term, one can generally remove the
products relating to ΔK, because ΔK is generally much
less than 1. Hence, a good approximation for TE is
given by:
⎡
⎤
⎛I
⎞
TE ≅ ±K ⎢ΔK log10 ⎜ LOG ⎟ ± 4( ± VLC ± VOSOUT )⎥
I
⎝ REF ⎠
⎢⎣
⎥⎦
As an example, consider the following situation:
Full-scale input = 5V
ILOG = 100µA
IREF = 100nA
K = 1 ±5% V/decade (note that the uncommitted amplifier is configured for a gain of 4)
VLC = ±5mV (obtained from the Electrical Characteristics table)
VOSOUT = ±2mV (typ)
TA = +25°C
Substituting into the total error approximation,
TE ≅ ± (1V/decade)(0.05log 10 (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-
www.maximintegrated.com
1.0
OUTPUT VOLTAGE (V)
Expanding this expression:
1.5
MAX4206 fig04
MAX4206
IREF = 10nA
0.5
0
-0.5
IREF = 100μA
IREF = 1μA
-1.0
-1.5
1n
10n
100n
1μ
10μ
100μ
1m
ILOG (A)
Figure 4. Ideal Transfer Function with Varying IREF
tributing to total error. For further accuracy, consider temperature 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 network 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 R COMP = 330Ω are suitable compensation values.
Maxim Integrated | 12
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
The MAX4206 bandwidth is proportional to the magnitude of the IREF and ILOG currents, whereas the noise is
inversely proportional to IREF and ILOG currents.
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.
Common Mode
Design Example
A common-mode input voltage, VCMVOUT, of 0.5V is
available at CMVOUT and can be used to bias the logging 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.
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,
Frequency Response and Noise Considerations
Adjusting the Logarithmic Intercept
Adjust the logarithmic intercept by changing the reference current, IREF. A resistor from REFISET to GND
(see Figures 5 and 6) adjusts the reference current,
according to the following equation:
V
RSET = REFISET
10 × IREF
where VREFISET is 0.5V. Select RSET 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,
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 singlesupply 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 output. 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 uncommitted LOGV2 amplifier and the following equation,
which refers to Figure 5:
⎛ K
⎞
R2 = R1⎜
− 1⎟
⎝ 0.25 ⎠
www.maximintegrated.com
Desired:
Single-Supply Operation
Logarithmic intercept: 100nA
RSET =
0.5V
= 500kΩ
10 × 100nA
Select R1 = 10kΩ:
⎛ 1V/ V ⎞
R2 = 10kΩ⎜
− 1⎟ = 30kΩ
⎝ 0.25 ⎠
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
I LOG is less than I REF . Bias the log and reference
amplifiers by connecting CMVOUT to CMVIN or connect 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 following equation:
⎛ R ⎞
VOS = VOSADJ ⎜1+ 2 ⎟
⎝ R1 ⎠
A resistive divider between REFVOUT, OSADJ, and
GND can be used to adjust VOSADJ (see Figure 6).
⎛ R4 ⎞
VOSADJ = VREFOUT ⎜
⎟
⎝ R3 + R4 ⎠
Maxim Integrated | 13
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
VCC
VCC
IIN
IIN
0.1μF
0.1μF
VCC
LOGV2
LOGIIN
CCOMP
100pF
REFIIN
MAX4206
0.1μF
CMVIN
0.1μF
REFISET
OSADJ
GND
RSET
500kΩ
VEE
R3
CMVOUT
OSADJ
ROS
0Ω
REFISET
LOGV1
REFVOUT
CMVIN
CMVOUT
LOGV1
0.1μF
R1
10kΩ
MAX4206
RCOMP
100Ω
RCOMP
100Ω
REFVOUT
SCALE
REFIIN
CCOMP
100pF
R1
10kΩ
R2
40kΩ
REFIOUT
RCOMP
100Ω
SCALE
CCOMP
100pF
VOUT
LOGIIN
CCOMP
100pF
R2
30kΩ
REFIOUT
RCOMP
100Ω
VCC
LOGV2
VOUT
RSET
50kΩ
R4
GND
VEE
0.1μF
VEE
Figure 5. Single-Supply Typical Operating Circuit
Figure 6. Dual-Supply Typical Operating Circuit
Scale Factor
Measuring Optical Absorbance
The scale factor, K, is the slope of the logarithmic output.
For the LOGV1 amplifier, K = 0.25V/decade. When operating from dual supplies, adjust the overall scale factor
for the MAX4206 using the uncommitted LOGV2 amplifier and the following equation, which refers to Figure 6:
A photodiode provides a convenient means of measuring 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).
⎛ K ⎞
R2 = R1 ⎜
⎟
⎝ 0.25 ⎠
Select R2 between 1kΩ and 100kΩ.
Design Example
Desired:
Dual-Supply Operation
Logarithmic intercept: 1µA
Overall scale factor = 1V/decade
0.5V
RSET =
= 50kΩ
10 × 1μA
Select R1 = 10kΩ:
⎛ 1V / decade ⎞
R2 = 10kΩ × ⎜
⎟ = 40kΩ
⎝
⎠
0.25
www.maximintegrated.com
In absorbance measurement instrumentation, a reference 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 sample 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 currents 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 overlapping “skirts” precede each photodiode. With proper filter selection, the MAX4206 output can vary monotonically
(ideally linearly) with optical frequency.
Maxim Integrated | 14
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
Photodiode Current Monitoring
Figure 8 shows the MAX4206 in a single-supply, opticalpower 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.
VCC
0.1μF
VCC
CMVIN
REFISET
REFIIN
REFVOUT
100pF
The MAX4206 drives capacitive loads of up to 50pF.
Reactive loads decrease phase margin and can produce 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 combination 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 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
www.maximintegrated.com
0.1μF
LOGV2
VCC
MAX4206
R2
100Ω
SCALE
LOGV1
R3
LOGIIN
Capacitive Loads
0.1μF
CMVOUT
R1
100pF
OSADJ
REFIOUT
100Ω
GND
VEE
R4
Figure 7. Measuring Optical Absorbance
noise immunity and a clean reference current. For lowcurrent 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 reference current are selectable. Refer to the MAX4206
Evaluation Kit data sheet for more information.
Chip Information
TRANSISTOR COUNT: 754
PROCESS: BiCMOS
Maxim Integrated | 15
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
VCC
2.2μH
+2.7V TO +76V
2.2μF
PHOTODIODE BIAS
0.22μF
0.1μF
BIAS
VCC
CLAMP
OUTPUT
REFVOUT
0.1μF
LOGV2
REFIOUT
MAX4007
REFIIN
100pF
SCALE
MAX4206
LOGV1
OSADJ
100Ω
REFISET
IAPD/10
IAPD
5MΩ
OUT
REF
100pF
GND
FIBER CABLE
CMVOUT
CMVIN
100Ω
0.1μF
APD
GND
TIA
VEE
TO LIMITING
AMPLIFIER
HIGH-SPEED DATA PATH
Figure 8. Logarithmic Current-Sensing Amplifier with Sourcing Input
Package Information
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a
“+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but the
drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
16 TQFN-EP
T1644-4
21-0139
90-0070
www.maximintegrated.com
Maxim Integrated | 16
Precision Transimpedance
Logarithmic Amplifier with Over
5 Decades of Dynamic Range
MAX4206
Revision History
REVISION
NUMBER
REVISION
DATE
2
5/15
DESCRIPTION
Updated Benefits and Features section
PAGES
CHANGED
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and
max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
© 2015 Maxim Integrated Products, Inc. | 17