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19-3071; Rev 1; 6/04
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
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 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 thin QFN package (4mm x 4mm x 0.8mm), and is specified for operation over the -40C to +85C extended temperature range.
KIT ATION EVALU ABLE AVAIL
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 10A Reference Current Source 0.5V Input Common-Mode Voltage Small 16-Pin Thin QFN Package (4mm x 4mm x 0.8mm) -40C to +85C Operating Temperature Range Evaluation Kit Available
MAX4206
Applications
Photodiode Current Monitoring Portable Instrumentation Medical Instrumentation Analog Signal Processing
PART MAX4206ETE
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 16 Thin QFN-EP*
*EP = Exposed paddle.
Typical Operating Circuit
VCC IIN
Pin Configuration
REFIOUT TOP VIEW (LEADS ON BOTTOM)
0.1F
VCC LOGV2 LOGIIN
VOUT
LOGIIN
CMVIN
REFIIN
CCOMP
R2 REFIOUT SCALE R1
16
15
14
13
RCOMP
REFIIN CCOMP RCOMP
N.C. REFVOUT GND VEE
1 2 3 4
12 11
CMVOUT REFISET VCC N.C.
MAX4206
CMVIN CMVOUT LOGV1 ROS REFISET OSADJ GND VEE
MAX4206
10 9
0.1F
REFVOUT 0.1F
5 LOGV1
6
OSADJ
7 SCALE
8 LOGV2
THIN QFN
RSET
________________________________________________________________ Maxim Integrated Products
1
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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 = +70C) 16-Pin Thin QFN (derate 16.9mW/C above +70C) ....1349mW Operating Temperature Range ...........................-40C to +85C Junction Temperature .....................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Supply Voltage Supply Current LOGIIN Current Range (Notes 3, 4) REFIIN Current Range (Notes 3, 4) Common-Mode Voltage Common-Mode Voltage Input Range SYMBOL VCC ICC ILOG IREF VCMVOUT VCMVIN IREF = 10nA, ILOG= 10nA to 1mA, K = 0.25V/decade (Note 4) TA = +25C TA = -40C to +85C (Note 4) TA = -40C to +85C TA = +25C, |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| TA = +25C TA = -40C to +85C (Note 4) TA = +25C TA = -40C to +85C (Note 4) 1.218 1.195 1 490 482 500 510 518 TA = +25C TA = -40C to +85C 237.5 231.25 80 250 (Note 2) TA = +25C TA = -40C to +85C Minimum Maximum Minimum Maximum 480 0.5 2 500 10 1 520 1.0 5 mV 10 262.5 268.75 mV/ decade V/ decade/ C 5 mV V/C 1.258 1.275 V mA mV 10 1 CONDITIONS MIN 2.7 3.9 TYP MAX 11.0 5 7 UNITS V mA nA mA nA mA mV V
Log Conformity Error
VLC
Logarithmic Slope (Scale Factor) Logarithmic Slope (Scale Factor) Temperature Drift Input Offset Voltage Input Offset Voltage Temperature Drift Voltage Reference Output Voltage Reference Output Current Current Reference Output Voltage
K
VIO VIOS VREFVOUT IREFVOUT VREFISET
1 6 1.238
2
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
DC ELECTRICAL CHARACTERISTICS--Single-Supply Operation (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER LOGV2 BUFFER Input Offset Voltage Input Bias Current Output Voltage Range VIO IB VOH VOL Output Short-Circuit Current Slew Rate Unity-Gain Bandwidth IOUT+ IOUTSR GBW TA = +25C TA = -40C to +85C (Note 4) (Note 4) RL to GND = 2k RL to GND = 2k Sourcing Sinking 0.2 0.01 VCC 0.2 0.08 34 58 12 5 mA V/s MHz 0.4 2 6 1 VCC 0.3 mV nA V SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX4206
AC ELECTRICAL CHARACTERISTICS--Single-Supply Operation
(VCC = +5V, VEE = GND = 0, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
PARAMETER LOGV2 Total Noise LOGV2 Spot Noise Density REFVOUT Total Noise REFVOUT Spot Noise Density REFISET Total Noise REFISET Spot Noise Density Small-Signal Unity-Gain Bandwidth SYMBOL CONDITIONS 0.1Hz to 10Hz, total output-referred noise, IREF = 10nA, ILOG = 100nA f = 5kHz, IREF = 10nA, ILOG = 100nA 1Hz to 10Hz, total output-referred noise f = 5kHz 1Hz to 10Hz, total output-referred noise f = 5kHz IREF = 1A, ILOG = 10A, RCOMP = 300, CCOMP = 32pF MIN TYP 17 0.8 3.3 266 0.67 23 1 MAX UNITS VRMS V/Hz VRMS nV/Hz VRMS nV/Hz MHz
DC ELECTRICAL CHARACTERISTICS--Dual-Supply Operation
(VCC = +5V, VEE = -5V, GND = 0, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Supply Voltage (Note 2) Supply Current LOGIIN Current Range (Notes 3, 4) REFIIN Current Range (Notes 3, 4) Common-Mode Voltage SYMBOL VCC VEE ICC ILOG IREF VCMVOUT TA = +25C TA = -40C to +85C Minimum Maximum Minimum Maximum 480 500 10 1 520 10 1 CONDITIONS MIN 2.7 -2.7 5 TYP MAX 5.5 -5.5 6 7.5 UNITS V mA nA mA nA mA mV
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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 = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Common-Mode Voltage Input Range SYMBOL VCMVIN IREF = 10nA, ILOG= 10nA to 1mA, K = 0.25V/decade (Note 4) TA = +25C TA = -40C to +85C TA = -40C to +85C TA = +25C, |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| TA = +25C TA = -40C to +85C (Note 4) 1.218 1.195 1 TA = +25C TA = -40C to +85C (Note 4) TA = +25C TA = -40C to +85C (Note 4) (Note 4) RL to GND = 2k RL to GND = 2k Sourcing Sinking VEE + 0.2 0.01 VCC 0.2 VEE + 0.08 34 58 12 5 mA V/s MHz 490 482 0.4 500 510 518 2 6 1 VCC 0.3 V VOL Output Short-Circuit Current Slew Rate Unity-Gain Bandwidth IOUT+ IOUTSR GBW TA = +25C TA = -40C to +85C 237.5 231.25 80 250 CONDITIONS MIN 0.5 2 TYP MAX 1.0 5 mV 10 262.5 268.75 mV/ decade V/ decade/ C 5 mV V/C 1.258 1.275 V mA mV UNITS V
Log Conformity Error
VLC
Logarithmic Slope (Scale Factor) Logarithmic Slope (Scale Factor) Temperature Drift Input Offset Voltage Input Offset Voltage Temperature Drift Voltage Reference Output Voltage Reference Output Current Current Reference Output Voltage LOGV2 BUFFER Input Offset Voltage Input Bias Current
K
VIO VIOS VREFVOUT IREFVOUT VREFISET
1 6 1.238
VIO IB VOH
mV nA
Output Voltage Range
4
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
AC ELECTRICAL CHARACTERISTICS--Dual-Supply Operation
(VCC = +5V, VEE = -5V, GND = 0, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
PARAMETER LOGV2 Total Noise LOGV2 Spot Noise Density REFVOUT Total Noise REFVOUT Spot Noise Density REFISET Total Noise REFISET Spot Noise Density Small-Signal Unity-Gain Bandwidth SYMBOL CONDITIONS 0.1Hz to 10Hz, total output-referred noise, IREF = 10nA, ILOG = 100nA f = 5kHz, IREF = 10nA, ILOG = 100nA 1Hz to 10Hz, total output-referred noise f = 5kHz 1Hz to 10Hz, total output-referred noise f = 5kHz IREF = 1A, ILOG = 10A, RCOMP = 300, CCOMP = 32pF MIN TYP 17 0.8 3.3 266 0.67 23 1 MAX UNITS VRMS V/Hz VRMS nV/Hz VRMS nV/Hz MHz
MAX4206
Note 1: Note 2: Note 3: Note 4:
All devices are 100% production tested at TA = +25C. 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 = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
VLOGV1 vs. ILOG
MAX4206 toc01
1.50 1.25 VLOGV1 (V) 1.00 0.75 0.50 0.25 0 -0.25
IREF = 10nA TA = -40C TO +85C VCC = +5V VEE = GND
1.50 1.25 VLOGV1 (V) 1.00 0.75 0.50 0.25 0 -0.25
IREF = 10nA TA = -40C TO +85C VCC = +5V VEE = -5V
MAX4206 toc02
1.50 1.25 VLOGV1 (V) 1.00 0.75 0.50 0.25 0 -0.25
IREF = 10nA TA = -40C TO +85C VCC = +2.7V VEE = GND
10n
100n
1
10 ILOG (A)
100
1m
10m
1n
10n
100n
1
10 100
1m
10m
10n
100n
1
10 ILOG (A)
100
1m
10m
ILOG (A)
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MAX4206 toc03
1.75
VLOGV1 vs. ILOG (IREF = 10nA)
1.75 1.75
VLOGV1 vs. ILOG
5
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
VLOGV1 vs. ILOG (IREF = 10nA TO 1mA)
MAX4206 toc04
VLOGV1 vs. IREF (ILOG = 10nA TO 1mA)
MAX4206 toc05
NORMALIZED LOG CONFORMANCE ERROR vs. ILOG
15 10 ERROR (mV) 5 0 -5 TA = -40C IREF = 10nA TA = -40C TO +85C VCC = +5V VEE = GND
MAX4206 toc06
2.00 1.75 1.50 1.25 VLOGV1 (V) 1.00 0.75 0.50 0.25 0 -0.25 -0.50 10n 100n 1 10 ILOG (A) 100 1m 1A 100nA 10nA 10A 1mA 100A
2.00 1.75 1.50 1.25 VLOGV1 (V) 1.00 0.75 0.50 0.25 0 -0.25 -0.50 10nA 100nA 1A 1n 10n 100n 10A 100A 1 IREF (A) 10 100 1mA
20
-10 -15 -20 1m 10n 100n 1 10 ILOG (A)
10m
100
1m
10m
NORMALIZED LOG CONFORMANCE ERROR vs. ILOG
MAX4206 toc07
NORMALIZED LOG CONFORMANCE ERROR vs. ILOG
MAX4206 toc08
NORMALIZED LOG CONFORMANCE ERROR vs. ILOG
15 10 ERROR (mV) 5 0 -5 IREF = 10nA SINGLE SUPPLY: VCC = +2.7V, +5V, 11V, VEE = GND DUAL SUPPLY: VCC = +5V VEE = -5V 10n 100n 1 10 ILOG (A) 100 1m 10m
MAX4206 toc09
20 15 10 ERROR (mV) 5 0 -5 TA = -40C IREF = 10nA TA = -40C TO +85C VCC = +5V VEE = -5V
20 15 10 ERROR (mV) 5 0 -5 TA = -40C IREF = 10nA TA = -40C TO +85C VCC = +2.7V VEE = GND
20
-10 -15 -20 1n 10n 100n 1
-10 -15 -20
-10 -15 -20 10n 100n 1 10 ILOG (A) 100 1m 10m
10 100
1m
10m
ILOG (A)
VLOGIIN - VCMVIN vs. ILOG
4 3 VLOGIIN - VCMVIN (mV) 2 1 0 -1 -2 -3 -4 -5 1n 10n 100n 1 10 100 1m 10m ILOG (A) 0.01 1 IREF = 10nA
MAX4206 toc10
VLOGV2 VOLTAGE-NOISE DENSITY vs. FREQUENCY
MAX4206 toc11
TOTAL WIDEBAND VOLTAGE NOISE AT VLOGV2 vs. ILOG
IREF = ILOG f = 1Hz TO 1MHz VOLTAGE NOISE (mVRMS) 4
MAX4206 toc12
5
10 10nA NOISE DENSITY (V/Hz) 100nA 1 1A
5
3
10A 0.1
2
1 IREF = ILOG 0 10 100 1k 10k 100k 1M 10M 10n 100n 1 10 100 1m FREQUENCY (Hz) ILOG (A)
6
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
ILOG PULSE RESPONSE (IREF = 100nA, VCC = 5V, VEE = GND)
MAX4206 toc13
ILOG PULSE RESPONSE (IREF = 100nA, VCC = 5V, VEE = -5V)
MAX4206 toc14
1.0V 0.75V VLOGV1 (V) 0.75V 0.50V 0.50V 0.25V 0.25V 0
100A TO 1mA
1.0V 0.75V
100A TO 1mA
10A TO 100A VLOGV1 (V)
0.75V 0.50V 0.50V 0.25V
10A TO 100A
1A TO 10A
1A TO 10A
100nA TO 1A
0.25V 0
100nA TO 1A
20s/div
20s/div
IREF PULSE RESPONSE (ILOG = 1mA)
MAX4206 toc15
LOGARITHMIC SLOPE DISTRIBUTION
MAX4206 toc16
30 1A TO 100nA 25 20 COUNT (%) 15 10 5 1mA TO 100A 0 240 245 250 255
1.0V 0.75V 0.75V VLOGV1 (V) 0.50V 0.50V 0.25V 0.25V 0
10A TO 1A
100A TO 10A
260
20s/div
SLOPE (mV/DECADE)
VREFVOUT DISTRIBUTION
MAX4206 toc17
INPUT OFFSET VOLTAGE DISTRIBUTION
INPUT OFFSET VOLTAGE = VLOGIIN - VCMVIN 14 12
MAX4206 toc18
25
RL = 100k
16
20
COUNT (%)
COUNT (%) 1.236 1.238 1.240 1.242 1.244
15
10 8 6 4 2
10
5
0 1.232 1.234
0 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 VREFVOUT (V) INPUT OFFSET VOLTAGE (mV)
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7
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
OUTPUT OFFSET VOLTAGE vs. TEMPERATURE
MAX4206 toc19
REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. TEMPERATURE
MAX4206 toc20
REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. LOAD CURRENT
1.29 REFERENCE OUTPUT VOLTAGE (V) 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.20
MAX4206 toc21
10 8 6 4 VLOGV1 (mV) 2 0 -2 -4 -6 -8 -10 -50 -25 50 TEMPERATURE (C) 0 25 75 IREF = 1A ILOG 1A
1.30 1.29 REFERENCE OUTPUT VOLTAGE (V) 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.20
1.30
100
-50
-25
50 TEMPERATURE (C)
0
25
75
100
-1.0
-0.5
0
0.5
1.0
LOAD CURRENT (mA)
REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. SUPPLY VOLTAGE
MAX4206 toc22
REFERENCE POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
-10 -20 REFERENCE PSRR (dB) -30 -40 -50 -60 -70 -80 -90 -100 VREFVOUT 200mV/div CREFVOUT = 0.1F IREFVOUT = 1mA
MAX4206 toc23
REFERENCE LINE-TRANSIENT RESPONSE
MAX4206 toc24
1.250 1.245 REFERENCE OUTPUT VOLTAGE (V) 1.240 1.235 1.230 1.225 1.220 1.215 1.210 1.205 1.200 -1.0 -0.5 0 0.5
0
VCC 2V/div 0V
1.238V
CREFVOUT = 0F 10 100 1k 10k 100k 1M 10s/div
1.0
LOAD CURRENT (mA)
FREQUENCY (Hz)
REFERENCE LOAD-TRANSIENT RESPONSE
MAX4206 toc25
REFERENCE TURN-ON TRANSIENT RESPONSE
MAX4206 toc26
CREFVOUT = 0F IREFVOUT 1mA/div 0mA VCC 2.5V/div 0V
VREFVOUT 100mV/div
1.238V
VREFVOUT 500mV/div 0V
100s/div
10s/div
8
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = GND = 0V, IREF = 1A, ILOG = 10A, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1M, TA = +25C, unless otherwise noted.)
SMALL-SIGNAL AC RESPONSE ILOG TO VLOGV1
MAX4206 toc27
SMALL-SIGNAL AC RESPONSE ILOG TO VLOGV1
ILOG = 1mA
MAX4206 toc28
SMALL-SIGNAL AC RESPONSE OF BUFFER
AV = 1V/V 0 NORMALIZED GAIN (dB) AV = 2V/V -3 AV = 4V/V -6
MAX4206 toc29
10 ILOG = 100A 0 NORMALIZED GAIN (dB) -10 -20 -30 -40 ILOG = 100nA -50 -60 100 1k 10k 100k 1M CCOMP = 33pF RCOMP = 330 ILOG = 1mA ILOG = 10A ILOG = 1A
10 0 NORMALIZED GAIN (dB) -10 -20 -30 -40 -50 -60 CCOMP = 100pF RCOMP = 1k ILOG = 100A ILOG = 10A ILOG = 1A
3
ILOG = 100nA
-9
-12 100 1k 10k 100k 1M 10M 10k 100k 1M FREQUENCY (Hz) 10M 100M FREQUENCY (Hz)
10M
FREQUENCY (Hz)
Pin Description
PIN 1, 9 2 3 4 5 6 7 8 10 11 12 13 14 15 16 NAME N.C. REFVOUT GND VEE LOGV1 OSADJ SCALE LOGV2 VCC REFISET CMVOUT REFIOUT REFIIN LOGIIN CMVIN No Connection. Not internally connected. 1.238V Reference Voltage Output. Bypass REFVOUT to GND with a 0 to 1F capacitor (optional). Ground Negative Power Supply. Bypass VEE to GND with a 0.1F capacitor. Logarithmic Amplifier Voltage Output 1. The output scale factor of LOGV1 is 0.25V/decade. Offset Adjust Input. When operating from a single power supply, current applied to OSADJ adjusts the output offset voltage (see the Output Offset section). Scale Factor Input. Adjust the output scale factor for LOGV2 using a resistive divider (see the Scale Factor section). Logarithmic Amplifier Voltage Output 2. Adjust the output scale factor for LOGV2 using a resistive divider (see the Scale Factor section). Positive Power Supply. Bypass VCC to GND with a 0.1F capacitor. Current Reference Adjust Input. A resistor (RSET), from REFISET to GND, adjusts the current at REFIOUT (see the Adjusting the Logarithmic Intercept section). 0.5V Common-Mode Voltage Reference Output. Bypass CMVOUT to GND with a 0.1F capacitor. Current Reference Output. The internal current reference output is available at REFIOUT. Current Reference Input. Apply an external reference current at REFIIN. IREFIIN is the reference current used by the logarithmic amplifier when generating LOGV1. Current Input to Logarithmic Amplifier. LOGIIN is typically connected to a photodiode anode or other external current source. Common-Mode Voltage Input. VCMVIN is the common-mode voltage for the input and reference amplifiers (see the Common Mode section). FUNCTION
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9
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
VCC REFVOUT CMVOUT
CURRENT MIRROR VCC LOGIIN CURRENT CORRECTION 1.238V VCC
REFIOUT
0.5V CMVIN VEE VCC REFIIN SUMMING AMPLIFIER AND TEMPERATURE COMPENSATION VCC REFISET LOGV2 SCALE
OSADJ VEE
VEE GND
MAX4206
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 depen10
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 I kT I = ln LOG - ln REF q IS IS kT I = ln LOG q IREF I kT = (ln(10)) log10 LOG q IREF ILOG = K x log10 IREF
(see Figure 3)
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
IDEAL TRANSFER FUNCTION WITH VARYING K
3 2 1 0 -1 -2 -3 -4 0.001
VEE
LOGIIN
VOUT = K LOG (ILOG/IREF)
CMVIN VEE VBE2
IREF REFIIN
VCC
NORMALIZED OUTPUT VOLTAGE (V)
K=1 K = 0.5 K = 0.25
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 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.
Referred-to-Input and Referred-to-Output Errors 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 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 K) log10 LOG BIAS1 4( VLC VOSOUT ) IREF - IBIAS2
Definitions
Transfer Function The ideal logarithmic amplifier transfer function is: I VIDEAL = K x 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.
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.
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MAX4206 fig03
ILOG
VCC
VBE1
4
11
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
IBIAS1 and IBIAS2 are currents in the order of 20pA, significantly smaller than ILOG and IREF, and can therefore be eliminated: I VLOGV 2 K(1 K) log10 LOG 4( VLC VOSOUT ) IREF
OUTPUT VOLTAGE (V) 1.5 1.0 0.5 0 -0.5 IREF = 100A -1.0 -1.5 IREF = 1A
IDEAL TRANSFER FUNCTION WITH VARYING IREF
MAX4206 fig04
Expanding this expression: I I VLOGV 2 K log10 LOG KK log10 LOG IREF IREF 4K(1 K)( VLC VOSOUT ) The first term of this expression is the ideal component of VLOGV1. The remainder of the expression is the TE: I TE KK log10 LOG 4K(1 K)( VLC VOSOUT ) IREF 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: I TE K K log10 LOG 4( VLC VOSOUT ) IREF As an example, consider the following situation: Full-scale input = 5V ILOG = 100A 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 = +25C Substituting into the total error approximation, TE (1V/decade)(0.05log 10 (100A/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-
IREF = 10nA
1n
10n
100n
1 ILOG (A)
10
100
1m
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 = 100. Where high bandwidth at low current is required, CCOMP = 32pF and R COMP = 330 are suitable compensation values.
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
Frequency Response and Noise Considerations 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.
MAX4206
Select R1 between 1k and 100k, with an ideal value of 10k. 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 = 0), the reference current, IREF, equals the log intercept of 100A. Therefore, RSET = Select R1 = 10k: 1V/ V R2 = 10k - 1 = 30k 0.25 0.5V = 500k 10 x 100nA
Common Mode
A common-mode input voltage, V CMVOUT, 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.
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 x IREF where VREFISET is 0.5V. Select RSET between 5k and 5M. REFIOUT current range is 10nA to 10A only.
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
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
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13
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
VCC VCC IIN 0.1F VCC LOGV2 LOGIIN CCOMP 100pF RCOMP 100 CCOMP 100pF RCOMP 100 REFIOUT REFIIN SCALE R1 10k VOUT R2 30k CCOMP 100pF RCOMP 100 CCOMP 100pF RCOMP 100 IIN 0.1F VCC LOGV2 LOGIIN REFIOUT REFIIN SCALE R1 10k LOGV1 REFVOUT 0.1F 0.1F ROS 0 REFISET RSET 500k GND VEE OSADJ RSET 50k CMVIN CMVOUT OSADJ REFISET GND VEE R4 R3 VOUT R2 40k
MAX4206
MAX4206
CMVIN CMVOUT LOGV1
REFVOUT 0.1F
VEE
0.1F
Figure 5. Single-Supply Typical Operating Circuit
Figure 6. Dual-Supply Typical Operating Circuit
Scale Factor 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: K R2 = R1 0.25 Select R2 between 1k and 100k. Design Example Desired: Dual-Supply Operation Logarithmic intercept: 1A Overall scale factor = 1V/decade 0.5V RSET = = 50k 10 x 1A Select R1 = 10k: 1V / decade R2 = 10k x = 40k 0.25
Measuring Optical Absorbance
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). 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.
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
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 = 5M) 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
MAX4206
0.1F REFISET REFIIN 100pF VCC 100
VCC
CMVIN CMVOUT REFVOUT LOGV2 0.1F R2 SCALE LOGV1 0.1F
MAX4206
Capacitive Loads
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.
LOGIIN 100pF OSADJ REFIOUT 100 GND VEE
R3
R1
R4
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.
Figure 7. Measuring Optical Absorbance
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.
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.
Layout and Bypassing
Bypass V CC and V EE to GND with ceramic 0.1F capacitors. Place the capacitors as close to the device as possible. Bypass REFVOUT and/or CMVOUT to GND with a 0.1F ceramic capacitor for increased TRANSISTOR COUNT: 754 PROCESS: BiCMOS
Chip Information
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15
Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4206
VCC +2.7V TO +76V 2.2H 2.2F PHOTODIODE BIAS 0.22F BIAS CLAMP REFVOUT 0.1F REFIOUT REFIIN 100pF 100 LOGV2 SCALE 0.1F
VCC OUTPUT
MAX4007
MAX4206
LOGV1 OSADJ REFISET
IAPD REF OUT
IAPD/10 100pF GND 100 GND TO LIMITING AMPLIFIER VEE
5M CMVOUT CMVIN 0.1F
FIBER CABLE
APD
TIA HIGH-SPEED DATA PATH
Figure 8. Logarithmic Current-Sensing Amplifier with Sourcing Input
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Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range
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
MAX4206
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
1
2
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
2
2
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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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