19-3071; Rev 0; 12/03 KIT ATION EVALU E L B A AVAIL Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 Ordering Information Applications Photodiode Current Monitoring PART Portable Instrumentation MAX4206ETE TEMP RANGE PIN-PACKAGE -40°C to +85°C 16 Thin QFN-EP* *EP = Exposed Paddle. Medical Instrumentation Analog Signal Processing Typical Operating Circuit Pin Configuration VCC IIN REFIIN LOGIIN CMVIN REFIOUT 0.1µF TOP VIEW (LEADS ON BOTTOM) VCC LOGV2 LOGIIN R2 CCOMP 16 15 14 13 REFIOUT RCOMP N.C. 1 12 CMVOUT REFVOUT 2 11 REFISET GND 3 10 VCC VEE 4 9 N.C. MAX4206 VOUT SCALE REFIIN CCOMP R1 MAX4206 RCOMP CMVIN CMVOUT LOGV1 REFVOUT 0.1µF 6 7 8 LOGV1 OSADJ SCALE LOGV2 0.1µF 5 THIN QFN ROS REFISET RSET OSADJ GND VEE ________________________________________________________________ Maxim Integrated Products 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. 1 MAX4206 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 purposes, 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 -40°C to +85°C extended temperature range. MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 Supply Voltage SYMBOL VCC Supply Current ICC LOGIIN Current Range (Notes 3, 4) ILOG REFIIN Current Range (Notes 3, 4) IREF Common-Mode Voltage Common-Mode Voltage Input Range Log Conformity Error Logarithmic Slope (Scale Factor) CONDITIONS (Note 2) MIN 2.7 TA = +25°C 3.9 TA = -40°C to +85°C Minimum Minimum TA = +25°C 500 ±2 TA = -40°C to +85°C 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 VREFISET 2 520 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 Current Reference Output Voltage mA ±10 TA = +25°C, |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| IREFVOUT 1 mV VIO Voltage Reference Output Current mA nA Input Offset Voltage VREFVOUT mA nA Maximum 0.5 Voltage Reference Output V 5 10 VCMVIN Logarithmic Slope (Scale Factor) Temperature Drift 11.0 1 480 K UNITS 10 Maximum IREF = 10nA, ILOG= 10nA to 1mA, K = 0.25V/decade (Note 4) MAX 7 VCMVOUT VLC TYP 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 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range (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 Input Offset Voltage VIO Input Bias Current IB Output Voltage Range Output Short-Circuit Current Slew Rate Unity-Gain Bandwidth TA = +25°C 0.4 TA = -40°C to +85°C (Note 4) 2 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 MIN TYP MAX VCC 2.7 5.5 VEE -2.7 -5.5 Supply Current ICC LOGIIN Current Range (Notes 3, 4) ILOG REFIIN Current Range (Notes 3, 4) IREF Common-Mode Voltage CONDITIONS VCMVOUT TA = +25°C 5 TA = -40°C to +85°C Minimum 7.5 10 1 10 mA mA nA Maximum 480 V nA Maximum Minimum 6 UNITS 500 1 mA 520 mV _______________________________________________________________________________________ 3 MAX4206 DC ELECTRICAL CHARACTERISTICS—Single-Supply Operation (continued) MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 4 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 _______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range (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.) 1.25 0.75 0.50 MAX4206 toc02 1.75 1.50 1.25 1.00 VLOGV1 (V) 1.00 VLOGV1 vs. ILOG IREF = 10nA TA = -40°C TO +85°C VCC = +5V VEE = -5V 1.50 VLOGV1 (V) VLOGV1 (V) 1.25 1.75 MAX4206 toc01 1.50 VLOGV1 vs. ILOG IREF = 10nA TA = -40°C TO +85°C VCC = +5V VEE = GND 0.75 0.50 0.75 0.50 0.25 0.25 0 0 0 -0.25 10n 100n 1µ 10µ ILOG (A) 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 1.75 -0.25 1n 10n 100n 1µ 10µ 100µ ILOG (A) 1m 10m 10n 100n 1µ 10µ 100µ 1m 10m ILOG (A) _______________________________________________________________________________________ 5 MAX4206 AC ELECTRICAL CHARACTERISTICS—Dual-Supply Operation 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.) 1.0 0.8 0.5 0.25 0.3 0 0 -0.25 1µ 10µ 100µ 1m 10µA 100µA 100nA 1µA -20 1n 1µ 10µ 100µ 100n 1µ 10µ 100µ 1m 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 10 5 0 -5 10n 100n 1µ 10µ 100µ 1m 0 -5 -20 10n 10m 5 -15 -20 -20 IREF = 10nA SINGLE SUPPLY: VCC = +2.7V, +5V, 11V, VEE = GND DUAL SUPPLY: VCC = +5V VEE = -5V 100n 1µ 10µ 100µ 1m 10n 10m 100n 1µ 10µ 100µ 1m ILOG (A) ILOG (A) ILOG (A) VLOGIIN - VCMVIN vs. ILOG VLOGV2 VOLTAGE-NOISE DENSITY vs. FREQUENCY TOTAL WIDEBAND VOLTAGE NOISE AT VLOGV2 vs. ILOG 10nA 2 1 0 -1 -2 100nA 1µA IREF = ILOG f = 1Hz TO 1MHz VOLTAGE NOISE (mVRMS) NOISE DENSITY (µV/√Hz) 3 5 MAX4206 toc11 10 MAX4206 toc10 IREF = 10nA 10m -10 TA = -40°C -15 -15 15 10 -10 TA = -40°C 20 MAX4206 toc08 20 ERROR (mV) -10 1 10µA 0.1 4 10m 3 2 1 -3 -4 IREF = ILOG 0 0.01 -5 1n 10n 100n 1µ 10µ 100µ ILOG (A) 6 10n 1m NORMALIZED LOG CONFORMANCE ERROR vs. ILOG -5 4 100n ILOG (A) 0 5 10n IREF (A) 5 1n -5 -15 ERROR (mV) 10 0 ILOG (A) IREF = 10nA TA = -40°C TO +85°C VCC = +5V VEE = -5V 15 10m MAX4206 toc07 20 100n 5 -10 10nA -0.5 10n ERROR (mV) 1mA -0.3 -0.50 ERROR (mV) 0.50 10 IREF = 10nA TA = -40°C TO +85°C VCC = +5V VEE = GND MAX4206 toc09 VLOGV1 (V) VLOGV1 (V) 100nA 10nA 0.75 1.5 15 1.3 1µA 1.00 1.8 MAX4206 toc12 100µA 10µA 1.25 20 MAX4206 toc05 1mA 1.50 2.0 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) VLOGIIN - VCMVIN (mV) MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 1m 10m 1 10 100 1k 10k 100k FREQUENCY (Hz) 1M 10M 10n 100n 1µ 10µ ILOG (A) _______________________________________________________________________________________ 100µ 1m Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 10µA TO 100µA 0.50V 0.50V 1µA TO 10µA 0.25V 1µA TO 10µA 0.25V 0.25V 0.25V 100nA TO 1µA 0 100nA TO 1µA 0 20µs/div 20µs/div IREF PULSE RESPONSE (ILOG = 1mA) LOGARITHMIC SLOPE DISTRIBUTION MAX4206 toc15 1µA TO 100nA MAX4206 toc16 1.0V 30 25 0.75V 0.50V 100µA TO 10µA 20 COUNT (%) 10µA TO 1µA 0.50V 15 10 0.25V 5 0.25V 1mA TO 100µA 0 0 240 245 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) 20µs/div 12 15 COUNT (%) COUNT (%) VLOGV1 (V) 0.75V 10 10 8 6 4 5 2 0 1.232 1.234 0 1.236 1.238 1.240 1.242 VREFVOUT (V) 1.244 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 INPUT OFFSET VOLTAGE (mV) _______________________________________________________________________________________ 7 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.) 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.) 1.27 1.26 1.25 1.24 1.23 1.22 1.40 1.35 1.30 1.25 1.20 1.15 1.10 1.21 1.05 1.20 1.00 -50 -25 0 25 50 TEMPERATURE (°C) 75 100 1.245 1.240 1.235 1.230 1.225 1.220 1.215 1.210 1.200 -1.0 -0.5 0 0.5 1.0 2 LOAD CURRENT (mA) CREFVOUT = 0.1µF IREFVOUT = 1mA -20 3 4 SUPPLY VOLTAGE (V) REFERENCE LINE-TRANSIENT RESPONSE MAX4206 toc23 MAX4206 toc22 0 REFERENCE PSRR (dB) 1.250 1.205 REFERENCE POWER-SUPPLY REJECTION RATIO vs. FREQUENCY -10 REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. SUPPLY VOLTAGE MAX4206 toc21 1.28 1.45 REFERENCE OUTPUT VOLTAGE (V) 1.29 MAX4206 toc20 1.50 MAX4206 toc19 1.30 REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. LOAD CURRENT REFERENCE OUTPUT VOLTAGE (V) REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. TEMPERATURE REFERENCE OUTPUT VOLTAGE (V) MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range VCC 2V/div -30 0V -40 -50 -60 -70 VREFVOUT 200mV/div -80 -90 1.238V CREFVOUT = 0F -100 10 100 1k 10k 100k 1M 10µs/div FREQUENCY (Hz) REFERENCE LOAD-TRANSIENT RESPONSE REFERENCE TURN-ON TRANSIENT RESPONSE MAX4206 toc24 MAX4206 toc25 CREFVOUT = 0F IREFVOUT 1mA/div 0mA VCC 2.5V/div 0V VREFVOUT 100mV/div 1.238V VREFVOUT 500mV/div 0V 100µs/div 8 10µs/div _______________________________________________________________________________________ 5 6 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range -20 ILOG = 10µA -30 ILOG = 1µA -40 ILOG = 100µA -10 ILOG = 10µA -20 ILOG = 1µA -30 ILOG = 100nA -40 ILOG = 100nA -50 10k 100k 1M AV = 4V/V -6 10M -12 100 1k 10k 100k 1M 10M 10k FREQUENCY (Hz) FREQUENCY (Hz) MAX4206 toc28 AV = 2V/V -3 CCOMP = 100pF RCOMP = 1kΩ -60 -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 toc27 ILOG = 100µA MAX4206 toc26 10 SMALL-SIGNAL AC RESPONSE OF BUFFER SMALL-SIGNAL AC RESPONSE ILOG TO VLOGV1 SMALL-SIGNAL AC RESPONSE ILOG TO VLOGV1 100k 1M 10M 100M FREQUENCY (Hz) Pin Description PIN NAME FUNCTION 1, 9 N.C. 2 REFVOUT No Connection. Not internally connected. 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 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. 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). _______________________________________________________________________________________ 9 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.) MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range VCC REFVOUT CMVOUT CURRENT MIRROR VCC CURRENT CORRECTION LOGIIN REFIOUT 1.238V VCC 0.5V CMVIN VEE REFISET VCC LOGV2 REFIIN VCC SUMMING AMPLIFIER AND TEMPERATURE COMPENSATION SCALE OSADJ VEE VEE MAX4206 GND 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 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) Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 4 CMVIN VEE IREF VCC VBE2 REFIIN NORMALIZED OUTPUT VOLTAGE (V) LOGIIN 3 VOUT = K LOG (ILOG/IREF) 2 1 0 -1 -2 -3 -4 0.001 VEE K=1 K = 0.5 K = 0.25 MAX4206 fig03 VCC MAX4206 ILOG IDEAL TRANSFER FUNCTION WITH VARYING K VBE1 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) 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. 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: I VIDEAL = K × log10 LOG I REF 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. Total Error Total error, TE, is defined as the deviation of the output voltage, VLOGV1, from the ideal transfer function (see the Ideal 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 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. ______________________________________________________________________________________ 11 IBIAS1 and IBIAS2 are currents on 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± ∆K) log10 LOG ± 4( ± VLC ± VOSOUT ) IREF Expanding this expression: I I VLOGV 2 ≅ K log10 LOG ± K∆K 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 total error, TE: 1.0 IREF = 10nA 0.5 0 -0.5 IREF = 100µA IREF = 1µA -1.0 -1.5 1n I TE ≅ ±K∆K 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 = 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 con12 MAX4206 fig04 1.5 OUTPUT VOLTAGE (V) MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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. ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 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, 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. 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 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 ______________________________________________________________________________________ 13 MAX4206 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 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range VCC VCC IIN IIN 0.1µF 0.1µF VCC LOGV2 LOGIIN CCOMP 100pF LOGV2 CCOMP 100pF 0.1µF CMVIN 0.1µF REFISET OSADJ GND RSET 500kΩ RSET 50kΩ VEE R3 CMVOUT OSADJ ROS 0Ω REFISET LOGV1 REFVOUT CMVIN CMVOUT LOGV1 0.1µF R1 10kΩ MAX4206 RCOMP 100Ω MAX4206 REFVOUT SCALE REFIIN CCOMP 100pF R1 10kΩ RCOMP 100Ω R2 40kΩ REFIOUT RCOMP 100Ω SCALE REFIIN R4 GND VEE VEE Figure 5. Single-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 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 14 VOUT LOGIIN CCOMP 100pF R2 30kΩ REFIOUT RCOMP 100Ω VCC VOUT 0.1µF Figure 6. Dual-Supply Typical Operating Circuit 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. ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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. 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. VCC 0.1µF VCC CMVIN REFISET REFIIN 0.1µF CMVOUT REFVOUT 100pF 0.1µF LOGV2 VCC R2 MAX4206 100Ω SCALE LOGV1 R3 LOGIIN 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 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 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. TRANSISTOR COUNT: 754 PROCESS: BiCMOS ______________________________________________________________________________________ 15 MAX4206 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range VCC +2.7V TO +76V 2.2µH 2.2µF PHOTODIODE BIAS 0.22µF 0.1µF BIAS VCC CLAMP OUTPUT 0.1µF REFVOUT LOGV2 REFIOUT SCALE REFIIN MAX4007 100pF 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 16 ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 24L QFN THIN.EPS PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 B 1 2 PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 B 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 © 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX4206 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.)