19-3070; Rev 1; 6/04 KIT ATION EVALU E L B A AVAIL Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range The MAX4207 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 dual ±2.7V to ±5.5V supplies and is capable of measuring five decades of input current across a 10nA to 1mA range. The MAX4207’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 MAX4207. The output-offset voltage and the adjustable scale factor are also set using external resistors. The MAX4207 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. Features ♦ ±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 ♦ Input Amplifiers Summing Nodes at Ground ♦ Small 16-Pin Thin QFN Package (4mm x 4mm x 0.8mm) ♦ -40°C to +85°C Operating Temperature Range ♦ Evaluation Kit Available (Order MAX4206EVKIT) Applications Ordering Information Photodiode Current Monitoring PART Portable Instrumentation MAX4207ETE Medical Instrumentation TEMP RANGE PIN-PACKAGE -40°C to +85°C 16 Thin QFN-EP* *EP = Exposed paddle. Analog Signal Processing Typical Operating Circuit VCC Pin Configuration IIN 0.1µF VCC LOGV2 CMVIN LOGIIN REFIIN REFIOUT TOP VIEW (LEADS ON BOTTOM) 16 15 14 13 LOGIIN R2 CCOMP REFIOUT RCOMP SCALE REFIIN CCOMP N.C. 1 12 CMVOUT REFVOUT 2 11 REFISET GND 3 10 VCC VEE 4 9 N.C. MAX4207 THIN QFN R1 MAX4207 RCOMP CMVIN LOGV1 REFVOUT CMVOUT R3 REFISET 8 LOGV2 7 SCALE OSADJ 6 LOGV1 5 VOUT RSET GND OSADJ VEE R4 VEE 0.1µF ________________________________________________________________ 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 MAX4207 General Description MAX4207 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 +6V 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—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 SYMBOL VEE (Note 2) -2.7 -5.5 ILOG Log Conformity Error Logarithmic Slope (Scale Factor) IREF TA = +25°C 5 TA = -40°C to +85°C Minimum 10 Minimum K mA V 0.5 ±2 V ±5 mV TA = -40°C to +85°C ±10 TA = +25°C -237.5 TA = -40°C to +85°C -231.25 -250 80 0.6 TA = +25°C, |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| Input Offset Voltage Temperature Drift VIOS |VCMVIN - VREFIIN|, |VCMVIN - VLOGIIN| -262.5 -268.75 TA = -40°C to +85°C VIO 2 1 nA 0 Input Offset Voltage VREFVOUT mA mA 0 TA = +25°C V 1 10 Maximum IREF = 10nA, ILOG= 10nA to 1mA, K = -0.25V/decade (Note 4) UNITS nA Maximum VCMVIN VLC 6 7.5 VCMVOUT Logarithmic Slope (Scale Factor) Temperature Drift Voltage Reference Output MAX 5.5 LOGIIN Current Range (Notes 3, 4) Common-Mode Voltage Input Range TYP 2.7 ICC Common-Mode Voltage MIN (Note 2) Supply Current REFIIN Current Range (Notes 3, 4) CONDITIONS VCC µV/ decade/ °C 5 6 TA = +25°C 1.218 TA = -40°C to +85°C (Note 4) 1.195 1.238 _______________________________________________________________________________________ mV/ decade mV µV/°C 1.258 1.275 V 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 = -40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL Voltage Reference Output Current IREFVOUT Current Reference Output Voltage VREFISET CONDITIONS MIN TYP MAX 1 TA = +25°C 490 TA = -40°C to +85°C (Note 4) 482 500 UNITS 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) 2 6 (Note 4) 0.01 1 VOH RL to GND = 2kΩ VCC 0.2 VCC 0.3 VOL RL to GND = 2kΩ Output Voltage Range Output Short-Circuit Current Slew Rate Unity-Gain Bandwidth mV nA V VEE + 0.2 VEE + 0.08 IOUT+ Sourcing 34 IOUT- Sinking 58 mA SR 12 V/µs GBW 5 MHz 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. _______________________________________________________________________________________ 3 MAX4207 DC ELECTRICAL CHARACTERISTICS—Dual-Supply Operation (continued) Typical Operating Characteristics (VCC = +5V, VEE = -5V, GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ, TA = +25°C, unless otherwise noted.) VLOGV1 vs. ILOG 0.50 0.25 0.75 0 0.50 -0.25 -0.50 -0.50 -0.75 -0.75 -1.00 0 -0.25 TA = -40°C TO +85°C VCC = +2.7V VEE = -2.7V -1.00 TA = -40°C TO +85°C -1.25 1µ 10µ 100µ ILOG (A) 1m 10m -0.75 1n 10n 1mA 1.5 100µA 10µ 100µ 1m 10m 1n 100nA 20 1µA 15 10nA 0.5 0 10µA 100nA TA = -40°C TO +85°C -20 -2.0 1m 10m 1n 0 -5 TA = -40°C TA = -40°C TO +85°C VCC = +2.7V VEE = -2.7V 1m 10m 15 10 0 -5 -10 5 0 -5 -10 ILOG = 1µA TA = -40°C TO +85°C -15 IREF = 10nA, 100nA, 1µA, 10µA, 100µA, 1mA -20 10m 1µ 10µ 100µ ILOG (A) 20 TA = -40°C -15 1m 10n 100n NORMALIZED LOG CONFORMANCE ERROR vs. ILOG ERROR (mV) 5 ERROR (mV) 10 5 1µ 10µ 100µ ILOG (A) 1n 10m MAX4207 toc08 15 10 10n 100n 1m 20 MAX4207 toc07 15 1n 1µ 10µ 100µ IREF (A) NORMALIZED LOG CONFORMANCE ERROR vs. IREF 20 -15 10n 100n MAX4207 toc09 1µ 10µ 100µ ILOG (A) NORMALIZED LOG CONFORMANCE ERROR vs. ILOG -20 TA = -40°C 1mA 10nA -2.0 -10 -5 -15 -1.5 10n 100n 0 100µA IREF = 10nA TO 1mA 1n 10m 5 -10 -1.0 -1.5 1m 10 -0.5 1µA -1.0 10µ 100µ NORMALIZED LOG CONFORMANCE ERROR vs. ILOG ERROR (mV) 0 -0.5 1µ VLOGV1 vs. IREF ILOG = 10nA TO 1mA VLOGV1 (V) 0.5 100n IREF (A) 1.0 10µA 10n ILOG (A) 1.5 1.0 VLOGV1 (V) 1µ 2.0 MAX4207 toc04 2.0 100n MAX4207 toc05 10n 100n VLOGV1 vs. ILOG 4 -0.50 -1.25 1n 0.25 MAX4207 toc06 -0.25 ILOG = 1µA TA = -40°C TO +85°C 1.00 VLOGV1 (V) 0 VLOGV1 (V) VLOGV1 (V) 0.25 1.25 MAX4207 toc02 MAX4207 toc01 0.50 VLOGV1 vs. IREF 0.75 MAX4207 toc03 VLOGV1 vs. ILOG 0.75 ERROR (mV) MAX4207 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range -20 1n 10n 100n 1µ 10µ 100µ IREF (A) 1m 10m 1n 10n 100n 1µ 10µ 100µ ILOG (A) _______________________________________________________________________________________ 1m 10m Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 10 10 5 5 0 -5 4 0 -5 3 2 1 0 -1 -2 -10 -10 -3 -15 -15 -4 -20 -5 -20 1n 10n 100n 1µ 10µ 100µ IREF (A) 1m 10m MAX4207 toc12 (VCC = +2.7V, VEE = -2.7V) (VCC = +5V, VEE = -5V) (VCC = +5.5V, VEE = -5.5V) 15 ERROR (mV) ERROR (mV) 20 VLOGIIN - VCMVIN (mV) ILOG = 10nA, 100nA, 1µA, 10µA, 100µA, 1mA 15 MAX4207 toc10 20 INPUT OFFSET VOLTAGE (VLOGIIN - VCMVIN vs. ILOG) NORMALIZED LOG CONFORMANCE ERROR vs. ILOG MAX4207 toc11 NORMALIZED LOG CONFORMANCE ERROR vs. IREF 1n 10n 100n 1µ 10µ 100µ ILOG (A) 1m 10m ILOG PULSE RESPONSE (IREF = 1µA) 1n 10n 100n 1m 10m IREF PULSE RESPONSE (ILOG = 1µA) MAX4207 toc13 MAX4207 toc14 1µA TO 100nA +0.25V 1µ 10µ 100µ ILOG (A) 0V 100µA TO 1mA 0.75V 0.50V 10µA TO 1µA 0V -0.25V 10µA TO 100µA 0.50V 0.25V 100µA TO 10µA -0.25V -0.50V 1µA TO 10µA 0.25V 0V -0.50V 1mA TO 100µA -0.75V 0V 100nA TO 1µA -0.25V 20µs/div 20µs/div _______________________________________________________________________________________ 5 MAX4207 Typical Operating Characteristics (continued) (VCC = +5V, VEE = -5V, 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 = -5V, GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ, TA = +25°C, unless otherwise noted.) VLOGV2 VOLTAGE-NOISE DENSITY vs. FREQUENCY TOTAL WIDEBAND VOLTAGE NOISE AT VLOGV2 vs. ILOG 1 10µA 0.1 MAX4207 toc17 4 30 25 COUNT (%) 1µA 35 VMAX4207 toc16 100nA f = 1Hz TO 1MHz IREF = ILOG VOLTAGE NOISE (mVRMS) NOISE DENSITY (µV/√Hz) 10nA LOGARITHMIC SLOPE DISTRIBUTION 5 MAX4207 toc15 10 3 2 20 15 10 1 5 IREF = ILOG 0.01 0 0 10 100 1k 10k 100k 1M 10M 10n 100n 1µ 10µ 240 1m 100µ 245 250 ILOG (A) SLOPE (mV/DECADE) VREFVOUT DISTRIBUTION INPUT OFFSET VOLTAGE DISTRIBUTION OFFSET VOLTAGE vs. TEMPERATURE RL = 100kΩ 25 INPUT OFFSET VOLTAGE = VLOGIIN - VCMVIN 20 20 MAX4207 toc20 25 MAX4207 toc18 30 260 255 FREQUENCY (Hz) MAX4207 toc19 IREF = 1µA ILOG = 1µA 16 12 8 COUNT (%) 20 15 VLOGV1 (mV) 1 COUNT (%) 15 10 4 0 -4 10 -8 5 5 -12 -16 0 1.234 1.236 1.238 1.240 1.242 -20 -1.0 -0.5 1.246 0 0.5 1.0 1.5 2.0 2.5 -50 -25 INPUT OFFSET VOLTAGE (mV) VREFVOUT (V) REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. TEMPERATURE 1.29 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21 0 25 50 TEMPERATURE (°C) REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. LOAD CURRENT 1.30 1.29 REFERENCE OUTPUT VOLTAGE (V) MAX4207 toc21 1.30 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.20 1.20 -50 -25 0 25 50 TEMPERATURE (°C) 6 3.0 MAX4207 toc22 0 1.232 REFERENCE OUTPUT VOLTAGE (V) MAX4207 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 75 100 -1.0 -0.5 0 0.5 LOAD CURRENT (mA) _______________________________________________________________________________________ 1.0 75 100 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range REFERENCE POWER-SUPPLY REJECTION RATIO vs. FREQUENCY REFERENCE OUTPUT VOLTAGE (VREFVOUT) vs. SUPPLY VOLTAGE (VCC - VEE) MAX4207 toc24 1.240 CREFVOUT = 0.1µF IREFVOUT = 1mA -10 -20 REFERENCE PSRR (dB) 1.235 1.230 1.225 1.220 1.215 VCC - VEE 5V/div -30 0V -40 -50 -60 -70 VREFVOUT 200mV/div -80 1.210 1.205 -90 1.200 -100 5 6 7 8 9 10 1.238V CREFVOUT = 0F 10 11 100 1k 10k 100k 1M 10µs/div FREQUENCY (Hz) SUPPLY VOLTAGE (V) REFERENCE LOAD-TRANSIENT RESPONSE SMALL-SIGNAL AC RESPONSE (ILOG TO VLOGV1) REFERENCE TURN-ON TRANSIENT RESPONSE MAX4207 toc26 MAX4207 toc27 10 ILOG = 100µA IREFVOUT 1mA/div 0mA VCC - VEE 5V/div 0V VREFVOUT 100mV/div VREFVOUT 500mV/div 1.24V NORMALIZED GAIN (dB) 0 ILOG = 1mA -10 -20 ILOG = 10µA -30 ILOG = 1µA -40 ILOG = 100nA CCOMP = 33pF RCOMP = 330Ω IREF = 10µA -50 0V CREFVOUT = 0F MAX4207 toc28 REFERENCE OUTPUT VOLTAGE (V) 1.245 REFERENCE LINE-TRANSIENT RESPONSE MAX4207 toc25 0 MAX4207 toc23 1.250 CREFVOUT = 0F -60 100µs/div 100 10µs/div 1k 10k 100k 1M 10M FREQUENCY (Hz) ILOG = 100µA -10 ILOG = 10µA -20 ILOG = 1µA -30 ILOG = 100nA -40 AV = 2V/V -3 AV = 4V/V -6 -9 CCOMP = 100pF RCOMP = 100Ω IREF = 10µA -50 AV = 1V/V 0 NORMALIZED GAIN (dB) NORMALIZED GAIN (dB) 0 3 MAX4207 toc29 ILOG = 1mA SMALL-SIGNAL AC RESPONSE OF BUFFER MAX4207 toc30 SMALL-SIGNAL AC RESPONSE (ILOG TO VLOGV1) 10 -12 -60 100 1k 10k 100k FREQUENCY (Hz) 1M MAX4207 Typical Operating Characteristics (continued) (VCC = +5V, VEE = -5V, GND = 0V, IREF = 1µA, ILOG = 10µA, LOGV2 = SCALE, LOGV1 = OSADJ, CMVIN = CMVOUT, RSET > 1MΩ, TA = +25°C, unless otherwise noted.) 10M 10k 100k 1M 10M 100M FREQUENCY (Hz) _______________________________________________________________________________________ 7 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4207 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. Apply a voltage at OSADJ to adjust the LOGV2 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 between SCALE, GND, and LOGV2 (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 0V Common-Mode Voltage Reference Output 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). 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. VCC REFVOUT CMVOUT CURRENT MIRROR VCC CURRENT CORRECTION LOGIIN REFIOUT 1.238V VCC 0.5V CMVIN VEE REFISET LOGV2 VCC REFIIN SUMMING AMPLIFIER AND TEMPERATURE COMPENSATION VCC SCALE OSADJ VEE GND MAX4207 VEE LOGV1 Figure 1. Functional Diagram 8 _______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range VCC VBE1 LOGIIN CMVIN VEE IREF VCC VBE2 REFIIN VEE where: k = scale factor (V/decade) ILOG = the input current at LOGIIN Figure 2. Simplified Model of a Logarithmic Amplifier 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: kT I 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 dependencies 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: IREF = the reference current at REFIIN The MAX4207 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 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 MAX4207’s output from the best-fit straight line of the VLOGV1 versus log (ILOG/IREF) curve. It is expressed as a percent of the full-scale output or an output voltage. Referred-to-Input and Referred-to-Output Errors The log nature of the MAX4207 insures that any additive error at LOGV1 corresponds to multiplicative error at the input, regardless of input level. _______________________________________________________________________________________ 9 MAX4207 ILOG 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 I = K × log10 LOG (see Figure 3) IREF IDEAL TRANSFER FUNCTION WITH VARYING K VOUT = K LOG (ILOG/IREF) K=1 K = 0.5 K = 0.25 1 0 -1 K = -0.25 K = -0.5 K = -1 -3 -4 0.001 IREF = 100µA 0.5 0 -0.5 -1.0 IREF = 10nA IREF = 1µA -1.5 0.01 0.1 1 10 100 1000 CURRENT RATIO (ILOG/IREF) Figure 3. Ideal Transfer Function with Varying K 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 TE 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 ) I I REF BIAS2 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 MAX4207 when equal currents are presented to ILOG and IREF. Because the MAX4207 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. 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 Expanding this expression: 10 K = -0.25 1.0 OUTPUT VOLTAGE (V) 2 -2 1.5 MAX4207 fig03 NORMALIZED OUTPUT VOLTAGE (V) 4 3 IDEAL TRANSFER FUNCTION WITH VARYING IREF MAX4207 fig04 MAX4207 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 1n 10n 100n 1µ 10µ 100µ 1m ILOG (A) Figure 4. Ideal Transfer Function with Varying IREF 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 TE: I TE ≅ ±K∆K log10 LOG ± 4K(1 + ∆K)( ± VLC ± VOSOUT ) I REF 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 ) 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), and TA = +25°C. ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 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 bias currents increase as I LOG and I REF 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 MAX4207 incorporates leakage current compensation and high-current correction circuits to compensate for these errors. Frequency Compensation The MAX4207’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 = 32pF and RCOMP = 330Ω. Frequency Response and Noise Considerations The MAX4207 bandwidth is proportional to the magnitude of the IREF and I LOG currents, whereas the noise is inversely proportional to IREF and ILOG currents. Common Mode A 0V common-mode input voltage, VCMVOUT, is available at CMVOUT and can be used to bias the logging and reference amplifier inputs by connecting CMVOUT to CMVIN. A voltage between 0 and 0.5V, connected to CMVIN, may be used to bias the logging and reference transistor collectors, thereby optimizing performance. Adjusting the Logarithmic Intercept Adjust the logarithmic intercept by changing the reference current, IREF. A resistor from REFISET to GND (see Figure 5) 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. Dual-Supply Operation The MAX4207 operates only from dual ±2.7 to ±5.5V supplies. The relationship of inputs to outputs is a function of IREF, relative to ILOG, and the configuration of the uncommitted amplifier. The uncommitted amplifier can be configured in either inverting or noninverting mode. In an inverting configuration, the uncommitted amplifier output, LOGV2, is positive and LOGV1 is negative when ILOG exceeds IREF. When operating in a noninverting configuration, LOGV2 and LOGV1 are both negative when ILOG exceeds IREF (see Table 1). An inverting configuration of the uncommitted buffer is recommended when large output offset voltage adjustments are required using OSADJ. By connecting CMVOUT and CMVIN, the log and reference amplifier inputs (LOGIIN and REFIIN) are biased at 0V. Applying the external voltage (0 to 0.5V) to CMVIN optimizes the application’s performance. VCC IIN 0.1µF CCOMP 32pF RCOMP 330Ω VCC LOGV2 VOUT LOGIIN R2 4kΩ REFIOUT CCOMP 32pF SCALE REFIIN MAX4207 RCOMP 330Ω R1 10kΩ LOGV1 REFVOUT CMVIN R3 CMVOUT REFISET RSET 50kΩ OSADJ GND VEE VEE R4 0.1µF Figure 5. Typical Operating Circuit ______________________________________________________________________________________ 11 MAX4207 Substituting into the TE approximation, TE ≅ ± (1V/decade)(0.05 log10 (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 contributing to total error. For further accuracy, consider temperature monitoring as part of the calibration process. MAX4207 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range Table 1. MAX4207 Example Configurations LOGV2 AMPLIFIER CONFIGURATION Inverting Noninverting INPUT CONDITIONS VLOGV1 ILOG > IREF (constant) Negative Positive ILOG < IREF (constant) Positive Negative ILOG > IREF (constant) Negative Negative ILOG < IREF (constant) Positive Positive Output Offset The inverting configuration utilized by the MAX4207 facilitates large output-offset voltage adjustments. 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 5). R4 VOSADJ = VREFOUT R3 + R4 Scale Factor The scale factor, K, is the slope of the logarithmic output. For the LOGV1 amplifier, K = -0.25V/decade. Adjust the overall scale factor for the MAX4207 using the uncommitted LOGV2 amplifier and the following equation, which refers to Figure 5: R2 = R1 K − 0.25 Select R2 between 1kΩ and 100kΩ. Design Example Desired: Logarithmic intercept: 1µA Overall scale factor = +1V/decade 0.5V RSET = = 50kΩ 10 × 1µA Select R1 = 10kΩ: R2 = 10kΩ × 12 1V / decade = 40kΩ − 0.25 VLOGV2 Photodiode Current Monitoring Figure 6 shows the MAX4207 in an optical-power measurement circuit, common in fiberoptic applications. The MAX4007 current monitor converts the sensed APD current to an output current that drives the MAX4207 LOGIIN input (APD current is scaled by 0.1). The MAX4007 also buffers the high-voltage APD voltages from the lower MAX4207 voltages. The MAX4207’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. 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 MAX4207’s internal current reference driving REFIIN. Alternatively, connect a photodiode to each of the MAX4207’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 MAX4207 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 MAX4207 output can vary monotonically (ideally linearly) with optical frequency. ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range MAX4207 VCC 2.2µH 2.7V TO 76V PHOTODIODE BIAS 2.2µF 0.22µF 0.1µF BIAS VCC CLAMP OUTPUT 0.1µF REFVOUT LOGV2 REFIOUT SCALE REFIIN MAX4007 MAX4207 32pF LOGV1 OSADJ 330Ω REFISET IAPD/10 IAPD 5MΩ LOGIIN OUT REF GND FIBER CABLE APD 32pF CMVOUT 330Ω CMVIN GND TIA TO LIMITING AMPLIFIER VEE VEE 0.1µF HIGH-SPEED DATA PATH Figure 6. Logarithmic Current-Sensing Amplifier with Sourcing Input Capacitive Loads VCC VCC CMVIN REFISET REFIIN CMVOUT REFVOUT 32pF LOGV2 VCC Power Dissipation MAX4207 330Ω The MAX4207 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. SCALE LOGV1 LOGIIN The LOGV1 and LOGV2 amplifiers are capable of sourcing or sinking in excess of 30mA. Ensure that the continuous power dissipation rating for the MAX4207 is not exceeded. TQFN Package 32pF OSADJ REFIOUT 330Ω GND VEE VEE 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 MAX4207’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. Figure 7. Measuring Optical Absorbance ______________________________________________________________________________________ 13 MAX4207 Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range Layout and Bypassing Bypass V CC and V EE to GND with ceramic 0.1µF capacitors. Place the capacitors as close to the device as possible. Bypass REFVOUT and/or CMVOUT to GND with a 0.1µF ceramic capacitor for increased noise immunity and a clean reference current. For lowcurrent operation, it is recommended to use metal guard rings around LOGIIN, REFIIN, and REFISET. Connect this guard ring to CMVOUT. 14 Chip Information TRANSISTOR COUNT: 754 PROCESS: BiCMOS ______________________________________________________________________________________ Precision Transimpedance Logarithmic Amplifier with Over 5 Decades of Dynamic Range 24L QFN THIN.EPS 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 ____________________ 15 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX4207 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.)