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general description the MAX4207 logarithmic amplifier computes the log ratio of an input current relative to a reference current (externally or internally generated) and provides a cor- responding voltage output with a default -0.25v/decade scale factor. the device operates from dual ?.7v to ?.5v supplies and is capable of measuring five decades of input current across a 10na to 1ma range. the MAX4207? 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 MAX4207. the output-offset voltage and the adjustable scale factor are also set using exter- nal 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? to +85? extended temper- ature range. applications photodiode current monitoring portable instrumentation medical instrumentation analog signal processing features ? ?.7v to ?.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 10? reference current source ? input amplifiers summing nodes at ground ? small 16-pin thin qfn package (4mm x 4mm x 0.8mm) ? -40? to +85? operating temperature range ? evaluation kit available (order max4206evkit) MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range ________________________________________________________________ maxim integrated products 1 ordering information max4206 v ee gnd refiin refiout logiin refiset scale logv2 logv1 osadj cmvin cmvout refvout v cc v ee v cc r comp c comp r set r1 r2 0.1 f i in v out 0.1 f r comp c comp r4 r3 t ypical operating circuit 19-3070; rev 0; 12/03 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. evaluation kit available part temp range pin-package MAX4207ete -40? to +85? 16 thin qfn-ep* *ep = exposed paddle. 16 15 14 13 cmvin logiin refiin refiout 9 10 11 12 n.c. v cc refiset cmvout 4 3 2 1 v ee gnd refvout n.c. 5678 logv1 osadj scale logv2 MAX4207 top view (leads on bottom) thin qfn pin configuration
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. (all voltages referenced to gnd, unless otherwise noted.) v cc ...........................................................................-0.3v to +6v v ee ............................................................................-6v to +0.3v supply voltage (v cc to v ee ) .............................................. +12v refvout ....................................................(v ee - 0.3v) to +3.0v osadj, scale, refiset ...........................(v ee - 0.3v) to +5.5v refiin, logiin ........................................(v ee - 0.3v) to v cmvin logv1, logv2, cmvout, refiout ......................................(v ee - 0.3v) to (v cc + 0.3v) cmvin............................................................(v ee - 0.3v) to +1v continuous current (refiin, logiin) ................................10ma continuous power dissipation (t a = +70?) 16-pin thin qfn (derate 16.9mw/? above +70?) ...1349mw operating temperature range ...........................-40? to +85? junction temperature .....................................................+150? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? dc electrical characteristics?ual-supply operation (v cc = +5v, v ee = -5v, gnd = 0, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = -40? to +85?. typical values are at t a = +25?, unless otherwise noted.) (note 1) parameter symbol conditions min typ max units v cc (note 2) 2.7 5.5 supply voltage v ee (note 2) -2.7 -5.5 v t a = +25? 5 6 supply current i cc t a = -40? to +85? 7.5 ma minimum 10 na logiin current range (notes 3, 4) i log maximum 1 ma minimum 10 na refiin current range (notes 3, 4) i ref maximum 1 ma common-mode voltage v cmvout 0v common-mode voltage input range v cmvin 0 0.5 v t a = +25? ? ? log conformity error v lc i ref = 10na, i log = 10na to 1ma, k = -0.25v/decade (note 4) t a = -40? to +85? ?0 mv t a = +25? -237.5 -250 -262.5 logarithmic slope (scale factor) k t a = -40? to +85? -231.25 - 268.75 mv/ decade logarithmic slope (scale factor) temperature drift t a = -40? to +85? 80 ?/ decade/ ? input offset voltage v io t a = +25?, |v cmvin - v refiin |, |v cmvin - v logiin | 0.6 5 mv input offset voltage temperature drift v ios |v cmvin - v refiin |, |v cmvin - v logiin |6 ?/? t a = +25? 1.218 1.238 1.258 voltage reference output v refvout t a = -40? to +85? (note 4) 1.195 1.275 v
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range _______________________________________________________________________________________ 3 dc electrical characteristics?ual-supply operation (continued) (v cc = +5v, v ee = -5v, gnd = 0, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = -40? to +85?. typical values are at t a = +25?, unless otherwise noted.) (note 1) parameter symbol conditions min typ max units voltage reference output current i refvout 1ma t a = +25? 490 500 510 current reference output voltage v refiset t a = -40? to +85? (note 4) 482 518 mv logv2 buffer t a = +25? 0.4 2 input offset voltage v io t a = -40? to +85? (note 4) 6 mv input bias current i b (note 4) 0.01 1 na v oh r l to gnd = 2k ? v cc - 0.2 v cc - 0.3 output voltage range v ol r l to gnd = 2k ? v ee + 0.2 v ee + 0.08 v i out+ sourcing 34 output short-circuit current i out- sinking 58 ma slew rate sr 12 v/? unity-gain bandwidth gbw 5 mhz ac electrical characteristics?ual-supply operation (v cc = +5v, v ee = -5v, gnd = 0, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = +25?, unless otherwise noted.) parameter symbol conditions min typ max units logv2 total noise 0.1hz to 10hz, total output-referred noise, i ref = 10na, i log = 100na 17 ? rms logv2 spot noise density f = 5khz, i ref = 10na, i log = 100na 0.8 ?/ hz refvout total noise 1hz to 10hz, total output-referred noise 3.3 ? rms refvout spot noise density f = 5khz 266 nv/ hz refiset total noise 1hz to 10hz, total output-referred noise 0.67 ? rms refiset spot noise density f = 5khz 23 nv/ hz small-signal unity-gain bandwidth i ref = 1?, i log = 10?, r comp = 300 ? , c comp = 32pf 1 mhz note 1: all devices are 100% production tested at t a = +25?. all temperature limits are guaranteed by design. note 2: guaranteed and functionally verified. note 3: log conformity error less than ?mv with scale factor = 0.25v/decade. note 4: guaranteed by design.
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 4 _______________________________________________________________________________________ t ypical operating characteristics (v cc = +5v, v ee = -5v, gnd = 0v, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = +25?, unless otherwise noted.) v logv1 vs. i log MAX4207 toc01 i log (a) v logv1 (v) 1m 100 10n 100n 10 1 -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 -1.25 1n 10m t a = -40 c to +85 c 1m 100 10n 100n 1 10 1n 10m v logv1 vs. i ref MAX4207 toc03 i ref (a) v logv1 (v) -0.50 -0.25 0 0.25 0.50 0.75 1.00 1.25 -0.75 i log = 1 a t a = -40 c to +85 c v logv1 vs. i log MAX4207 toc04 i log (a) v logv1 (v) 1m 100 10n 100n 10 1 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 -2.0 1n 10m 100 a 1ma 10 a 1 a i ref = 10na to 1ma 100na 10na v logv1 vs. i ref MAX4207 toc05 i ref (a) v logv1 (v) 1m 100 10n 100n 10 1 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 -2.0 1n 10m 100 a 1ma 10 a 1 a 100na 10na i log = 10na to 1ma normalized log conformance error vs. i ref MAX4207 toc08 i ref (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m i log = 1 a t a = -40 c to +85 c normalized log conformance error vs. i log MAX4207 toc09 i log (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m i ref = 10na, 100na, 1 a, 10 a, 100 a, 1ma 1m 100 10n 100n 1 10 1n 10m v logv1 vs. i log MAX4207 toc02 i log (a) v logv1 (v) -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 -1.25 t a = -40 c to +85 c v cc = +2.7v v ee = -2.7v normalized log conformance error vs. i log MAX4207 toc06 i log (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m t a = -40 c t a = -40 c t a = -40 c to +85 c normalized log conformance error vs. i log MAX4207 toc07 i log (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m t a = -40 c to +85 c v cc = +2.7v v ee = -2.7v
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range _______________________________________________________________________________________ 5 normalized log conformance error vs. i ref MAX4207 toc10 i ref (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m i log = 10na, 100na, 1 a, 10 a, 100 a, 1ma 1ma normalized log conformance error vs. i log MAX4207 toc11 i log (a) error (mv) 1m 100 10n 100n 10 1 -15 -10 -5 0 5 10 15 20 -20 1n 10m (v cc = +2.7v, v ee = -2.7v) (v cc = +5v, v ee = -5v) (v cc = +5.5v, v ee = -5.5v) input offset voltage (v logiin - v cmvin vs. i log ) MAX4207 toc12 i log (a) v logiin - v cmvin (mv) 1m 100 10n 100n 10 1 -4 -2 -3 -1 0 1 2 3 4 -5 1n 10m i log pulse response (i ref = 1 a) MAX4207 toc13 20 s/div +0.25v -0.25v -0.50v -0.75v 1 a to 100na 10 a to 1 a 100 a to 10 a 1ma to 100 a 0v 0v -0.25v -0.50v i ref pulse response (i log = 1 a) MAX4207 toc14 20 s/div 0.75v 0.25v 0v -0.25v 100 a to 1ma 10 a to 100 a 1 a to 10 a 100na to 1 a 0.50v 0.50v 0.25v 0v t ypical operating characteristics (continued) (v cc = +5v, v ee = -5v, gnd = 0v, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = +25?, unless otherwise noted.)
v logv2 voltage-noise density vs. frequency MAX4207 toc15 frequency (hz) 1m 100k 10k 1k 100 10 0.1 1 10 0.01 110m noise density ( v/ hz) i ref = i log 100na 10na 1 a 10 a total wideband voltage noise at v logv2 vs. i log vMAX4207 toc16 i log (a) voltage noise (mv rms ) 100 10 1 100n 1 2 3 4 5 0 10n 1m f = 1hz to 1mhz i ref = i log logarithmic slope distribution MAX4207 toc17 slope (mv/decade) count (%) 255 250 245 5 10 15 20 25 30 35 0 240 260 v refvout distribution MAX4207 toc18 v refvout (v) count (%) 1.242 1.240 1.238 1.236 1.234 5 10 15 20 25 30 0 1.232 1.246 r l = 100k ? input offset voltage distribution MAX4207 toc19 input offset voltage (mv) count (%) 2.5 2.0 1.5 1.0 0.5 0 -0.5 5 10 15 20 25 0 -1.0 3.0 input offset voltage = v logiin - v cmvin reference output voltage (v refvout ) vs. temperature MAX4207 toc20 temperature ( c) reference output voltage (v) 75 50 25 0 -25 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 1.20 -50 100 MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 6 _______________________________________________________________________________________ t ypical operating characteristics (continued) (v cc = +5v, v ee = -5v, gnd = 0v, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = +25?, unless otherwise noted.) reference output voltage (v refvout ) vs. load current MAX4207 toc21 load current (ma) reference output voltage (v) 0.5 0 -0.5 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.00 -1.0 1.0 reference output voltage (v refvout ) vs. supply voltage (v cc - v ee ) MAX4207 toc22 supply voltage (v) reference output voltage (v) 10 9 8 7 6 1.205 1.210 1.215 1.220 1.225 1.230 1.235 1.240 1.245 1.250 1.200 511
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range _______________________________________________________________________________________ 7 t ypical operating characteristics (continued) (v cc = +5v, v ee = -5v, gnd = 0v, i ref = 1?, i log = 10?, logv2 = scale, logv1 = osadj, cmvin = cmvout, r set > 1m ? , t a = +25?, unless otherwise noted.) 0 -100 10 100 1k 10k 100k 1m reference power-supply rejection ratio vs. frequency -80 MAX4207 toc23 frequency (hz) reference psrr (db) -60 -40 -20 -90 -70 -50 -30 -10 c refvout = 0.1 f i refvout = 1ma reference line-transient response MAX4207 toc24 10 s/div 0v v cc - v ee 5v/div v refvout 200mv/div 1.238v c refvout = 0f reference load-transient response MAX4207 toc25 100 s/div 0ma i refvout 1ma/div v refvout 100mv/div 1.24v c refvout = 0f small-signal ac response (i log to v logv1 ) MAX4207 toc28 frequency (hz) normalized gain (db) 1m 100k 10k 1k -50 -40 -30 -20 -10 0 10 -60 100 10m i log = 100 a i log = 1ma i log = 10 a i log = 1 a i log = 100na c comp = 100pf r comp = 100 ? i ref = 10 a small-signal ac response of buffer MAX4207 toc29 frequency (hz) normalized gain (db) 10m 1m 100k -9 -6 -3 0 3 -12 10k 100m a v = 2v/v a v = 4v/v a v = 1v/v reference turn-on transient response MAX4207 toc26 10 s/div 0v v cc - v ee 5v/div v refvout 500mv/div 0v c refvout = 0f small-signal ac response (i log to v logv1 ) MAX4207 toc27 frequency (hz) normalized gain (db) 1m 100k 10k 1k -50 -40 -30 -20 -10 0 10 -60 100 10m i log = 100 a i log = 1ma i log = 10 a c comp = 33pf r comp = 330 ? i ref = 10 a i log = 1 a i log = 100na
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 8 _______________________________________________________________________________________ MAX4207 logiin cmvin v cc refiin v cc current correction v cc logv1 scale osadj logv2 v cc v cc refiset current mirror cmvout refvout refiout gnd v ee 0.5v 1.238v v ee v ee summing amplifier and temperature compensation figure 1. functional diagram pin name function 1, 9 n.c. no connection. not internally connected. 2 refvout 1.238v reference voltage output. bypass refvout to gnd with a 0 to 1? capacitor (optional). 3 gnd ground 4v ee negative power supply. bypass v ee to gnd with a 0.1? 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 v cc positive power supply. bypass v cc to gnd with a 0.1? capacitor. 11 refiset current reference adjust input. a resistor, r set , 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. i refiin 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. v cmvin is the common-mode voltage for the input and reference amplifiers (see the common mode section ) . pin description
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range _______________________________________________________________________________________ 9 detailed description theory figure 2 shows a simplified model of a logarithmic amplifier. two transistors convert the currents applied at logiin and refiin to logarithmic voltages accord- ing to the following equation: where: v be = base-emitter voltage of a bipolar transistor k = 1.381 x 10 -23 j/k t = absolute temperature (k) q = 1.602 x 10 ?9 c i c = collector current i s = reverse saturation current the logarithmic amplifier compares v be1 to the refer- ence voltage v be2 , which is a logarithmic voltage for a known reference current, i ref . the temperature depen- dencies of a logarithmic amplifier relate to the thermal voltage, (kt/q), and i s . matched transistors eliminate the i s temperature dependence of the amplifier in the following manner: where: k = scale factor (v/decade) i log = the input current at logiin i ref = 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? scale factor, maintaining a constant slope over temperature. definitions transfer function the ideal logarithmic amplifier transfer function is: adjust k (see the scale factor section) to increase the transfer-function slope as illustrated in figure 3. adjust i ref 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? output from the best-fit straight line of the v logv1 versus log (i log /i ref ) 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 addi- tive error at logv1 corresponds to multiplicative error at the input, regardless of input level. vk i i ideal log ref = ? ? ? ? ? ? log 10 vvv kt q i i kt q i i kt q i i i i kt q i i out be be log s ref s log s ref s log ref = ? = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = ? ? ? ? ? ? ? ? ? ? 12 ln ln ln ln ln ? ? ? ? ? ? ? ? ? ? ? = ? ? ? ? ? ? () ? ? ? ? ? ? ? ? ? ? ? ? ? ? = ? ? ? ? ? ? kt q i i k i i log ref log ref ln( ) log log 10 10 10 v kt q i i be c s = ? ? ? ? ? ? ? ? ? ? ? ? ln logiin cmvin v cc refiin v cc v be1 v be2 v ee v ee i log i ref figure 2. simplified model of a logarithmic amplifier (see figure 3)
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 10 ______________________________________________________________________________________ total error total error, te, is defined as the deviation of the output voltage, v logv1 , from the ideal transfer function (see the ideal transfer function section): total error is a combination of the associated gain, input offset current, input bias current, output offset voltage, and transfer characteristic nonlinearity (log conformity) errors: where v lc and v osout are the log conformity and out- put offset voltages, respectively. output offset is defined as the offset occurring at the output of the MAX4207 when equal currents are presented to i log and i ref . because the MAX4207 is configured with a gain of k = -0.25v/decade, a 4 should multiply the (? lc ? osout ) term, if v lc and v osout were derived from this default configuration. i bias1 and i bias2 are currents on the order of 20pa, significantly smaller than i log and i ref , and can there- fore be eliminated: expanding this expression: the first term of this expression is the ideal component of v logv1 . the remainder of the expression is the total error, te: in the second term, one can generally remove the products relating to ? k, because ? k is generally much less than 1. hence, a good approximation for te is given by: as an example, consider the following situation: full-scale input = 5v i log = 100? i ref = 100na k = 1 ?% v/decade (note that the uncommitted ampli- fier is configured for a gain of 4) v lc = ?mv (obtained from the electrical characteristics table) v osout = ?mv (typ), and t a = +25?. te k k i i vv log ref lc osout ? ? ? ? ? ? ? () ? ? ? ? ? ? ? ? ? log 10 4 te k k i i kkvv log ref lc osout ? ? ? ? ? ? ? + () ?? log ( ) 10 41 vk i i kk i i kkvv logv log ref log ref lc osout 210 10 41 ? ? ? ? ? ? ? ? ? ? ? ? ? + () log log () ? ? vkk i i vv logv log ref lc osout 210 14 ? ? ? ? ? ? ? () ? ? ? ? ? ? ? ? () log ? vkk ii ii vv logv log bias ref bias lc osout 210 1 2 14 = ? ? ? ? ? ? () ? ? ? ? ? ? ? ? () log ? - - vvte logv ideal 1 = ideal transfer function with varying k MAX4207 fig03 current ratio (i log /i ref ) normalized output voltage (v) 100 110 0.1 0.01 -2 -3 -1 0 1 2 3 4 -4 0.001 1000 v out = k log (i log /i ref ) k = 1 k = 0.5 k = 0.25 k = -0.25 k = -0.5 k = -1 figure 3. ideal transfer function with varying k ideal transfer function with varying i ref MAX4207 fig04 i log (a) output voltage (v) 100 1 10 100n 10n -1.0 -0.5 0 0.5 1.0 1.5 -1.5 1n 1m k = -0.25 i ref = 100 a i ref = 10na i ref = 1 a figure 4. ideal transfer function with varying i ref
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range ______________________________________________________________________________________ 11 substituting into the total error approximation, te ? (1v/decade)(0.05 log 10 (100?/100na) ? (?mv ?mv) = ?0.15v ?(?mv)] as a worst case, one finds te ? ?78mv or ?.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. applications information input current range five decades of input current across a 10na to 1ma range are acceptable for i log and i ref . the effects of bias currents increase as i log and i ref fall below 10na. bandwidth decreases at low i log values (see the frequency response and noise considerations section). as i log and i ref 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? frequency response is a function of the input current magnitude and the selected compensation network at logiin and refiin. the compensation net- work comprised of c comp and r comp ensures stability over the specified range of input currents by introducing an additional pole/zero to the system. for the typical application, select c comp = 32pf and r comp = 330 ? . frequency response and noise considerations the MAX4207 bandwidth is proportional to the magnitude of the i ref and i log currents, whereas the noise is inversely proportional to i ref and i log currents. common mode a 0v common-mode input voltage, v cmvout , is avail- able 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 refer- ence current, i ref . a resistor from refiset to gnd (see figure 5) adjusts the reference current, according to the following equation: where v refiset is 0.5v. select r set between 5k ? and 5m ? . refiout current range is 10na to 10? only. dual-supply operation the MAX4207 operates only from dual ?.7 to ?.5v sup- plies. the relationship of inputs to outputs is a function of i ref , relative to i log , and the configuration of the uncom- mitted amplifier. the uncommitted amplifier can be con- figured in either inverting or noninverting mode. in an inverting configuration, the uncommitted amplifier output, logv2, is positive and logv1 is negative when i log exceeds i ref . when operating in a noninverting configu- ration, logv2 and logv1 are both negative when i log exceeds i ref (see table 1). an inverting configuration of the uncommitted buffer is recommended when large out- put offset voltage adjustments are required using osadj. by connecting cmvout and cmvin, the log and refer- ence amplifier inputs (logiin and refiin) are biased at 0v. applying the external voltage (0 to 0.5v) to cmvin optimizes the application? performance. r v i set refiset ref = 10 MAX4207 v ee v ee gnd refiin refiout logiin refiset scale logv2 logv1 cmvin cmvout v cc v cc r comp 330 ? c comp 32pf r set 50k ? r1 10k ? r2 4k ? 0.1 f i in 0.1 f v out r comp 330 ? c comp 32pf osadj refvout r4 r3 figure 5. typical operating circuit
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 12 ______________________________________________________________________________________ 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: a resistive divider between refvout, osadj, and gnd can be used to adjust v osadj (see figure 5). 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 uncom- mitted logv2 amplifier and following equation, which refers to figure 5: select r 2 between 1k ? and 100k ? . design example desired: logarithmic intercept: 1? overall scale factor = +1v/decade select r 1 = 10k ? : 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? inter- nal current reference sources 10na (r set = 5m ? ) to the refiin input. this configuration sets the logarithmic inter- cept 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 measur- ing optical power, as diode current is proportional to the incident optical power. measure absolute optical power using a single photodiode connected at logiin, with the MAX4207? internal current reference driving refiin. alternatively, connect a photodiode to each of the MAX4207? logging inputs, logiin and refiin, to measure relative optical power (figure 7). in absorbance measurement instrumentation, a refer- ence light source is split into two paths. the unfiltered path is incident upon the photodiode of the reference channel, refiin. the other path passes through a sam- ple of interest, with the resulting filtered light incident on the photodiode of the second channel, logiin. the MAX4207 outputs provide voltages proportional to the log ratio of the two optical powers?n indicator of the optical absorbance of the sample. in wavelength-locking applications, often found in fiberoptic communication modules, two photodiode cur- rents provide a means of determining whether a given optical channel is tuned to the desired optical frequen- cy. in this application, two bandpass optical filters with overlapping ?kirts?precede each photodiode. with proper filter selection, the MAX4207 output can vary monotonically (ideally linearly) with optical frequency. rk v decade k 210 1 025 40 = = ? ?? / . r v a k set = = 05 10 1 50 . ? rr k 21 025 = ? . vv r rr osadj refout = + ? ? ? ? ? ? 4 34 vv r r os osadj =+ ? ? ? ? ? ? 1 2 1 table 1. MAX4207 example configurations logv2 amplifier configuration input conditions v logv1 v logv2 i log > i ref (constant) negative positive inverting i log < i ref (constant) positive negative i log > i ref (constant) negative negative noninverting i log < i ref (constant) positive positive
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range ______________________________________________________________________________________ 13 capacitive loads the MAX4207 drives capacitive loads of up to 50pf. reactive loads decrease phase margin and can pro- duce excessive ringing and oscillation. use an isolation resistor in series with logv1 or logv2 to reduce the effect of large capacitive loads. recall that the combi- nation of the capacitive load and the small isolation resistor limits ac performance. power dissipation the logv1 and logv2 amplifiers are capable of sourcing or sinking in excess of 30ma. ensure that the continuous power dissipation rating for the MAX4207 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 MAX4207? exposed pad is internally connected to v ee , and can either be connected to the pc board v ee plane or left unconnected. ensure that only v ee traces are routed under the exposed paddle. v ee gnd refvout cmvin refiout cmvout refiset v cc scale logv2 osadj logv1 refiin logiin fiber cable apd 2.7v to 76v 2.2 h 2.2 f0.2 2 f bias ref clamp out tia gnd to limiting amplifier high-speed data path 32pf v cc 5m ? 32pf 330 ? 330 ? output photodiode bias 0.1 f i apd /10 i apd 0.1 f MAX4207 max4007 0.1 f v ee figure 6. logarithmic current-sensing amplifier with sourcing input figure 7. measuring optical absorbance MAX4207 v ee gnd refiin logiin v cc cmvin refiout cmvout refiset v cc v cc scale logv1 logv2 osadj refvout 32pf 330 ? 330 ? 32pf v ee
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range 14 ______________________________________________________________________________________ layout and bypassing bypass v cc and v ee to gnd with ceramic 0.1? capacitors. place the capacitors as close to the device as possible. bypass refvout and/or cmvout to gnd with a 0.1? ceramic capacitor for increased noise immunity and a clean reference current. for low- current operation, it is recommended to use metal guard rings around logiin, refiin, and refiset. connect this guard ring to cmvout. chip information transistor count: 754 process: bicmos
MAX4207 precision transimpedance logarithmic amplifier with over 5 decades of dynamic range maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it 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 2003 maxim integrated products printed usa is a registered trademark of maxim integrated products. 24l qfn thin.eps b 1 2 21-0139 package outline 12,16,20,24l qfn thin, 4x4x0.8 mm b 2 2 21-0139 package outline 12,16,20,24l qfn thin, 4x4x0.8 mm package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .)

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