LOG101AIDR

LOG101 LOG 101 SBOS242B – MAY 2002 – REVISED JUNE 2004 Precision LOGARITHMIC AND LOG RATIO AMPLIFIER FEATURES q EASY-TO-USE COMPLETE CORE FUNCTION q HIGH ACCURACY: 0.01% FSO Over 5 Decades q WIDE INPUT DYNAMIC RANGE: 7.5 Decades, 100pA to 3.5mA q LOW QUIESCENT CURRENT: 1mA q WIDE SUPPLY RANGE: ±4.5V to ±18V DESCRIPTION The LOG101 is a versatile integrated circuit that computes the logarithm or log ratio of an input current relative to a reference current. The LOG101 is tested over a wide dynamic range of input signals. In log ratio applications, a signal current can come from a photodiode, and a reference current from a resistor in series with a precision external reference. The output signal at VOUT is trimmed to 1V per decade of input current allowing seven decades of input current dynamic range. Low DC offset voltage and temperature drift allow accurate measurement of low-level signals over a wide environmental temperature range. The LOG101 is specified over the temperature range –5° C to +75 ° C, with operation over –40°C to +85°C. Note: Protected under US Patent #6,667,650; other patents pending. APPLICATIONS q LOG, LOG RATIO COMPUTATION: Communication, Analytical, Medical, Industrial, Test, and General Instrumentation q PHOTODIODE SIGNAL COMPRESSION AMPS q ANALOG SIGNAL COMPRESSION IN FRONT OF ANALOG-TO-DIGITAL (A/D) CONVERTERS I2 V+ 4 I1 1 VOUT = (1V) • LOG (I1/I2) CC 8 LOG101 Q1 Q2 A1 A2 R2 3 VOUT R1 5 GND V– 6 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 2002-2004, Texas Instruments Incorporated www.ti.com ABSOLUTE MAXIMUM RATINGS(1) Supply Voltage, V+ to V– .................................................................... 36V Input Voltage .................................................... (V–) – 0.5 to (V+) + 0.5V Input Current ................................................................................... ±10mA Output Short-Circuit(2) .............................................................. Continuous Operating Temperature .................................................... –40°C to +85°C Storage Temperature ..................................................... –55°C to +125°C Junction Temperature .................................................................... +150°C Lead Temperature (soldering, 10s) ............................................... +300°C NOTES: (1) Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. (2) Short-circuit to ground. ELECTROSTATIC DISCHARGE SENSITIVITY This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. PIN DESCRIPTION Top View I1 NC VOUT V+ 1 2 3 4 LOG101 8 7 6 5 I2 NC GND V– SO NC = No Internal Connection PACKAGE/ORDERING INFORMATION(1) PACKAGE DESIGNATOR D SPECIFIED TEMPERATURE RANGE –5°C to +75°C PACKAGE MARKING LOG101 ORDERING NUMBER LOG101AID LOG101AIDR TRANSPORT MEDIA, QUANTITY Rails, 100 Tape and Reel, 2500 PRODUCT LOG101AID PACKAGE-LEAD SO -8 " " " " " NOTE: (1) For the most current package and ordering information, see the Package Option Addendum located at the end of this data sheet. ELECTRICAL CHARACTERISTICS At TA = +25°C, VS = ±5V, and ROUT = 10kΩ, unless otherwise noted. Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. LOG101AID PARAMETER CORE LOG FUNCTION IIN / VOUT Equation LOG CONFORMITY ERROR(1) Initial over Temperature GAIN(3) Initial Value Gain Error vs Temperature INPUT, A1 and A2 Offset Voltage vs Temperature vs Power Supply (PSRR) Input Bias Current vs Temperature Voltage Noise Current Noise Common-Mode Voltage Range (Positive) (Negative) Common-Mode Rejection Ratio (CMRR) OUTPUT, A2 (VOUT) Output Offset, VOSO, Initial vs Temperature Full-Scale Output (FSO) Short-Circuit Current 1nA to 100µA (5 decades) 100pA to 3.5mA (7.5 decades) 1nA to 100µA (5 decades) 100pA to 3.5mA (7.5 decades)(2) 1nA to 100µA 1nA to 100µA TMIN to TMAX CONDITION MIN TYP VO = (1V) • log (I1/I2) 0.01 0.06 0.0001 0.0005 1 0.15 0.003 ±0.3 ±2 5 ±5 Doubles Every 10°C 3 30 4 (V+) – 2 (V+) – 1.5 (V–) + 2 (V–) + 1.2 105 ±3 ±2 (V–) + 1.2 ±18 0.2 MAX UNITS V % % %/ °C %/ °C V/decade % %/ °C mV µV/°C µV/V pA µVrms nV/√Hz fA/√Hz V V dB ±15 (V+) – 1.5 mV ±1 0.01 ±1.5 50 TMIN to TMAX VS = ±4.5V to ±18V TMIN to TMAX f = 10Hz to 10kHz f = 1kHz f = 1kHz TMIN to TMAX VS = ±5V µV/°C V mA 2 LOG101 www.ti.com SBOS242B ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. LOG101AID PARAMETER TOTAL Initial ERROR(4)(5) CONDITION I1 or I2 remains fixed while other varies. Min to Max I1 or I2 = 3.5mA I1 or I2 = 1mA I1 or I2 = 100µA I1 or I2 = 10µA I1 or I2 = 1µA I1 or I2 = 100nA I1 or I2 = 10nA I1 or I2 = 1nA I1 or I2 = 350pA I1 or I2 = 100pA I1 or I2 = 3.5mA I1 or I2 = 1mA I1 or I2 = 100µA I1 or I2 = 10µA I1 or I2 = 1µA I1 or I2 = 100nA I1 or I2 = 10nA I1 or I2 = 1nA I1 or I2 = 350pA I1 or I2 = 100pA I1 or I2 = 3.5mA I1 or I2 = 1mA I1 or I2 = 100µA I1 or I2 = 10µA I1 or I2 = 1µA I1 or I2 = 100nA I1 or I2 = 10nA I1 or I2 = 1nA I1 or I2 = 350pA I1 or I2 = 100pA MIN TYP MAX UNITS vs Temperature ±1.2 ±0.4 ±0.1 ±0.05 ±0.05 ±0.09 ±0.2 ±0.3 ±0.1 ±0.3 ±3.0 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.25 ±0.1 ±0.1 ±75 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 vs Supply mV mV mV mV mV mV mV mV mV mV mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ °C mV/ V mV/ V mV/ V mV/ V mV/ V mV/ V mV/ V mV/V mV/ V mV/ V FREQUENCY RESPONSE, CORE LOG(6) BW, 3dB I2 = 10nA I2 = 1µA I2 = 10µA I2 = 1mA Step Response Increasing I2 = 1µA to 1mA I2 = 100nA to 1µA I2 = 10nA to 100nA Decreasing I2 = 1mA to 1µA I2 = 1µA to 100nA I2 = 100nA to 10nA POWER SUPPLY Operating Range Quiescent Current TEMPERATURE RANGE Specified Range, TMIN to TMAX Operating Range Storage Range Thermal Resistance, θJA SO-8 CC = 4500pF CC = 150pF CC = 150pF CC = 50pF 0.1 38 40 45 kHz kHz kHz kHz CC = 150pF CC = 150pF CC = 150pF CC = 150pF CC = 150pF CC = 150pF VS IO = 0 ±4.5 11 7 110 45 20 550 ±18 ±1.5 75 85 125 150 µs µs µs µs µs µs V mA °C °C °C °C/W ±1 –5 –40 –55 NOTES: (1) Log Conformity Error is peak deviation from the best-fit straight line of VOUT versus log (I1 / I2) curve expressed as a percent of peak-to-peak full-scale. (2) May require higher supply for full dynamic range. (3) Output core log function is trimmed to 1V output per decade change of input current. (4) Worst-case Total Error for any ratio of I1 /I2 is the largest of the two errors, when I1 and I2 are considered separately. (5) Total I1 + I2 should be kept below 4.5mA on ±5V supply. (6) Bandwidth (3dB) and transient response are a function of both the compensation capacitor and the level of input current. LOG101 SBOS242B www.ti.com 3 TYPICAL CHARACTERISTICS At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. NORMALIZED TRANSFER FUNCTION 4.0 1 ONE CYCLE OF NORMALIZED TRANSFER FUNCTION 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 6 8 10 Normalized Output Voltage (V) 2.0 1.0 0.0 –1.0 –2.0 –3.0 –4.0 0.0001 0.001 0.01 0.1 1 10 100 1k 10k Current Ratio, I1/I2 Normalized Output Voltage (V) 3.0 VOUT = 1V LOG (I1/I2) Current Ratio, I1/I2 120 100 TOTAL ERROR vs INPUT CURRENT 5.8 4.8 GAIN ERROR (I2 = 1µA) +85°C +75°C +75°C Gain Error (%) Total Error (mV) 80 60 40 +25°C 20 0 100pA 1nA 10nA 100nA –5°C 3.8 +25°C 2.8 1.8 0.8 –0.2 100pA 1nA –5°C to –40°C 1µA 10µA 100µA 1mA 10mA 10nA 100nA 1µA 10µA 100µA 1mA 10mA Input Current (I1 or I2) Input Current (I1 or I2) 100M 10M 1M 100k CC (pF) MINIMUM VALUE OF COMPENSATION CAPACITOR 3dB Frequency Response (Hz) 3dB FREQUENCY RESPONSE 1M 10µA 100k 10k 1k 100 10 1 0.1 100pA 1nA 10nA 100nA I2 1µA 10µA 100µA 1mA 1nA 10nA I1 = 1nA 100µA 1µA 100µA 100µA 1mA I1 = 1mA Select CC for I1 min. and I2 max. Values below 2pF may be ignored. I1 = 100pA I1 = 1nA I1 = 10nA I1 = 100nA 1µA I1 = 10µA 100µA 1mA 1nA 10nA 100nA 1µA I2 10µA 100µA 1mA 10mA CC =1 F 0p 100µA 10k 1k 100 10 1 100pA µ o1 At 0µ 1 10nA A 1µA 1mA to 10µA 100nA 10nA CC =1 00 0 pF CC µF =1 I1 = 1nA 4 LOG101 www.ti.com SBOS242B TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. 17 15 LOG CONFORMITY vs INPUT CURRENT 350 300 LOG CONFORMITY vs TEMPERATURE 7 Decades (100pA to 1mA) 6 Decades (1nA to 1mA) 11 9 7 5 3 1 –1 100pA 1nA 10nA 100nA 1µA +85°C Log Conformity (m%) Log Conformity (mV) 13 250 200 150 100 50 0 –40 –30 –20 –10 0 5 Decades (1nA to 100µA) +75°C –40°C to +25°C 10µA 100µA 1mA 10 20 30 40 50 60 70 80 90 Input Current (I1 or I2) Temperature (°C) APPLICATION INFORMATION The LOG101 is a true logarithmic amplifier that uses the base-emitter voltage relationship of bipolar transistors to compute the logarithm, or logarithmic ratio of a current ratio. Figure 1 shows the basic connections required for operation of the LOG101. In order to reduce the influence of lead inductance of power-supply lines, it is recommended that each supply be bypassed with a 10µF tantalum capacitor in parallel with a 1000pF ceramic capacitor, as shown in Figure 1. Connecting the capacitors as close to the LOG101 as possible will contribute to noise reduction as well. INPUT CURRENT RANGE To maintain specified accuracy, the input current range of the LOG101 should be limited from 100pA to 3.5mA. Input currents outside of this range may compromise LOG101 performance. Input currents larger than 3.5mA result in increased nonlinearity. An absolute maximum input current rating of 10mA is included to prevent excessive power dissipation that may damage the logging transistor. On ±5V supplies, the total input current (I1 + I2) is limited to 4.5mA. Due to compliance issues internal to the LOG101, to accommodate larger total input currents, supplies should be increased. Currents smaller than 100pA will result in increased errors due to the input bias currents of op amps A1 and A2 (typically 5pA). The input bias currents may be compensated for, as shown in Figure 2. The input stages of the amplifiers have FET inputs, with input bias current doubling every 10°C, which makes the nulling technique shown practical only where the temperature is fairly stable. R2 10kΩ V+ V– R1 1MΩ 1 I1 LOG101 V+ 10µF 1000pF 4 1 6 LOG101 8 I1 I2 5 CC 3 VOUT 5 3 VOUT 8 I2 R1' > 1MΩ 4 6 GND 10µF V– 1000pF CC V– R2' 10kΩ V+ FIGURE 1. Basic Connections of the LOG101. FIGURE 2. Bias Current Nulling. LOG101 SBOS242B www.ti.com 5 SETTING THE REFERENCE CURRENT When the LOG101 is used to compute logarithms, either I1 or I2 can be held constant and becomes the reference current to which the other is compared. VOUT is expressed as: VOUT = (1V) • log (I1/I2) (1) V+ V+ I1 = 2.5nA to 1mA REF3025 2.5V 1GΩ to 2.5kΩ 100kΩ I2 = 2.5nA 10MΩ +25mV +2.5V 100Ω 8 3 CC 5 V– 6 GND LOG101 1 4 3 VOUT IREF can be derived from an external current source (such as shown in Figure 3), or it may be derived from a voltage source with one or more resistors. When a single resistor is used, the value may be large depending on IREF. If IREF is 10nA and +2.5V is used: RREF = 2.5V/10nA = 250MΩ IREF 2N2905 RREF +15V 6V IN834 2N2905 OPA335 Chopper Op Amp –2.5V (2) FIGURE 5. Current Source with Offset Compensation. at different levels of input signals. Smaller input currents require greater gains to maintain full dynamic range, and will slow the frequency response of the LOG101. –15V 3.6kΩ IREF = 6V RREF FREQUENCY COMPENSATION Frequency compensation for the LOG101 is obtained by connecting a capacitor between pins 3 and 8. The size of the capacitor is a function of the input currents, as shown in the Typical Characteristic Curves (Minimum Value of Compensation Capacitor). For any given application, the smallest value of the capacitor which may be used is determined by the maximum value of I2 and the minimum value of I1. Larger values of CC will make the LOG101 more stable, but will reduce the frequency response. In an application, highest overall bandwidth can be achieved by detecting the signal level at VOUT, then switching in appropriate values of compensation capacitors. FIGURE 3. Temperature Compensated Current Source. A voltage divider may be used to reduce the value of the resistor, as shown in Figure 4. When using this method, one must consider the possible errors caused by the amplifier’s input offset voltage. The input offset voltage of amplifier A1 has a maximum value of 1.5mV, making VREF a suggested value of 100mV. VREF = 100mV R1 +5V R2 R3 > > R 2 IREF R3 1 VOS + – A1 NEGATIVE INPUT CURRENTS The LOG101 will function only with positive input currents (conventional current flows into pins 1 and 8). In situations where negative input currents are needed, the circuits in Figures 6, 7, and 8 may be used. FIGURE 4. T Network for Reference Current. Figure 5 shows a low-level current source using a series resistor. The low offset op-amp reduces the effect of the LOG101’s input offset voltage. IIN QA QB National LM394 FREQUENCY RESPONSE The frequency response curve seen in the Typical Characteristic Curves is shown for constant DC I1 and I2 with a small signal AC current on one input. The 3dB frequency response of the LOG101 is a function of the magnitude of the input current levels and of the value of the frequency compensation capacitor. See Typical Characteristic Curve “3dB Frequency Response” for details. The transient response of the LOG101 is different for increasing and decreasing signals. This is due to the fact that a log amp is a nonlinear gain element and has different gains D1 D2 OPA703 IOUT FIGURE 6. Current Inverter/Current Source. 6 LOG101 www.ti.com SBOS242B +5V +3.3V 1/2 OPA2335 1.5kΩ 1.5kΩ +5V TLV271 or 1 OPA2335 2 V+ I1 Sample λ1 Light λ 1 Source λ 1´ I2 BSH203 4 1 3 VOUT D1 LOG101 8 5 CC 6 10nA to 1mA Back Bias +3.3V 1/2 OPA2335 D2 10nA to 1mA Pin 1 or Pin 8 Photodiode LOG101 V– FIGURE 9. Absorbance Measurement. FIGURE 7. Precision Current Inverter/Current Source. OPERATION ON SINGLE SUPPLY Many applications do not have the dual supplies required to operate the LOG101. Figure 10 shows the LOG101 configured for operation with a single +5V supply. VOLTAGE INPUTS The LOG101 gives the best performance with current inputs. Voltage inputs may be handled directly with series resistors, but the dynamic input range is limited to approximately three decades of input voltage by voltage noise and offsets. The transfer function of Equation (13) applies to this configuration. Single Supply +5V 4 APPLICATION CIRCUITS LOG RATIO One of the more common uses of log ratio amplifiers is to measure absorbance. A typical application is shown in Figure 9. Absorbance of the sample is A = logλ1´/ λ1 If D1 and D2 are matched A ∝ (1V) logI1 / I2 DATA COMPRESSION In many applications the compressive effects of the logarithmic transfer function are useful. For example, a LOG101 preceding a 12-bit Analog-to-Digital (A/D) converter can produce the dynamic range equivalent to a 20-bit converter. (3) 1µF I1 1 3 VOUT LOG101 6 8 I2 5 CC (4) 2 1µF 3 TPS(1) 4 5 1 1µF NOTE: (1) TPS60402DBV negative charge pump. –5V FIGURE 10. Single +5V Power-Supply Operation. 1.5kΩ 100kΩ +5V 10nA to 1mA Back Bias +3.3V +3.3V 1/2 OPA2335 +5V 1.5kΩ 100kΩ Photodiode 1/2 OPA2335 1.5kΩ 100kΩ 100kΩ 10nA to 1mA Pin 1 or Pin 8 LOG101 FIGURE 8. Precision Current Inverter/Current Source. LOG101 SBOS242B www.ti.com 7 INSIDE THE LOG101 Using the base-emitter voltage relationship of matched bipolar transistors, the LOG101 establishes a logarithmic function of input current ratios. Beginning with the base-emitter voltage defined as: VBE I = VT ln C IS kT where : VT = q also VOUT = VL VOUT = R1 + R 2 R1 (9) (10) (1) R1 + R 2 I n VT log 1 R1 I2 k = Boltzman’s constant = 1.381 • 10–23 T = Absolute temperature in degrees Kelvin q = Electron charge = 1.602 • 10–19 Coulombs IC = Collector current IS = Reverse saturation current From the circuit in Figure 11, we see that: or VOUT = (1V) • log I1 I2 (11) I1 Q1 + – VBE – VBE Q2 + I2 VOUT VL = VBE1 – VBE 2 Substituting (1) into (2) yields: VL = VT1 ln I1 IS1 – VT2 ln I2 IS 2 A2 (2) I1 A1 1 2 VOUT = (1V) • LOG I2 I1 I2 R2 VL R1 (3) If the transistors are matched and isothermal and VTI = VT2, then (3) becomes: FIGURE 11. Simplified Model of a Log Amplifier. (4) (5) (6) (7) It should be noted that the temperature dependance associated with VT = kT/q is internally compensated on the LOG101 by making R1 a temperature sensitive resistor with the required positive temperature coefficient. I I VL = VT1 ln 1 – ln 2  IS IS   I VL = VT ln 1 and sin ce I2 ln x = 2.3 log10 x I VL = n VT log 1 I2 where n = 2.3 (8) DEFINITION OF TERMS TRANSFER FUNCTION The ideal transfer function is: VOUT = 1V • log (I1/I2) (5) VOUT (V) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5 –2.0 –2.5 –3.0 –3.5 100 pA 1nA 10n A 100 nA µA 1 10µ A 100 µA 1m A A 10m I1 A 0p 10 I 2 = nA 1 I 2 = 0n A 1 I= 2 Figure 12 shows the graphical representation of the transfer over valid operating range for the LOG101. ACCURACY Accuracy considerations for a log ratio amplifier are somewhat more complicated than for other amplifiers. This is because the transfer function is nonlinear and has two inputs, each of which can vary over a wide dynamic range. The accuracy for any combination of inputs is determined from the total error specification. A 0n 10 I 2 = µA 1 I 2 = 0µA 1 I 2 = 00µA 1 I 2 = mA 1 I= 2 VOUT = (1V) • LOG (I1/I2) FIGURE 12. Transfer Function with Varying I 2 and I1. 8 LOG101 www.ti.com SBOS242B TOTAL ERROR The total error is the deviation (expressed in mV) of the actual output from the ideal output of VOUT = 1V • log (I1/I2). Thus, VOUT(ACTUAL) = VOUT(IDEAL) ± Total Error. (6) to I2 is shown in Figure 13. The OPA703 is configured as a level shifter with inverting gain and is used to scale the photodiode current directly into the A/D converter input voltage range. The wide dynamic range of the LOG101 is also useful for measuring avalanche photodiode current (APD) (see Figure 14). It represents the sum of all the individual components of error normally associated with the log amp when operated in the current input mode. The worst-case error for any given ratio of I1/I2 is the largest of the two errors when I1 and I2 are considered separately. Temperature can affect total error. LOG CONFORMITY For the LOG101, log conformity is calculated the same as linearity and is plotted I1 /I2 on a semi-log scale. In many applications, log conformity is the most important specification. This is because bias current errors are negligible (5pA compared to input currents of 100pA and above) and the scale factor and offset errors may be trimmed to zero or removed by system calibration. This leaves log conformity as the major source of error. Log conformity is defined as the peak deviation from the best fit straight line of the VOUT versus log (I1/I2) curve. This is expressed as a percent of ideal full-scale output. Thus, the nonlinearity error expressed in volts over m decades is: (7) VOUT(NONLIN) = 1V/dec • 2NmV where N is the log conformity error, in percent. ERRORS RTO AND RTI As with any transfer function, errors generated by the function itself may be Referred-to-Output (RTO) or Referred-toInput (RTI). In this respect, log amps have a unique property: Given some error voltage at the log amp’s output, that error corresponds to a constant percent of the input regardless of the actual input level. USING A LARGER REFERENCE VOLTAGE REDUCES OFFSET ERRORS Using a larger reference voltage to create the reference current minimizes errors due to the LOG101’s input offset voltage. Maintaining an increasing output voltage as a function of increasing photodiode current is also important in many optical sensing applications. All zeros from the A/D converter output represent zero or low-scale photodiode current. Inputting the reference current into I1, and designing IREF such that it is as large or larger than the expected maximum photodiode current is accomplished using this requirement. The LOG101 configured with the reference current connecting I1 and the photodiode current connecting INDIVIDUAL ERROR COMPONENTS The ideal transfer function with current input is: VOUT = (1V) • log I1 I2 (8) The actual transfer function with the major components of error is: (9) I –I VOUT = (1V) (1 ± ∆K ) log 1 B1 ± 2Nm ± VOS O I2 – IB 2 IREF = VREF R1 VOUT = VREF – R2 R3 • (1V)LOG ( IREF IPHOTO ) CC VOUT 3 R2 R3 IREF VMIN to VMAX R1 I1 1 Q1 Q2 OPA703 A2 A1 R2 A/D Converter VREF IPHOTO I2 8 R3 LOG101 6 IMIN to IMAX FIGURE 13. Technique for Using Full-Scale Reference Current Such that V OUT Increases with Increasing Photodiode Current. LOG101 SBOS242B www.ti.com 9 +15V to +60V 500Ω ISHUNT Irx = 1µA to 1mA Receiver 5kΩ +5V APD I to V Converter 5kΩ 10Gbits/sec INA168 SOT23-5 2 IOUT 1 IOUT = 0.1 • ISHUNT CC 1.2kΩ 1kΩ +5V 4 1 Q1 Q2 OPA703 A2 A1 100µA 25kΩ VOUT = 2.5V to 0V 3 8 REF3025 2.5V LOG101 SO-8 5 –5V 6 FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current. The individual component of error is: ∆K = gain accuracy (0.15%, typ), as specified in the specification table. IB1 = bias current of A1 (5pA, typ) IB2 = bias current of A2 (5pA, typ) N = log conformity error (0.01%, 0.06%, typ) 0.01% for n = 5, 0.06% for n = 7 VOSO = output offset voltage (3mV, typ) n = number of decades over which N is specified: Example: what is the error when I1 = 1µA and I2 = 100nA VOUT = (1 ± 0.0015) log (10) Where VOUT EOS1 V1 – IB1 ± R1 R1 = (1V) (1 ± ∆K ) log ± 2Nn ± VOSO EOS2 V2 – IB2 ± R2 R2 Since the ideal output is 1.000V, the error as a percent of reading is % error = 0.005055 • 100% = 0.5% 1 (12) For the case of voltage inputs, the actual transfer function is (13) EOS1 E and OS2 are considered to be zero for large R1 R2 10 −6 − 5 • 10 −12 ± (2)(0.0001)5 ± 3.0mV 10 −7 − 5 • 10 −12 = 1.005055V (11) values of resistance from external input current sources. 10 LOG101 www.ti.com SBOS242B PACKAGE OPTION ADDENDUM www.ti.com 9-Dec-2004 PACKAGING INFORMATION Orderable Device LOG101AID LOG101AIDR (1) Status (1) ACTIVE ACTIVE Package Type SOIC SOIC Package Drawing D D Pins Package Eco Plan (2) Qty 8 8 100 2500 None None Lead/Ball Finish CU SNPB CU SNPB MSL Peak Temp (3) Level-3-235C-168 HR Level-3-235C-168 HR The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) Eco Plan - May not be currently available - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. None: Not yet available Lead (Pb-Free). Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Green (RoHS & no Sb/Br): TI defines "Green" to mean "Pb-Free" and in addition, uses package materials that do not contain halogens, including bromine (Br) or antimony (Sb) above 0.1% of total product weight. (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDECindustry standard classifications, and peak solder temperature. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. 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