LOG101 LOG 101 SBOS242B – MAY 2002 – REVISED JUNE 2004 Precision LOGARITHMIC AND LOG RATIO AMPLIFIER FEATURES DESCRIPTION ● EASY-TO-USE COMPLETE CORE FUNCTION The LOG101 is a versatile integrated circuit that computes the logarithm or log ratio of an input current relative to a reference current. ● HIGH ACCURACY: 0.01% FSO Over 5 Decades ● WIDE INPUT DYNAMIC RANGE: 7.5 Decades, 100pA to 3.5mA ● LOW QUIESCENT CURRENT: 1mA 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. ● WIDE SUPPLY RANGE: ±4.5V to ±18V The output signal at VOUT is trimmed to 1V per decade of input current allowing seven decades of input current dynamic range. APPLICATIONS ● LOG, LOG RATIO COMPUTATION: Communication, Analytical, Medical, Industrial, Test, and General Instrumentation ● PHOTODIODE SIGNAL COMPRESSION AMPS ● ANALOG SIGNAL COMPRESSION IN FRONT OF ANALOG-TO-DIGITAL (A/D) CONVERTERS I2 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. CC VOUT = (1V) • LOG (I1/I2) V+ 4 8 I1 LOG101 1 Q1 Q2 3 A2 A1 VOUT R2 R1 5 6 GND V– 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. Copyright © 2002-2004, Texas Instruments Incorporated 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. www.ti.com ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Supply Voltage, V+ to V– .................................................................... 36V Input Voltage .................................................... (V–) – 0.5 to (V+) + 0.5V Input Current ................................................................................... ±10mA 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. 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 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. 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. PIN DESCRIPTION Top View SO I1 1 8 I2 NC 2 7 NC VOUT 3 6 GND V+ 4 5 V– LOG101 NC = No Internal Connection PACKAGE/ORDERING INFORMATION(1) SPECIFIED TEMPERATURE RANGE PACKAGE MARKING ORDERING NUMBER TRANSPORT MEDIA, QUANTITY LOG101AID LOG101AIDR Rails, 100 Tape and Reel, 2500 PRODUCT PACKAGE-LEAD PACKAGE DESIGNATOR LOG101AID SO -8 D –5°C to +75°C LOG101 " " " " " 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 Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. At TA = +25°C, VS = ±5V, and ROUT = 10kΩ, unless otherwise noted. LOG101AID PARAMETER CONDITION 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 2 MIN TYP MAX VO = (1V) • log (I1/I2) 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) 0.01 0.06 0.0001 0.0005 1nA to 100µA 1nA to 100µA TMIN to TMAX 1 0.15 0.003 TMIN to TMAX VS = ±4.5V to ±18V TMIN to TMAX f = 10Hz to 10kHz f = 1kHz f = 1kHz TMIN to TMAX VS = ±5V ±3 ±2 (V–) + 1.2 V 0.2 ±1 0.01 ±0.3 ±2 5 ±5 Doubles Every 10°C 3 30 4 (V+) – 2 (V+) – 1.5 (V–) + 2 (V–) + 1.2 105 ±1.5 50 % % %/ °C %/ °C V/decade % %/ °C mV µV/°C µV/V pA µVrms nV/√Hz fA/√Hz V V dB ±15 (V+) – 1.5 ±18 UNITS mV µV/°C V mA 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) vs Temperature vs Supply CONDITION MIN MAX UNITS ±75 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±3.0 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.25 ±0.1 ±0.1 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 CC = 4500pF CC = 150pF CC = 150pF CC = 50pF 0.1 38 40 45 kHz kHz kHz kHz CC = 150pF CC = 150pF CC = 150pF 11 7 110 µs µs µs CC = 150pF CC = 150pF CC = 150pF 45 20 550 µs µs µs 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 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 VS IO = 0 TEMPERATURE RANGE Specified Range, TMIN to TMAX Operating Range Storage Range Thermal Resistance, θJA SO-8 TYP ±1.2 ±0.4 ±0.1 ±0.05 ±0.05 ±0.09 ±0.2 ±0.3 ±0.1 ±0.3 ±4.5 ±1 –5 –40 –55 ±18 ±1.5 V mA 75 85 125 °C °C °C °C/W 150 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. ONE CYCLE OF NORMALIZED TRANSFER FUNCTION NORMALIZED TRANSFER FUNCTION 4.0 VOUT = 1V LOG (I1/I2) Normalized Output Voltage (V) Normalized Output Voltage (V) 3.0 1 2.0 1.0 0.0 –1.0 –2.0 –3.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 –4.0 0.0001 0.001 0.01 0.1 1 10 100 1k 1 10k 2 Current Ratio, I1/I2 6 4 10 8 Current Ratio, I1/I2 TOTAL ERROR vs INPUT CURRENT 120 3 GAIN ERROR (I2 = 1µA) 5.8 +85°C 100 4.8 +75°C Gain Error (%) Total Error (mV) +75°C 80 60 40 +25°C 3.8 +25°C 2.8 –5°C to –40°C 1.8 –5°C 20 0.8 0 100pA 1nA 10nA 100nA 1µA –0.2 100pA 1nA 10µA 100µA 1mA 10mA 10nA 100nA 1µA Input Current (I1 or I2) 1M Select CC for I1 min. and I2 max. Values below 2pF may be ignored. 10µA I1 = 100pA I1 = 1nA 100k CC (pF) 1M 3dB Frequency Response (Hz) 10M I1 = 10nA 10k I1 = 100nA 1µA 1k 100 I1 = 10µA 100µA 1mA 10 1 100pA 1nA 10nA 100nA 1µA 100k 10k 1k 100µA 100µA 1mA I1 = 1mA 1µA 100µA CC =1 F 0p 100µA 1nA A 1µA 1mA to 10µA 10nA 100 I1 = 1nA 10 1 1µ to µA 0 1 10nA 0 00 CC =1 100nA 10nA pF CC I1 = 1nA µF =1 0.1 10µA 100µA 1mA 10mA 100pA 1nA 10nA 100nA 1µA 10µA 100µA 1mA I2 I2 4 10mA 3dB FREQUENCY RESPONSE MINIMUM VALUE OF COMPENSATION CAPACITOR 100M 10µA 100µA 1mA Input Current (I1 or I2) LOG101 www.ti.com SBOS242B TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. LOG CONFORMITY vs INPUT CURRENT 17 15 7 Decades (100pA to 1mA) 300 13 Log Conformity (m%) Log Conformity (mV) LOG CONFORMITY vs TEMPERATURE 350 +85°C 11 9 7 +75°C 5 –40°C to +25°C 250 6 Decades (1nA to 1mA) 200 150 5 Decades (1nA to 100µA) 100 3 50 1 –1 100pA 1nA 10nA 100nA 1µA 10µA 100µA 0 –40 –30 –20 –10 0 1mA Input Current (I1 or I2) APPLICATION INFORMATION 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. 10µF 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. V+ 1000pF 4 R2 10kΩ 1 V– V+ 6 3 LOG101 VOUT R1 1MΩ 5 1 8 I2 3 VOUT I1 5 LOG101 CC 6 GND 8 10µF I2 1000pF R1' > 1MΩ 4 V– CC V– FIGURE 1. Basic Connections of the LOG101. R2' 10kΩ V+ FIGURE 2. Bias Current Nulling. LOG101 SBOS242B 30 40 50 60 70 80 90 INPUT CURRENT RANGE 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. I1 10 20 Temperature (°C) www.ti.com 5 SETTING THE REFERENCE CURRENT V+ V+ 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. I1 = 2.5nA to 1mA REF3025 4 2.5V 3 1 VOUT 1GΩ to 2.5kΩ VOUT is expressed as: LOG101 100kΩ I2 = 2.5nA VOUT = (1V) • log (I1/I2) 10MΩ (1) 8 +25mV 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: 5 3 +2.5V CC 100Ω 6 GND V– OPA335 Chopper Op Amp –2.5V (2) RREF = 2.5V/10nA = 250MΩ FIGURE 5. Current Source with Offset Compensation. IREF 2N2905 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. RREF 3.6kΩ 2N2905 +15V –15V 6V IN834 IREF = FREQUENCY COMPENSATION 6V RREF 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. In an application, highest overall bandwidth can be achieved by detecting the signal level at VOUT, then switching in appropriate values of compensation capacitors. VREF = 100mV R1 R3 1 +5V R2 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. VOS + – IREF NEGATIVE INPUT CURRENTS A1 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. R3 >> R2 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. QA IIN 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. D1 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 6 QB D2 OPA703 IOUT FIGURE 6. Current Inverter/Current Source. LOG101 www.ti.com SBOS242B V+ +5V TLV271 or 1 OPA2335 2 +3.3V 4 I1 1 1/2 OPA2335 VOUT 10nA to 1mA LOG101 λ 1´ λ1 +5V 1/2 OPA2335 Back Bias D1 Sample 1.5kΩ 1.5kΩ 3 I2 Light λ 1 Source BSH203 +3.3V 10nA to 1mA Pin 1 or Pin 8 8 6 5 D2 CC LOG101 Photodiode V– FIGURE 9. Absorbance Measurement. FIGURE 7. Precision Current Inverter/Current Source. OPERATION ON SINGLE 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. 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. Single Supply +5V 4 3 1 APPLICATION CIRCUITS I1 LOG RATIO VOUT LOG101 6 One of the more common uses of log ratio amplifiers is to measure absorbance. A typical application is shown in Figure 9. 8 5 I2 CC Absorbance of the sample is A = logλ1´/ λ1 (3) If D1 and D2 are matched A ∝ (1V) logI1 / I2 (4) 1µF 3 2 5 TPS(1) 1µF DATA COMPRESSION 4 1 –5V 1µF NOTE: (1) TPS60402DBV negative charge pump. 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. FIGURE 10. Single +5V Power-Supply Operation. 1.5kΩ 100kΩ 100kΩ +5V 10nA to 1mA +3.3V Back Bias +3.3V 1/2 OPA2335 +5V 1.5kΩ 1/2 OPA2335 Photodiode 1.5kΩ 100kΩ 100kΩ LOG101 10nA to 1mA Pin 1 or Pin 8 FIGURE 8. Precision Current Inverter/Current Source. LOG101 SBOS242B www.ti.com 7 INSIDE THE LOG101 also 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 VOUT = VL VOUT = (1) k = Boltzman’s constant = 1.381 • 10–23 (9) R1 + R 2 I n VT log 1 R1 I2 VOUT = (1V) • log or T = Absolute temperature in degrees Kelvin R1 + R 2 R1 (10) I1 I2 (11) q = Electron charge = 1.602 • 10–19 Coulombs IC = Collector current IS = Reverse saturation current I1 From the circuit in Figure 11, we see that: VL = VBE1 – VBE 2 Q1 I1 IS1 – VT2 ln 1 2 A1 IS 2 I I VL = VT1 ln 1 – ln 2 I I S S I1 VL = VT ln and sin ce I2 R2 VL R1 FIGURE 11. Simplified Model of a Log Amplifier. (4) 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. (5) ln x = 2.3 log10 x (6) I VL = n VT log 1 I2 (7) where n = 2.3 (8) DEFINITION OF TERMS 3.5 TRANSFER FUNCTION 3.0 The ideal transfer function is: 2.5 VOUT = 1V • log (I1/I2) I1 I2 I2 (3) If the transistors are matched and isothermal and VTI = VT2, then (3) becomes: A 0p 10 I 2 = nA 1 I 2 = 0n A 1 I = 2.0 1.5 (5) 2 1.0 VOUT (V) Figure 12 shows the graphical representation of the transfer over valid operating range for the LOG101. 0.5 ACCURACY –0.5 –1.5 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. pA 100 1nA 0.0 –1.0 8 A2 VBE VOUT = (1V) • LOG I2 VOUT + VBE I1 (2) I2 Q2 – + Substituting (1) into (2) yields: VL = VT1 ln – –2.0 –2.5 –3.0 A 10n A 0n 10 I 2 = µA 1 I 2 = 0µA 1 I 2 = 00µA 1 I 2 = mA 1 I = nA µA 1 100 10µ A 100 µA A 1m A 10m I1 VOUT = (1V) • LOG (I1/I2) 2 –3.5 FIGURE 12. Transfer Function with Varying I 2 and I1. LOG101 www.ti.com SBOS242B 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. 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. The wide dynamic range of the LOG101 is also useful for measuring avalanche photodiode current (APD) (see Figure 14). (6) 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. 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. 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) USING A LARGER REFERENCE VOLTAGE REDUCES OFFSET ERRORS VOUT(NONLIN) = 1V/dec • 2NmV where N is the log conformity error, in percent. 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 IREF = VREF VOUT = VREF – R1 R2 R3 • (1V)LOG ( INDIVIDUAL ERROR COMPONENTS The ideal transfer function with current input is: VOUT = (1V) • log 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 IPHOTO ) R2 CC R3 VOUT VMIN to VMAX 3 IREF R1 (8) I1 I2 I1 Q1 1 Q2 A/D Converter OPA703 A2 VREF A1 IPHOTO R2 I2 R3 8 IMIN to IMAX LOG101 6 FIGURE 13. Technique for Using Full-Scale Reference Current Such that V OUT Increases with Increasing Photodiode Current. LOG101 SBOS242B www.ti.com 9 ISHUNT +15V to +60V 500Ω Irx = 1µA to 1mA Receiver 5kΩ 5kΩ 10Gbits/sec +5V APD INA168 SOT23-5 I to V Converter IOUT = 0.1 • ISHUNT 1 2 IOUT CC 1.2kΩ 1kΩ +5V 4 1 3 Q1 Q2 OPA703 VOUT = 2.5V to 0V A2 A1 100µA 25kΩ 8 LOG101 REF3025 2.5V SO-8 6 5 –5V FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current. The individual component of error is: Since the ideal output is 1.000V, the error as a percent of reading is ∆K = gain accuracy (0.15%, typ), as specified in the specification table. % error = IB1 = bias current of A1 (5pA, typ) IB2 = bias current of A2 (5pA, typ) 0.005055 • 100% = 0.5% 1 (12) For the case of voltage inputs, the actual transfer function is N = log conformity error (0.01%, 0.06%, typ) 0.01% for n = 5, 0.06% for n = 7 VOUT VOSO = output offset voltage (3mV, typ) n = number of decades over which N is specified: EOS1 V1 – IB1 ± R1 R1 = (1V) (1 ± ∆K ) log ± 2Nn ± VOSO EOS2 V2 – IB2 ± R2 R2 Example: what is the error when I1 = 1µA and I2 = 100nA VOUT = (1 ± 0.0015) log 10 −6 − 5 • 10 −12 ± (2)(0.0001)5 ± 3.0mV 10 −7 − 5 • 10 −12 = 1.005055V 10 (10) Where (13) EOS1 E and OS2 are considered to be zero for large R1 R2 values of resistance from external input current sources. (11) LOG101 www.ti.com SBOS242B PACKAGE OPTION ADDENDUM www.ti.com 9-Dec-2004 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Drawing Pins Package Eco Plan (2) Qty LOG101AID ACTIVE SOIC D 8 100 None CU SNPB Level-3-235C-168 HR LOG101AIDR ACTIVE SOIC D 8 2500 None CU SNPB Level-3-235C-168 HR Lead/Ball Finish MSL Peak Temp (3) (1) 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. 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