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LOG112 Datasheet(PDF) 7 Page - Texas Instruments |
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LOG112 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 20 page LOG112, 2112 7 SBOS246D www.ti.com FIGURE 1. Basic Connections of the LOG112. APPLICATION INFORMATION The LOG112 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 and Figure 2 show the basic connections required for operation of the LOG112 and LOG2112. In order to reduce the influence of lead inductance of power-supply lines, it is recommended that each supply be bypassed with a 10 µF tantalum capacitor in parallel with a 1000pF ceramic capacitor, as shown in Figure 1 and Figure 2. Connecting the capacitors as close to the LOG112 and LOG2112 as possible will contribute to noise reduction as well. INPUT CURRENT RANGE To maintain specified accuracy, the input current range of the LOG112 and LOG2112 should be limited from 100pA to 3.5mA. Input currents outside of this range may compromise the LOG112 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 input transistor. On ±5V supplies, the total input current (I 1 + I2) is limited to 4.5mA. Due to compliance issues internal to the LOG112 and LOG2112, to accommodate larger total input currents, supplies should be increased. SETTING THE REFERENCE CURRENT When the LOG112 and LOG2112 are used to compute loga- rithms, either I1 or I2 can be held constant to become the reference current to which the other is compared. VLOGOUT is expressed as: VLOGOUT = (0.5V)LOG (I1/IREF) (1) IREF can be derived from an external current source (such as that 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 (2) 14 1 5 8 6 910 13 V– V+ 10 µF LOG112 1000pF 10 µF 1000pF I 1 I 2 V LOGOUT V REF 11 V REF – GND V CM – IN C C 15 16 5 6 11 8 V– V+ 10 µF LOG2112 1000pF 10 µF 1000pF I 1B I 2B 1 2 I 2A I 1A V LOGOUTA 9 V REF 12 V LOGOUTB C CB C CA FIGURE 2. Basic Connections of the LOG2112. FIGURE 3. Temperature Compensated Current Source. 2N2905 I REF R REF 2N2905 +15V –15V I REF = 6V R REF 3.6k Ω 6V IN834 FIGURE 4. T Network for Reference Current. A 1 + R 2 R 1 +5V R 3 V REF = 100mV R 3 >> R2 I REF – V OS 1 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. |
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