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LOG2112AIDWRE4 Datasheet(PDF) 11 Page - Texas Instruments |
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LOG2112AIDWRE4 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 20 page  LOG112, 2112 11 SBOS246D www.ti.com also VV RR R OUT L = + 12 1 (9) V RR R nV I I OUT T = + 12 1 1 2 log (10) or V V LOG I I OUT = (. ) 05 1 2 (11) Using the base-emitter voltage relationship of matched bipolar transistors, the LOG112 establishes a logarith- mic function of input current ratios. Beginning with the base-emitter voltage defined as: VV I I where V kT q BE T C S T == ln : (1) k = Boltzmann’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 12: VV V LBE BE = 12 – (2) Substituting (1) into (2) yields: VV I I V I I LT S T S = 1 1 1 2 2 2 ln – ln (3) If the transistors are matched and isothermal and VTI = VT2, then (3) becomes: VV I I I I LT SS = 1 12 ln – ln (4) VV I I and ce LT = ln sin 1 2 (5) ln . log xx = 23 10 (6) Vn V I I LT = log 1 2 (7) where n = 2.3 (8) INSIDE THE LOG112 FIGURE 13. Simplified Model of a Log Amplifier. A 2 A 1 I 1 Q 1 Q 2 I 2 I 1 I 2 ++ –– R 2 V OUT V L R 1 V BE1 V BE2 V OUT = (0.5V)LOG I 1 I 2 NOTE: R1 is a metal resistor used to compensate for gain over temperature. DEFINITION OF TERMS TRANSFER FUNCTION The ideal transfer function is: VLOGOUT = (0.5V)LOG (I1/I2) Figure 14 shows the graphical representation of the transfer over valid operating range for the LOG112 and LOG2112. ACCURACY Accuracy considerations for a log ratio amplifier are some- what 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. TOTAL ERROR The total error is the deviation (expressed in mV) of the actual output from the ideal output of VLOGOUT = (0.5V)LOG (I1/I2). Thus, VLOGOUT(ACTUAL) = VLOGOUT(IDEAL) ± Total Error (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. FIGURE 14. Transfer Function with Varying I2 and I1. 3.0 3.5 2.0 2.5 1.0 1.5 0.5 0 –3.0 –3.5 –2.0 –2.5 –1.0 –0.5 –1.5 1nA 10nA 100nA 1µ A 10µ A 100 µA 1mA 10m A 100pA I 2 = 100pA I 2 = 1nA I 2 = 10nA I 2 = 100nA I 2 = 1µ A I 2 = 10 µA I 2 = 100 µA I 2 = 1m A I 1 V LOGOUT = (0.5V)LOG (I1/I2) |
Similar Part No. - LOG2112AIDWRE4 |
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Similar Description - LOG2112AIDWRE4 |
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