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LOG102AIDG4 Datasheet(PDF) 9 Page - Texas Instruments |
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LOG102AIDG4 Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 19 page  www.ti.com LOG102 9 SBOS211B A 2 A 1 I 1 Q 1 Q 2 V OUT = (1V) LOG I 1 I 2 I 2 I 1 I 2 ++ –– R 2 V OUT V L R 1 V BE1 V BE2 also VV RR R I I V RR R nV I I OUT L OUT T = + = = + 12 1 2 12 1 2 1 1 log log Using the base-emitter voltage relationship of matched bipolar transistors, the LOG102 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 : 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 11, we see that VV V LBE BE = 12 – Substituting (1) into (2) yields VV I I V I I LT S T S = 1 1 1 2 2 2 ln – ln If the transistors are matched and isothermal and VTI = VT2, then (3) becomes: VV I I I I VV I I and ce xx Vn V I I LT SS LT LT = = = = 1 12 1 2 10 1 2 23 ln – ln ln sin ln . log log where n = 2.3 INSIDE THE LOG102 (1) (2) (3) (4) (5) (6) (7) (9) (10) (11) FIGURE 11. Simplified Model of Log Amplifier. (8) or It should be noted that the temperature dependance associated with VT = kT/q is internally compensated on the LOG102 by making R1 a temperature sensitive resis- tor with the required positive temperature coefficient. USING A LARGER REFERENCE VOLTAGE REDUCES OFFSET ERRORS Using a larger reference voltage to create the reference current minimizes errors due to the LOG102’s input offset voltage. Maintaining an increasing output voltage as a func- tion 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 LOG102 configured with the reference current connecting I1 and the photodiode current connecting to I2 is shown in Figure 12. A3 is configured as a level shifter with inverting gain and is used to scale the photodiode current directly into the A/D input voltage range. The wide dynamic range of the LOG102 is useful for measuring avalanche photodiode current (APD) (see Figure 13). Not Recommended For New Designs |
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