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LOG112 Datasheet(PDF) 9 Page - Texas Instruments |
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LOG112 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 14 page LOG112, 2112 9 SBOS246C www.ti.com FIGURE 8. Precision Current Inverter/Current Source. 1k Ω +5V +5V 1.5k Ω 1.5k Ω 100k Ω 10nA to 1mA LOG112 10nA to 1mA 100k Ω 100k Ω +3.3V +3.3V 1/2 OPA2335 1/2 OPA2335 Photodiode Back Bias Pin 1 or Pin 14 100k Ω VOLTAGE INPUTS The LOG112 and LOG2112 give the best performances 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. ACHIEVING HIGHER ACCURACY WITH HIGHER INPUT CURRENTS As input current to the LOG112 increases, output accuracy degrades. For a 4.5mA input current on ±5V supplies and a 10mA input current on ±12V supplies, total output error can be between 15% and 25%. Applying a common-mode volt- age to VCM of at least +1V and up to 2.5V, brings the log transistors out of saturation and reduces output error to approximately 10%. To avoid forward biasing a photodiode, return the cathode to the VCM pin, as shown in Figure 9. To reverse bias the photodiode, apply a more positive voltage to the cathode than the anode. APPLICATION CIRCUITS LOG RATIO One of the more common uses of log ratio amplifiers is to measure absorbance. See Figure 10 for a typical application. Absorbance of the sample is A = log λ 1´/ λ1 (3) If D1 and D2 are matched A ∝ (0.5V) logI1/I2 (4) FIGURE 9. Extending Input Current Level and Improving Accuracy by Applying a Common-Mode Voltage. A 2 A 1 Q 1 Q 2 C C I 1 V REF – GND V O3 V+ = +5V GND LOG112 A 3 R 2 R 1 V REF 100k Ω +2.5V 150k Ω R REF +IN3 V LOGOUT –IN3 V– = –5V V CM I 2 V REF 65 3 4 7 9 10 13 11 8 14 1 +1.5V |
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