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LOG104AID Datasheet(PDF) 9 Page - Texas Instruments |
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LOG104AID Datasheet(HTML) 9 Page - Texas Instruments |
9 / 13 page www.ti.com LOG104 9 SBOS243B TOTAL ERROR The total error is the deviation (expressed in mV) of the actual output from the ideal output of VOUT = 0.5V • log(I1/I2). Thus, VOUT(ACTUAL) = VOUT(IDEAL) ± Total Error. 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. ERRORS RTO AND RTI As with any transfer function, errors generated by the func- tion itself may be Referred-to-Output (RTO) or Referred-to- Input (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. (6) FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current. A 1 –5V SO-8 5 LOG104 Q 1 2 Q 2 I OUT I OUT = 0.1 • ISHUNT 6 1.2k Ω Receiver Irx = 1 µA to 1mA I to V Converter APD 10Gbits/sec 1k Ω C C A 2 OPA703 V OUT = 2.5V to 0V I SHUNT 100 µA 500 Ω 5k Ω 5k Ω +15V to +60V +5V 25k Ω INA168 SOT23-5 REF3025 2.5V +5V 1 1 8 4 3 MEASURING AVALANCHE PHOTODIODE CURRENT The wide dynamic range of the LOG104 is useful for measuring avalanche photodiode current (APD), as shown in Figure 13. LOG CONFORMITY For the LOG104, 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 specifica- tion. This is true 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. 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: VOUT (NONLIN) = 0.5V/dec • 2Nm V where N is the log conformity error, in percent. (7) |
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