AD548 compensation is provided by resistors R8 and R15 that have a positive 3500 ppm/°C temperature coefficient. The transfer function for the output voltage is: V = 1V log (I ) OUT 10 2 / I1 Frequency compensation is provided by R11, R12, C1, and C2. Small signal bandwidth is approximately 300 kHz at input cur- rents above 100 µA and will proportionally decrease with lower signal levels. D1, D2, R13, and R14 compensate for the effects of the two logging transistors’ ohmic emitter resistance. To trim this circuit, set the two input currents to 10 µA and adjust VOUT to zero by adjusting the potentiometer on A3. Then set I2 to 1 µA and adjust the scale factor such that the output voltage is 1 V by trimming potentiometer R10. Offset adjustment for A1 and A2 is provided to increase the accuracy of the voltage inputs. This circuit ensures a 1% log conformance error over an input current range of 300 pA to 1 mA, with low level accuracy limited by the AD548’s input current. The low level input voltage accuracy of this circuit is limited by the input offset voltage and drift of the AD548. Figure 10. Log Ratio Amplifier –10– REV. D
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