AD548

–10–

REV. D

Figure 10. Log Ratio Amplifier

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

OUT

= 1V log

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

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.