AD548

–8–

REV. C

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Figure 30. Log Ratio Amplifier

compensation is provided by resistors R8 and R15, which 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 ad-

voltage is 1 V by trimming potentiometer R10. Offset adjust-

ment 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 volt-

age accuracy of this circuit is limited by the input offset voltage

and drift of the AD548.

TO-99 (H) Package

SOIC (R) Package

Plastic Mini-DIP (N) Package

Cerdip (Q) Package