AD548
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
TO-99 (H) Package
Plastic Mini-DIP (N) Package
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
10
(I
2
/
I
1
)
Cerdip (Q) Package
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-
just V
OUT
to zero by adjusting the potentiometer on A3. Then
set I
2
to 1
µA
and adjust the scale factor such that the output
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.
SOIC (R) Package
–8–
REV. C
PRINTED IN U.S.A.
C999a–19–12/86
AD [ ANALOG DEVICES ]
