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:
VOUT = 1V log10 ( I2 / 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